JBoss DNA uses a simple abstraction layer to isolate it from the security infrastructure used within an application. A SecurityContext represents the context of an authenticated user, and is defined as an interface:

public interface SecurityContext {

    /**
     * Get the name of the authenticated user.
     * @return the authenticated user's name
     */
    String getUserName();

    /**
     * Determine whether the authenticated user has the given role.
     * @param roleName the name of the role to check
     * @return true if the user has the role and is logged in; false otherwise
     */
    boolean hasRole( String roleName );

    /**
     * Logs the user out of the authentication mechanism.
     * For some authentication mechanisms, this will be implemented as a no-op.
     */
    void logout();
}

Every ExecutionContext has a SecurityContext instance, though the top-level (default) execution context does not represent an authenticated user. But you can create a subcontext for a user authenticated via JAAS:

ExecutionContext context = ...
String username = ...
char[] password = ...
String jaasRealm = ...
SecurityContext securityContext = new JaasSecurityContext(jaasRealm, username, password);
ExecutionContext userContext = context.with(securityContext);

In the case of JAAS, you might not have the password but would rather prompt the user. In that case, simply create a subcontext with a different security context:

ExecutionContext context = ...
String jaasRealm = ...
CallbackHandler callbackHandler = ...
ExecutionContext userContext = context.with(new JaasSecurityContext(jaasRealm, callbackHandler);

Of course if your application has a non-JAAS authentication and authorization system, you can simply provide your own implementation of SecurityContext:

ExecutionContext context = ...
SecurityContext mySecurityContext = ...
ExecutionContext myAppContext = context.with(mySecurityContext);

These ExecutionContext then represent the authenticated user in any component that uses the context.

@Immutable
public class ServletSecurityContext implements SecurityContext {

    private final String userName;
    private final HttpServletRequest request;

    /**
     * Create a {@link ServletSecurityContext} with the supplied 
     * {@link HttpServletRequest servlet information}.
     * 
     * @param request the servlet request; may not be null
     */
    public ServletSecurityContext( HttpServletRequest request ) {
        this.request = request;
        this.userName = request.getUserPrincipal() != null ? request.getUserPrincipal().getName() : null;
    }

    /**
     * Get the name of the authenticated user.
     * @return the authenticated user's name
     */
    public String getUserName() {
        return userName;
    }

    /**
     * Determine whether the authenticated user has the given role.
     * @param roleName the name of the role to check
     * @return true if the user has the role and is logged in; false otherwise
     */
    boolean hasRole( String roleName ) {
        request.isUserInRole(roleName);
    }

    /**
     * Logs the user out of the authentication mechanism.
     * For some authentication mechanisms, this will be implemented as a no-op.
     */
    public void logout() {
    }
}

HttpServletRequest request = ...
Repository repository = engine.getRepository("my repository");
SecurityContext securityContext = new ServletSecurityContext(httpServletRequest);
ExecutionContext servletContext = context.with(securityContext);

As we saw earlier, every ExecutionContext has a registry of namespaces. Namespaces are used throughout the graph API (as we'll see soon), and the prefix associated with each namespace makes for more readable string representations. The namespace registry tracks all of these namespaces and prefixes, and allows registrations to be added, modified, or removed. The interface for the NamespaceRegistry shows how these operations are done:

public interface NamespaceRegistry {

    /**
     * Return the namespace URI that is currently mapped to the empty prefix.
     * @return the namespace URI that represents the default namespace, 
     * or null if there is no default namespace
     */
    String getDefaultNamespaceUri();

    /**
     * Get the namespace URI for the supplied prefix.
     * @param prefix the namespace prefix
     * @return the namespace URI for the supplied prefix, or null if there is no 
     * namespace currently registered to use that prefix
     * @throws IllegalArgumentException if the prefix is null
     */
    String getNamespaceForPrefix( String prefix );

    /**
     * Return the prefix used for the supplied namespace URI.
     * @param namespaceUri the namespace URI
     * @param generateIfMissing true if the namespace URI has not already been registered and the 
     *        method should auto-register the namespace with a generated prefix, or false if the  
     *        method should never auto-register the namespace
     * @return the prefix currently being used for the namespace, or "null" if the namespace has 
     *         not been registered and "generateIfMissing" is "false"
     * @throws IllegalArgumentException if the namespace URI is null
     * @see #isRegisteredNamespaceUri(String)
     */
    String getPrefixForNamespaceUri( String namespaceUri, boolean generateIfMissing );

    /**
     * Return whether there is a registered prefix for the supplied namespace URI.
     * @param namespaceUri the namespace URI
     * @return true if the supplied namespace has been registered with a prefix, or false otherwise
     * @throws IllegalArgumentException if the namespace URI is null
     */
    boolean isRegisteredNamespaceUri( String namespaceUri );

    /**
     * Register a new namespace using the supplied prefix, returning the namespace URI previously 
     * registered under that prefix.
     * @param prefix the prefix for the namespace, or null if a namesapce prefix should be generated 
     *        automatically
     * @param namespaceUri the namespace URI
     * @return the namespace URI that was previously registered with the supplied prefix, or null if the 
     *         prefix was not previously bound to a namespace URI
     * @throws IllegalArgumentException if the namespace URI is null
     */
    String register( String prefix, String namespaceUri );

    /**
     * Unregister the namespace with the supplied URI.
     * @param namespaceUri the namespace URI
     * @return true if the namespace was removed, or false if the namespace was not registered
     * @throws IllegalArgumentException if the namespace URI is null
     * @throws NamespaceException if there is a problem unregistering the namespace
     */
    boolean unregister( String namespaceUri );

    /**
     * Obtain the set of namespaces that are registered.
     * @return the set of namespace URIs; never null
     */
    Set<String> getRegisteredNamespaceUris();

    /**
     * Obtain a snapshot of all of the {@link Namespace namespaces} registered at the time this method 
     * is called. The resulting set is immutable, and will not reflect changes made to the registry.
     * @return an immutable set of Namespace objects reflecting a snapshot of the registry; never null
     */
    Set<Namespace> getNamespaces();
}

This interfaces exposes Namespace objects that are immutable:

@Immutable
interface Namespace extends Comparable<Namespace> {
    /**
     * Get the prefix for the namespace
     * @return the prefix; never null but possibly the empty string
     */
    String getPrefix();

    /**
     * Get the URI for the namespace
     * @return the namespace URI; never null but possibly the empty string
     */
    String getNamespaceUri();
}

JBoss DNA actually uses several implementations of NamespaceRegistry, but you can even implement your own and create ExecutionContexts that uses it:

NamespaceRegistry myRegistry = ...
ExecutionContext contextWithMyRegistry = context.with(myRegistry);

JBoss DNA is designed around extensions: sequencers, connectors, MIME type detectors, and class loader factories. The core part of JBoss DNA is relatively small and has few dependencies, while many of the "interesting" components are extensions that plug into and are used by different parts of the core or by layers above (such as the JCR implementation). The core doesn't really care what the extensions do or what external libraries they require, as long as the extension fulfills its end of the extension contract.

This means that you only need the core modules of JBoss DNA on the application classpath, while the extensions do not have to be on the application classpath. And because the core modules of JBoss DNA have few dependencies, the risk of JBoss DNA libraries conflicting with the application's are lower. Extensions, on the other hand, will likely have a lot of unique dependencies. By separating the core of JBoss DNA from the class loaders used to load the extensions, your application is isolated from the extensions and their dependencies.

But in this case, how does JBoss DNA load all the extension classes? You may have noticed earlier that ExecutionContext implements the ClassLoaderFactory interface with a single method:

public interface ClassLoaderFactory {
    /**
     * Get a class loader given the supplied classpath.  The meaning of the classpath 
     * is implementation-dependent.
     * @param classpath the classpath to use
     * @return the class loader; may not be null
     */
    ClassLoader getClassLoader( String... classpath );
}

This means that any component that has a reference to an ExecutionContext has the ability to create a class loader with a supplied class path. As we'll see later, the connectors and sequencers are all defined with a class and optional class path. This is where that class path comes in.

The actual meaning of the class path, however, is a function of the implementation. JBoss DNA uses a StandardClassLoaderFactory that just loads the classes using the Thread's current context class loader (or, if there is none, delegates to the class loader that loaded the StandardClassLoaderFactory class). Of course, it's possible to implement other ClassLoaderFactory with other implementations. Then, just create a subcontext with your implementation:

ClassLoaderFactory myClassLoaderFactory = ...
ExecutionContext contextWithMyClassLoaderFactories = context.with(myClassLoaderFactory);

JBoss DNA often needs the ability to determine the MIME type for some binary content. When uploading content into a repository, we may want to add the MIME type as metadata. Or, we may want to make some processing decisions based upon the MIME type. So, JBoss DNA created a small pluggable framework for determining the MIME type by using the name of the file (e.g., extensions) and/or by reading the actual content.

JBoss DNA defines a MimeTypeDetector interface that abstracts the implementation that actually determines the MIME type given the name and content. If the detector is able to determine the MIME type, it simply returns it as a string. If not, it merely returns null. Note, however, that a detector must be thread-safe. Here is the interface:

@ThreadSafe
public interface MimeTypeDetector {

    /**
     * Returns the MIME-type of a data source, using its supplied content and/or its supplied name, 
     * depending upon the implementation. If the MIME-type cannot be determined, either a "default" 
     * MIME-type or null may be returned, where the former will prevent earlier 
     * registered MIME-type detectors from being consulted.
     * 
     * @param name The name of the data source; may be null.
     * @param content The content of the data source; may be null.
     * @return The MIME-type of the data source, or optionally null 
     * if the MIME-type could not be determined.
     * @throws IOException If an error occurs reading the supplied content.
     */
    String mimeTypeOf( String name, InputStream content ) throws IOException;
}

To use a detector, simply invoke the method and supply the name of the content (e.g., the name of the file, with the extension) and the InputStream to the actual binary content. The result is a String containing the MIME type (e.g., "text/plain") or null if the MIME type cannot be determined. Note that the name or InputStream may be null, making this a very versatile utility.

Once again, you can obtain a MimeTypeDetector from the ExecutionContext. JBoss DNA provides and uses by default an implementation that uses only the name (the content is ignored), looking at the name's extension and looking for a match in a small listing (loaded from the org/jboss/dna/graph/mime.types loaded from the classpath). You can add extensions by copying this file, adding or correcting the entries, and then placing your updated file in the expected location on the classpath.

Of course, you can always use a different MimeTypeDetector by creating a subcontext and supplying your implementation:

MimeTypeDetector myDetector = ...
ExecutionContext contextWithMyDetector = context.with(myDetector);

Two other components are made available by the ExecutionContext. The PropertyFactory is an interface that can be used to create Property instances, which are used throughout the graph API. The ValueFactories interface provides access to a number of different factories for different kinds of property values. These will be discussed in much more detail in the next chapter. But like the other components that are in an ExecutionContext, you can create subcontexts with different implementations:

PropertyFactory myPropertyFactory = ...
ExecutionContext contextWithMyPropertyFactory = context.with(myPropertyFactory);

and

ValueFactories myValueFactories = ...
ExecutionContext contextWithMyValueFactories = context.with(myValueFactories);

Of course, implementing your own factories is a pretty advanced topic, and it will likely be something you do not need to do in your application.

In this chapter, we introduced the ExecutionContext as a representation of the environment in which many of the JBoss DNA components operate. ExecutionContext provides a very simple but powerful way to inject commonly-needed facilities throughout the system.

In the next chapter, we'll dive into Graph API and will introduce the notion of nodes, paths, names, and properties, that are so essential and used throughout JBoss DNA.

JBoss DNA uses names to identify quite a few different types of objects. As we'll soon see, each property of a node is given by a name, and each segment in a path is comprised of a name. Therefore, names are a very important concept.

JBoss DNA names consist of a local part and are qualified with a namespaces. The local part can consist of any character, and the namespace is identified by a URI. Namespaces were introduced in the previous chapter and are managed by the ExecutionContext's namespace registry. Namespaces help reduce the risk of clashes in names that have an equivalent same local part.

All names are immutable, which means that once a Name object is created, it will never change. This characteristic makes it much easier to write thread-safe code - the objects never change and therefore require no locks or synchronization to guarantee atomic reads. This is a technique that is more and more often found in newer languages and frameworks that simplify concurrent operations.

Name is also a interface rather than a concrete class:

@Immutable
public interface Name extends Comparable<Name>, Serializable, Readable {

    /**
     * Get the local name part of this qualified name.
     * @return the local name; never null
     */
    String getLocalName();

    /**
     * Get the URI for the namespace used in this qualified name.
     * @return the URI; never null but possibly empty
     */
    String getNamespaceUri();
}

This means that you need to use a factory to create Name instances.

The use of a factory may seem like a disadvantage and unnecessary complexity, but there actually are several benefits. First, it hides the concrete implementations, which is very appealing if an optimized implementation can be chosen for particular situations. It also simplifies the usage, since Name only has a few methods. Third, it allows the factory to cache or pool instances where appropriate to help conserve memory. Finally, the very same factory actually serves as a conversion mechanism from other forms. We'll actually see more of this later in this chapter, when we talk about other kinds of property values.

The factory for creating Name objects is called NameFactory and is available within the ExecutionContext, via the getValueFactories() method. But before we see that, let's first discuss how names are represented as strings.

We'll see how names are used later one, but one more point to make: Name is both serializable and comparable, and all implementations should support equals(...) and hashCode() so that Name can be used as a key in a hash-based map. Name also extends the Readable interface, which we'll learn more about later in this chapter.

Another important concept in JBoss DNA's graph model is that of a path, which provides a way of locating a node within a hierarchy. JBoss DNA's Path object is an immutable ordered sequence of Path.Segment objects. A small portion of the interface is shown here:

@Immutable
public interface Path extends Comparable<Path>, Iterable<Path.Segment>, Serializable, Readable {

    /**
     * Return the number of segments in this path.
     * @return the number of path segments
     */
    public int size();

    /**
     * Return whether this path represents the root path.
     * @return true if this path is the root path, or false otherwise
     */
    public boolean isRoot();

    /**
     * {@inheritDoc}
     */
    public Iterator<Path.Segment> iterator();

    /**
     * Obtain a copy of the segments in this path. None of the segments are encoded.
     * @return the array of segments as a copy
     */
    public Path.Segment[] getSegmentsArray();

    /**
     * Get an unmodifiable list of the path segments.
     * @return the unmodifiable list of path segments; never null
     */
    public List<Path.Segment> getSegmentsList();
    /**
     * Get the last segment in this path.
     * @return the last segment, or null if the path is empty
     */
    public Path.Segment getLastSegment();

    /**
     * Get the segment at the supplied index.
     * @param index the index
     * @return the segment
     * @throws IndexOutOfBoundsException if the index is out of bounds
     */
    public Path.Segment getSegment( int index );

    /**
     * Return an iterator that walks the paths from the root path down to this path. This method 
     * always returns at least one path (the root returns an iterator containing itself).
     * @return the path iterator; never null
     */
    public Iterator<Path> pathsFromRoot();

    /**
     * Return a new path consisting of the segments starting at beginIndex index (inclusive). 
     * This is equivalent to calling path.subpath(beginIndex,path.size()-1).
     * @param beginIndex the beginning index, inclusive.
     * @return the specified subpath
     * @exception IndexOutOfBoundsException if the beginIndex is negative or larger 
     *            than the length of this Path object
     */
    public Path subpath( int beginIndex );

    /**
     * Return a new path consisting of the segments between the beginIndex index (inclusive)
     * and the endIndex index (exclusive).
     * @param beginIndex the beginning index, inclusive.
     * @param endIndex the ending index, exclusive.
     * @return the specified subpath
     * @exception IndexOutOfBoundsException if the beginIndex is negative, or 
     *            endIndex is larger than the length of this Path 
     *            object, or beginIndex is larger than endIndex.
     */
    public Path subpath( int beginIndex, int endIndex );

    ...
}   

There are actually quite a few methods (not shown above) for obtaining related paths: the path of the parent, the path of an ancestor, resolving a path relative to this path, normalizing a path (by removing "." and ".." segments), finding the lowest common ancestor shared with another path, etc. There are also a number of methods that compare the path with others, including determining whether a path is above, equal to, or below this path.

Each Path.Segment is an immutable pair of a Name and same-name-sibling (SNS) index. When two sibling nodes have the same name, then the first sibling will have SNS index of "1" and the second will be given a SNS index of "2". (This mirrors the same-name-sibling index behavior of JCR paths.)

@Immutable
public static interface Path.Segment extends Cloneable, Comparable<Path.Segment>, Serializable, Readable {

    /**
     * Get the name component of this segment.
     * @return the segment's name
     */
    public Name getName();

    /**
     * Get the index for this segment, which will be 1 by default.
     * @return the index
     */
    public int getIndex();

    /**
     * Return whether this segment has an index that is not "1"
     * @return true if this segment has an index, or false otherwise.
     */
    public boolean hasIndex();

    /**
     * Return whether this segment is a self-reference (or ".").
     * @return true if the segment is a self-reference, or false otherwise.
     */
    public boolean isSelfReference();

    /**
     * Return whether this segment is a reference to a parent (or "..")
     * @return true if the segment is a parent-reference, or false otherwise.
     */
    public boolean isParentReference();
}
		

Like Name, the only way to create a Path or a Path.Segment is to use the PathFactory, which is available within the ExecutionContext via the getValueFactories() method.

The JBoss DNA graph model allows nodes to hold multiple properties, where each property is identified by a unique Name and may have one or more values. Like many of the other classes used in the graph model, Property is an immutable object that, once constructed, can never be changed and therefore provides a consistent snapshot of the state of a property as it existed at the time it was read.

JBoss DNA properties can hold a wide range of value objects, including normal Java strings, names, paths, URIs, booleans, longs, doubles, decimals, binary content, dates, UUIDs, references to other nodes, or any other serializable object. All but three these are the standard Java classes: dates are represented by an immutable DateTime class; binary content is represented by an immutable Binary interface patterned after the proposed interface of the same name in JSR-283; and is an immutable interface patterned after the corresponding interface is JSR-170 and JSR-283.

The Property interface defines methods for obtaining the name and property values:

@Immutable
public interface Property extends Iterable<Object>, Comparable<Property>, Readable {

    /**
     * Get the name of the property.
     * 
     * @return the property name; never null
     */
    Name getName();

    /**
     * Get the number of actual values in this property.
     * @return the number of actual values in this property; always non-negative
     */
    int size();

    /**
     * Determine whether the property currently has multiple values.
     * @return true if the property has multiple values, or false otherwise.
     */
    boolean isMultiple();

    /**
     * Determine whether the property currently has a single value.
     * @return true if the property has a single value, or false otherwise.
     */
    boolean isSingle();

    /**
     * Determine whether this property has no actual values. This method may return true 
     * regardless of whether the property has a single value or multiple values.
     * This method is a convenience method that is equivalent to size() == 0.
     * @return true if this property has no values, or false otherwise
     */
    boolean isEmpty();

    /**
     * Obtain the property's first value in its natural form. This is equivalent to calling
     * isEmpty() ? null : iterator().next()
     * @return the first value, or null if the property is {@link #isEmpty() empty}
     */
    Object getFirstValue();

    /**
     * Obtain the property's values in their natural form. This is equivalent to calling iterator().
     * A valid iterator is returned if the property has single valued or multi-valued.
     * The resulting iterator is immutable, and all property values are immutable.
     * @return an iterator over the values; never null
     */
    Iterator<?> getValues();

    /**
     * Obtain the property's values as an array of objects in their natural form.
     * A valid iterator is returned if the property has single valued or multi-valued, or a
     * null value is returned if the property is {@link #isEmpty() empty}.
     * The resulting array is a copy, guaranteeing immutability for the property.
     * @return the array of values
     */
    Object[] getValuesAsArray();
}
		

Creating Property instances is done by using the PropertyFactory object owned by the ExecutionContext. This factory defines methods for creating properties with a Name and various representation of values, including variable-length arguments, arrays, Iterator, and .

When it comes to using the property values, JBoss DNA takes a non-traditional approach. Many other graph models (including JCR) mark each property with a data type and then require all property values adhere to this data type. When the property values are obtained, they are guaranteed to be of the correct type. However, many times the property's data type may not match the data type expected by the caller, and so a conversion may be required and thus has to be coded.

The JBoss DNA graph model uses a different tact. Because callers almost always have to convert the values to the types they can handle, JBoss DNA skips the steps of associating the Property with a data type and ensuring the values match. Instead, JBoss DNA simply provides a very easy mechanism to convert the property values to the type desired by the caller. In fact, the conversion mechanism is exactly the same as the factories that create the values in the first place.

JBoss DNA properties can hold a variety of types of value objects: strings, names, paths, URIs, booleans, longs, doubles, decimals, binary content, dates, UUIDs, references to other nodes, or any other serializable object. To assist in the creation of these values and conversion into other types, JBoss DNA defines a ValueFactory interface. This interface is parameterized with the type of value that is being created, but defines methods for creating those values from all of the other known value types:

public interface ValueFactory<T> {

    /**
     * Get the PropertyType of values created by this factory.
     * @return the value type; never null
     */
    PropertyType getPropertyType();

		/*
		 * Methods to create a value by converting from another value type.
		 * If the supplied value is the same type as returned by this factory,
		 * these methods simply return the supplied value.
		 * All of these methods throw a ValueFormatException if the supplied value
		 * could not be converted to this type.
		 */
    T create( String value ) throws ValueFormatException;
    T create( String value, TextDecoder decoder ) throws ValueFormatException;
    T create( int value ) throws ValueFormatException;
    T create( long value ) throws ValueFormatException;
    T create( boolean value ) throws ValueFormatException;
    T create( float value ) throws ValueFormatException;
    T create( double value ) throws ValueFormatException;
    T create( BigDecimal value ) throws ValueFormatException;
    T create( Calendar value ) throws ValueFormatException;
    T create( Date value ) throws ValueFormatException;
    T create( DateTime value ) throws ValueFormatException;
    T create( Name value ) throws ValueFormatException;
    T create( Path value ) throws ValueFormatException;
    T create(  value ) throws ValueFormatException;
    T create( URI value ) throws ValueFormatException;
    T create( UUID value ) throws ValueFormatException;
    T create( byte[] value ) throws ValueFormatException;
    T create( Binary value ) throws ValueFormatException, IoException;
    T create( InputStream stream, long approximateLength ) throws ValueFormatException, IoException;
    T create( Reader reader, long approximateLength ) throws ValueFormatException, IoException;
    T create( Object value ) throws ValueFormatException, IoException;

    /*
     * Methods to create an array of values by converting from another array of values.
     * If the supplied values are the same type as returned by this factory,
     * these methods simply return the supplied array.
     * All of these methods throw a ValueFormatException if the supplied values
     * could not be converted to this type.
		 */
    T[] create( String[] values ) throws ValueFormatException;
    T[] create( String[] values, TextDecoder decoder ) throws ValueFormatException;
    T[] create( int[] values ) throws ValueFormatException;
    T[] create( long[] values ) throws ValueFormatException;
    T[] create( boolean[] values ) throws ValueFormatException;
    T[] create( float[] values ) throws ValueFormatException;
    T[] create( double[] values ) throws ValueFormatException;
    T[] create( BigDecimal[] values ) throws ValueFormatException;
    T[] create( Calendar[] values ) throws ValueFormatException;
    T[] create( Date[] values ) throws ValueFormatException;
    T[] create( DateTime[] values ) throws ValueFormatException;
    T[] create( Name[] values ) throws ValueFormatException;
    T[] create( Path[] values ) throws ValueFormatException;
    T[] create( [] values ) throws ValueFormatException;
    T[] create( URI[] values ) throws ValueFormatException;
    T[] create( UUID[] values ) throws ValueFormatException;
    T[] create( byte[][] values ) throws ValueFormatException;
    T[] create( Binary[] values ) throws ValueFormatException, IoException;
    T[] create( Object[] values ) throws ValueFormatException, IoException;

    /**
     * Create an iterator over the values (of an unknown type). The factory converts any 
     * values as required.  This is useful when wanting to iterate over the values of a property,
     * where the resulting iterator exposes the desired type.
     * @param values the values
     * @return the iterator of type T over the values, or null if the supplied parameter is null
     * @throws ValueFormatException if the conversion from an iterator of objects could not be performed
     * @throws IoException If an unexpected problem occurs during the conversion.
     */
    Iterator<T> create( Iterator<?> values ) throws ValueFormatException, IoException;
    Iterable<T> create( <?> valueIterable ) throws ValueFormatException, IoException;
}
	

This makes it very easy to convert one or more values (of any type, including mixtures) into corresponding value(s) that are of the desired type. For example, converting the first value of a property (regardless of type) to a String is simple:

ValueFactory<String> stringFactory = ...
Property property = ...
String value = stringFactory.create( property.getFirstValue() );
		

Likewise, iterating over the values in a property and converting them is just as easy:

ValueFactory<String> stringFactory = ...
Property property = ...
for ( String value : stringFactory.create(property) ) {
    // do something with the values
}
		

What we've glossed over so far, however, is how to obtain the correct ValueFactory for the desired type. If you remember back to the previous chapter, ExecutionContext has a getValueFactories() method that return a ValueFactories interface:

This interface exposes a ValueFactory for each of the types, and even has methods to obtain a ValueFactory given the PropertyType enumeration. So, the previous examples could be expanded a bit:

ValueFactory<String> stringFactory = context.getValueFactories().getStringFactory();
Property property = ...
String value = stringFactory.create( property.getFirstValue() );
		

and

ValueFactory<String> stringFactory = context.getValueFactories().getStringFactory();
Property property = ...
for ( String value : stringFactory.create(property) ) {
    // do something with the values
}
		

You might have noticed that several of the ValueFactories methods return subinterfaces of ValueFactory. These add type-specific methods that are more commonly needed in certain cases. For example, here is the NameFactory interface:

public interface NameFactory extends ValueFactory<Name> {

    Name create( String namespaceUri, String localName );
    Name create( String namespaceUri, String localName, TextDecoder decoder );

    NamespaceRegistry getNamespaceRegistry();
}
		

and here is the DateTimeFactory interface, which adds methods for creating DateTime values for the current time as well as for specific instants in time:

public interface DateTimeFactory extends ValueFactory<DateTime> {

    /**
     * Create a date-time instance for the current time in the local time zone.
     */
    DateTime create();

    /**
     * Create a date-time instance for the current time in UTC.
     */
    DateTime createUtc();

    DateTime create( DateTime original, long offsetInMillis );
    DateTime create( int year, int monthOfYear, int dayOfMonth,
                     int hourOfDay, int minuteOfHour, int secondOfMinute, int millisecondsOfSecond );
    DateTime create( int year, int monthOfYear, int dayOfMonth,
                     int hourOfDay, int minuteOfHour, int secondOfMinute, int millisecondsOfSecond,
                     int timeZoneOffsetHours );
    DateTime create(	int year, int monthOfYear, int dayOfMonth,
                     int hourOfDay, int minuteOfHour, int secondOfMinute, int millisecondsOfSecond,
                     int timeZoneOffsetHours, String timeZoneId );
}
		

The PathFactory interface defines methods for creating relative and absolute Path objects using combinations of other Path objects and Names and Path.Segments, and introduces methods for creating Path.Segment objects:

public interface PathFactory extends ValueFactory<Path> {

    Path createRootPath();
    Path createAbsolutePath( Name... segmentNames );
    Path createAbsolutePath( Path.Segment... segments );
    Path createAbsolutePath( <Path.Segment> segments );

    Path createRelativePath();
    Path createRelativePath( Name... segmentNames );
    Path createRelativePath( Path.Segment... segments );
    Path createRelativePath( <Path.Segment> segments );

    Path create( Path parentPath, Path childPath );
    Path create( Path parentPath, Name segmentName, int index );
    Path create( Path parentPath, String segmentName, int index );
    Path create( Path parentPath, Name... segmentNames );
    Path create( Path parentPath, Path.Segment... segments );
    Path create( Path parentPath, <Path.Segment> segments );
    Path create( Path parentPath, String subpath );

    Path.Segment createSegment( String segmentName );
    Path.Segment createSegment( String segmentName, TextDecoder decoder );
    Path.Segment createSegment( String segmentName, int index );
    Path.Segment createSegment( Name segmentName );
    Path.Segment createSegment( Name segmentName, int index );
}

And finally, the BinaryFactory defines methods for creating Binary objects from a variety of binary formats, as well as a method that looks for a cached Binary instance given the supplied secure hash:

public interface BinaryFactory extends ValueFactory<Binary> {

    /**
     * Create a value from the binary content given by the supplied input, the approximate length, 
     * and the SHA-1 secure hash of the content. If the secure hash is null, then a secure hash is
     * computed from the content. If the secure hash is not null, it is assumed to be the hash for 
     * the content and may not be checked.
     */
    Binary create( InputStream stream, long approximateLength, byte[] secureHash ) 
                          throws ValueFormatException, IoException;
    Binary create( Reader reader, long approximateLength, byte[] secureHash ) 
                          throws ValueFormatException, IoException;

    /**
     * Create a binary value from the given file.
     */
    Binary create( File file ) throws ValueFormatException, IoException;

    /**
     * Find an existing binary value given the supplied secure hash. If no such binary value exists, 
     * null is returned. This method can be used when the caller knows the secure hash (e.g., from 
     * a previously-held Binary object), and would like to reuse an existing binary value 
     * (if possible) rather than recreate the binary value by processing the stream contents. This is
     * especially true when the size of the binary is quite large.
     * 
     * @param secureHash the secure hash of the binary content, which was probably obtained from a
     *        previously-held Binary object; a null or empty value is allowed, but will always 
     *        result in returning null
     * @return the existing Binary value that has the same secure hash, or null if there is no 
     *        such value available at this time
     */
    Binary find( byte[] secureHash );
}
		

JBoss DNA provides efficient implementations of all of these interfaces: the ValueFactory interfaces and subinterfaces; the Path, Path.Segment, Name, Binary, DateTime, and interfaces; and the ValueFactories interface return by the ExecutionContext. In fact, some of these interfaces have multiple implementations that are optimized for specific but frequently-occurring conditions.

As shown above, the Name, Path.Segment, Path, and Property interfaces all extend the Readable interface, which defines a number of getString(...) methods that can produce a (readable) string representation of of that object. Recall that all of these objects contain names with namespace URIs and local names (consisting of any characters), and so obtaining a readable string representation will require converting the URIs to prefixes, escaping certain characters in the local names, and formatting the prefix and escaped local name appropriately. The different getString(...) methods of the Readable interface accept various combinations of NamespaceRegistry and TextEncoder parameters:

@Immutable
public interface Readable {

    /**
     * Get the string form of the object. A default encoder is used to encode characters.
     * @return the encoded string
     */
    public String getString();

    /**
     * Get the encoded string form of the object, using the supplied encoder to encode characters.
     * @param encoder the encoder to use, or null if the default encoder should be used
     * @return the encoded string
     */
    public String getString( TextEncoder encoder );

    /**
     * Get the string form of the object, using the supplied namespace registry to convert any 
     * namespace URIs to prefixes. A default encoder is used to encode characters.
     * @param namespaceRegistry the namespace registry that should be used to obtain the prefix
     *        for any namespace URIs
     * @return the encoded string
     * @throws IllegalArgumentException if the namespace registry is null
     */
    public String getString( NamespaceRegistry namespaceRegistry );

    /**
     * Get the encoded string form of the object, using the supplied namespace registry to convert 
     * the any namespace URIs to prefixes.
     * @param namespaceRegistry the namespace registry that should be used to obtain the prefix for 
     *        the namespace URIs
     * @param encoder the encoder to use, or null if the default encoder should be used
     * @return the encoded string
     * @throws IllegalArgumentException if the namespace registry is null
     */
    public String getString( NamespaceRegistry namespaceRegistry,
                             TextEncoder encoder );

    /**
     * Get the encoded string form of the object, using the supplied namespace registry to convert 
     * the names' namespace URIs to prefixes and the supplied encoder to encode characters, and using 
     * the second delimiter to encode (or convert) the delimiter used between the namespace prefix 
     * and the local part of any names.
     * @param namespaceRegistry the namespace registry that should be used to obtain the prefix 
     *        for the namespace URIs in the names
     * @param encoder the encoder to use for encoding the local part and namespace prefix of any names, 
     *        or null if the default encoder should be used
     * @param delimiterEncoder the encoder to use for encoding the delimiter between the local part 
     *        and namespace prefix of any names, or null if the standard delimiter should be used
     * @return the encoded string
     */
    public String getString( NamespaceRegistry namespaceRegistry,
                             TextEncoder encoder, TextEncoder delimiterEncoder );
}
		

We've seen the NamespaceRegistry in the previous chapter, but we've haven't yet talked about the TextEncoder interface. A TextEncoder merely does what you'd expect: it encodes the characters in a string using some implementation-specific algorithm. JBoss DNA provides a number of TextEncoder implementations, including:

All of these classes also implement the TextDecoder interface, which defines a method that decodes an encoded string using the opposite transformation.

Of course, you can provide alternative implementations, and supply them to the appropriate getString(...) methods as required.

In addition to Path objects, nodes can be identified by one or more identification properties. These really are just Property instances with names that have a special meaning (usually to connectors). JBoss DNA also defines a Location class that encapsulates:

So, when a client knows the path and/or the identification properties, they can create a Location object and then use that to identify the node. Location is a class that can be instantiated through factory methods on the class:

public abstract class Location implements <Property>, Comparable<Location> {

    public static Location create( Path path ) { ... }
    public static Location create( UUID uuid ) { ... }
    public static Location create( Path path, UUID uuid ) { ... }
    public static Location create( Path path, Property idProperty ) { ... }
    public static Location create( Path path, Property firstIdProperty, 
                                     Property... remainingIdProperties ) { ... }
    public static Location create( Path path, <Property idProperties ) { ... }
    public static Location create( Property idProperty ) { ... }
    public static Location create( Property firstIdProperty, 
                                     Property... remainingIdProperties ) { ... }
    public static Location create( <Property> idProperties ) { ... }
    public static Location create( List<Property> idProperties ) { ... }
    ...
}		

Like many of the other classes and interfaces, Location is immutable and cannot be changed once created. However, there are methods on the class to create a copy of the Location object with a different Path, a different UUID, or different identification properties:

public abstract class Location implements <Property>, Comparable<Location> {
    ...
    public Location with( Property newIdProperty );
    public Location with( Path newPath );
    public Location with( UUID uuid );
    ...
}		

One more thing about locations: we'll see later in the next chapter how they are used to make requests to the connectors. When creating the requests, clients usually have an incomplete location (e.g., a path but no identification properties). When processing the requests, connectors provide an actual location that contains the path and all identification properties. If actual Location objects are then reused in subsequent requests by the client, the connectors will have the benefit of having both the path and identification properties and may be able to more efficiently locate the identified node.

JBoss DNA's Graph API was designed as a lightweight public API for working with graph information. The Graph class is the primary class in API, and each instance represents a single, independent view of a single graph. Graph instances don't maintain state, so every request (or batch of requests) operate against the underlying graph and the return immutable snapshots of the requested state at the time the request was made.

There are several ways to obtain a Graph instance, as we'll see in later chapters. For the time being, the important thing to understand is what a Graph instance represents and how it interacts with the underlying content to return representations of portions of that underlying graph content.

The Graph class basically represents an internal domain specific language (DSL), designed to be easy to use in an application. The Graph API makes extensive use of interfaces and method chaining, so that methods return a concise interface that has only those methods that make sense at that point. In fact, this should be really easy if your IDE has code completion. Just remember that under the covers, a Graph is just building Request objects, submitting them to the connector, and then exposing the results.

The next few subsections describe how to use a Graph instance.

Workspace current = graph.getCurrentWorkspace();

Set<String> workspaceNames = graph.getWorkspaces();

graph.useWorkspace("myWorkspace");

graph.useWorkspace(null);

graph.createWorkspace().named("newWorkspace");

graph.createWorkspace().namedSomethingLike("newWorkspace");

graph.createWorkspace().clonedFrom("original").named("something");

graph.createWorkspace().clonedFrom("original").namedSomethingLike("something");

String current = graph.getCurrentWorkspaceName();

Workspace current = graph.getCurrentWorkspace();
String name = current.getName();
Location rootLocation = current.getRoot();

Path path = ...
Map<Name,Property> propertiesByName = graph.getPropertiesByName().on(path);

Path path = ...
for ( Property property : graph.getProperties().on(path) ) {
	  ...
}

Path path = ...
for ( Location child : graph.getChildren().of(path) ) {
    Path childPath = child.getPath();
	  ...
}

Path path = ...
Location location = ...
Property idProp1 = ...
Property idProp2 = ...
UUID uuid = ...
graph.move(path).into(idProp1, idProp2);
graph.copy(path).into(location);
graph.delete(uuid);
graph.delete(idProp1,idProp2);

Path path = ...
String path2 = ...
Location location = ...
Property idProp1 = ...
Property idProp2 = ...
UUID uuid = ...
graph.batch().move(path).into(idProp1, idProp2)
       .and().copy(path2).into(location)
       .and().delete(uuid)
       .execute();
Results results = graph.batch().read(path2)
                           .and().readChildren().of(idProp1,idProp2)
                           .and().readSugraphOfDepth(3).at(uuid2)
                           .execute();
for ( Location child : results.getNode(path2) ) {
    ...
}

JBoss DNA Graph objects operate upon the underlying graph content, but we haven't really talked about how that works. Recall that the Graph objects don't maintain any stateful representation of the content, but instead submit requests to the underlying graph and return representations of the requested portions of the content. This section focuses on what those requests look like, since they'll actually become very important when working with connectors in the next chapter.

A graph Request is an encapsulation of a command that is to be executed by the underlying graph owner (typically a connector). Request objects can take many different forms, as there are different classes for each kind of request. Each request contains the information needed to complete the processing, and it also is the place where the results (or error) are recorded.

The Graph object creates the Request objects using Location objects to identify the node (or nodes) that are the subject of the request. The Graph can either submit the request immediately, or it can batch multiple requests together into "units of work". The submitted requests are then processed by the underlying system (e.g., connector) and returned back to the Graph object, which then extracts and returns the results.

There are actually quite a few different types of Request classes:

ChangeRequest is a subclass of Request that provides a base class for all the requests that request a change be made to the content. As we'll see later, these ChangeRequest objects also get reused by the observation system.

And there are also requests that deal with changing workspaces (and thus extend ChangeRequest):

Although there are over a dozen different kinds of requests, we do anticipate adding more in future releases. For example, DNA will likely support searching repository content in sources through an additional subclass of Request. Getting the version history for a node will likely be another kind of request added in an upcoming release.

This section covered the different kinds of Request classes. The next section provides a easy way to encapsulate how a component should responds to these requests, and after that we'll see how these Request objects are also used in the observation framework.

JBoss DNA connectors are typically the components that receive these Request objects. We'll dive deep into connectors in the next chapter, but before we do there is one more component related to Requests that should be discussed.

The RequestProcessor class is an abstract class that defines a process(...) method for each concrete Request subclass. In other words, there is a process(CompositeRequest) method, a process(ReadNodeRequest) method, and so on. This makes it easy to implement behavior that responds to the different kinds of Request classes: simply subclass the RequestProcessor, override all of the abstract methods, and optionally overriding any of the other methods that have a default implementation.

The JBoss DNA graph model also incorporates an observation framework that allows components to register and be notified when changes occur within the content owned by a graph.

Many event frameworks define the listeners and sources as interfaces. While this is often useful, it requires the implementations properly address the thread-safe semantics of managing and calling the listeners. The JBoss DNA observation framework uses abstract or concrete classes to minimize the effort required for implementing ChangeObserver or Observable. These abstract classes provide implementations for a number of utility methods (such as the unregister() method on ChangeObserver) that also save effort and code.

However, one of the more important reasons for providing classes is that ChangeObserver uses weak references to track the Observable instances, and the ChangeObservers class uses weak references for the listeners. This means that an observer does not prevent Observable instances from being garbage collected, nor do observers prevent Observable instances from being garbage collected. These abstract class provide all this functionality for free.

As we'll see in the next chapter, each connector is responsible for propagating the ChangeRequest objects to the connector's Observer. But that's not the only use of . We'll also see later how the sequencing system uses to monitor for changes in the graph content to determine which, if any, sequencers should be run. And, the JCR implementation also uses the observation framework to propagate those changes to JCR clients.

In this chapter, we introduced JBoss DNA's graph model and showed the different kinds of objects used to represent nodes, paths, names, and properties. We saw how all of these objects are actually immutable, and how the low-level Graph API uses this characteristic to provide a stateless and thread-safe interface for working with repository content using the request model used to read, update, and change content.

Next, we'll dive into the connector framework, which builds on top of the graph model and request model, allowing JBoss DNA to access the graph content stored in many different kinds of systems.

With JBoss DNA, your applications use the JCR API to work with the repository, but the DNA repository transparently fetches the information from different kinds of repositories and storage systems, not just a single purpose-built store. This is fundamentally what makes JBoss DNA different.

How does JBoss DNA do this? At the heart of JBoss DNA and it's JCR implementation is a simple graph-based connector system. Essentially, JBoss DNA's JCR implementation uses a single connector to access all content:

Figure 6.1. JBoss DNA's JCR implementation delegates to a connector

That single connector could use an in-memory repository, a JBoss Cache instance (including those that are clustered and replicated), or a federated repository where content from multiple sources is unified.

Figure 6.2. JBoss DNA can put JCR on top of multiple kinds of systems

Really, the federated connector gives us all kinds of possibilities, since we can use that connector on top of lots of connectors to other individual sources. This simple connector architecture is fundamentally what makes JBoss DNA so powerful and flexible. Along with a good library of connectors, which is what we're planning to create.

For instance, we want to build a connector to other JCR repositories, and another that accesses the local file system. We've already started on a Subversion connector, which will allow JCR to access the files in a SVN repository (and perhaps push changes into SVN through a commit). And of course we want to create a connector that accesses data and metadata from relational databases. For more information, check out our roadmap. Of course, if we don't have a connector to suit your needs, you can write your own.

Figure 6.3. Future JBoss DNA connectors

It's even possible to put a different API layer on top of the connectors. For example, the new New I/O (JSR-203) API offers the opportunity to build new file system providers. This would be very straightforward to put on top of a JCR implementation, but it could be made even simpler by putting it on top of a DNA connector. In both cases, it'd be a trivial mapping from nodes that represent files and folders into JSR-203 files and directories, and events on those nodes could easily be translated into JSR-203 watch events. Then, simply choose a DNA connector and configure it to use the source you want to use.

Figure 6.4. Virtual File System with JBoss DNA

Before we go further, let's define some terminology regarding connectors.

As an example, consider that we want JBoss DNA to give us access through JCR to the schema information contained in a relational databases. We first have to develop a connector that allows us to interact with relational databases using JDBC. That connector would contain a JdbcRepositorySource Java class that implements RepositorySource, and that has all of the various JavaBean properties for setting the name of the driver class, URL, username, password, and other properties. (Or we might have a JavaBean property that defines the JNDI name where we can find a JDBC DataSource instance pointing to our JDBC database.)

Our new connector would also have a JdbcRepositoryConnection Java class that implements the RepositoryConnection interface. This class would probably wrap a JDBC database connection, and would implement the execute(...) method such that the nodes exposed by the connector describe the database schema of the database. For example, the connector might represent each database table as a node with the table's name, with properties that describe the table (e.g., the description, whether it's a temporary table), and with child nodes that represent each of the columns, keys and constraints.

To use our connector in an application that uses JBoss DNA, we need to create an instance of the JdbcRepositorySource for each database instance that we want to access. If we have 3 MySQL databases, 9 Oracle databases, and 4 PostgreSQL databases, then we'd need to create a total of 16 JdbcRepositorySource instances, each with the properties describing a single database instance. Those sources are then available for use by JBoss DNA components, including the JCR implementation.

So, we've so far learned what a connector is and how they're used to establish connections to the underlying sources and access the content in those sources. Next we'll show how connectors expose the notion of workspaces, and describe how to create your own connectors.

A number of connectors are already available in JBoss DNA, and are outlined in detail later in the document. Note that we do want to build more connectors in the upcoming releases.

There may come a time when you want to tackle creating your own connector. Maybe the connectors we provide out-of-the-box don't work with your source. Maybe you want to use a different cache system. Maybe you have a system that you want to make available through a JBoss DNA repository. Or, maybe you're a contributor and want to help us round out our library with a new connector. No matter what the reason, creating a new connector is pretty straightforward, as we'll see in this section.

Creating a custom connector involves the following steps:

Let's go through each one of these steps in more detail.

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-graph</artifactId>

  <version>0.5</version>

</dependency>

     

<!-- DNA-related unit testing utilities and classes -->

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-graph</artifactId>

  <version>0.5</version>

  <type>test-jar</type>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-common</artifactId>

  <version>0.5</version>

  <type>test-jar</type>

  <scope>test</scope>

</dependency>

<!-- Unit testing -->

<dependency>

  <groupId>junit</groupId>

  <artifactId>junit</artifactId>

  <version>4.4</version>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>org.hamcrest</groupId>

  <artifactId>hamcrest-library</artifactId>

  <version>1.1</version>

  <scope>test</scope>

</dependency>

<!-- Logging with Log4J -->

<dependency>

  <groupId>org.slf4j</groupId>

  <artifactId>slf4j-log4j12</artifactId>

  <version>1.4.3</version>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>log4j</groupId>

  <artifactId>log4j</artifactId>

  <version>1.2.14</version>

  <scope>test</scope>

</dependency>

     

/**
 * A connection to a repository source.
 * <p>
 * These connections need not support concurrent operations by multiple threads.
 * </p>
 */
@NotThreadSafe
public interface RepositoryConnection {

    /**
     * Get the name for this repository source. This value should be the same as that returned
     * by the same RepositorySource that created this connection.
     * 
     * @return the identifier; never null or empty
     */
    String getSourceName();

    /**
     * Return the transactional resource associated with this connection. The transaction manager 
     * will use this resource to manage the participation of this connection in a distributed transaction.
     * 
     * @return the XA resource, or null if this connection is not aware of distributed transactions
     */
    XAResource getXAResource();

    /**
     * Ping the underlying system to determine if the connection is still valid and alive.
     * 
     * @param time the length of time to wait before timing out
     * @param unit the time unit to use; may not be null
     * @return true if this connection is still valid and can still be used, or false otherwise
     * @throws InterruptedException if the thread has been interrupted during the operation
     */
    boolean ping( long time, TimeUnit unit ) throws InterruptedException;

    /**
     * Get the default cache policy for this repository. If none is provided, a global cache policy
     * will be used.
     * 
     * @return the default cache policy
     */
    CachePolicy getDefaultCachePolicy();

    /**
     * Execute the supplied commands against this repository source.
     * 
     * @param context the environment in which the commands are being executed; never null
     * @param request the request to be executed; never null
     * @throws RepositorySourceException if there is a problem loading the node data
     */
    void execute( ExecutionContext context, Request request ) throws RepositorySourceException;

    /**
     * Close this connection to signal that it is no longer needed and that any accumulated 
     * resources are to be released.
     */
    void close();
}

public void execute( final ExecutionContext context,
                     final Request request ) throws RepositorySourceException {
    String sourceName = // from the RepositorySource
    Observer observer = // from the RepositoryContext
    RequestProcessor processor = new CustomRequestProcessor(sourceName,context,observer);
    try {
        processor.process(request);
    } finally {
        processor.close();	// sends the accumulated ChangeRequests as a Changes to the Observer
    }
}

In this chapter, we covered all the aspects of JBoss DNA connectors, including the connector API, how DNA's JCR implementation works with connectors, what connectors are available (and how to use them), and how to write your own connector. So now that you know how to set up and use JBoss DNA repositories, the next chapter describes the sequencing framework and how to build your own custom sequencers. After that, we'll get into how to configure JBoss DNA and use JCR.

Sequencers are just POJOs that implement a specific interface, and their job is to process a stream of data (supplied by JBoss DNA) to extract meaningful content that usually takes the form of a structured graph. Exactly what content is up to each sequencer implementation. For example, JBoss DNA comes with an image sequencer that extracts the simple metadata from different kinds of image files (e.g., JPEG, GIF, PNG, etc.). Another example is the Compact Node Definition (CND) sequencer that processes the CND files to extract and produce a structured representation of the node type definitions, property definitions, and child node definitions contained within the file.

Sequencers are configured to identify the kinds of nodes that the sequencers can work against. When content in the repository changes, JBoss DNA looks to see which (if any) sequencers might be able to run on the changed content. If any sequencer configurations do match, those sequencers are run against the content, and the structured graph output of the sequencers is then written back into the repository (at a location dictated by the sequencer configuration). And once that information is in the repository, it can be easily found and accessed via the standard JCR API.

In other words, JBoss DNA uses sequencers to help you extract more meaning from the artifacts you already are managing, and makes it much easier for applications to find and use all that valuable information. All without your applications doing anything extra.

The StreamSequencer interface defines the single method that must be implemented by a sequencer:

public interface StreamSequencer {

    /**
     * Sequence the data found in the supplied stream, placing the output 
     * information into the supplied map.
     *
     * @param stream the stream with the data to be sequenced; never null
     * @param output the output from the sequencing operation; never null
     * @param context the context for the sequencing operation; never null
     */
    void sequence( InputStream stream, SequencerOutput output,  context );
}

Implementations are responsible for processing the content in the supplied InputStream content and generating structured content using the supplied SequencerOutput interface. The provides additional details about the information that is being sequenced, including the location and properties of the node being sequenced, the MIME type of the node being sequenced, and a Problems object where the sequencer can record problems that aren't severe enough to warrant throwing an exception. The also provides access to the ValueFactories that can be used to create Path, Name, and any other value objects.

The SequencerOutput interface is fairly easy to use, and its job is to hide from the sequencer all the specifics about where the output is being written. Therefore, the interface has only a few methods for implementations to call. Two methods set the property values on a node, while the other sets references to other nodes in the repository. Use these methods to describe the properties of the nodes you want to create, using relative paths for the nodes and valid JCR property names for properties and references. JBoss DNA will ensure that nodes are created or updated whenever they're needed.

public interface SequencerOutput {

  /**
   * Set the supplied property on the supplied node.  The allowable
   * values are any of the following:
   *   - primitives (which will be autoboxed)
   *   - String instances
   *   - String arrays
   *   - byte arrays
   *   - InputStream instances
   *   - Calendar instances
   *
   * @param nodePath the path to the node containing the property; 
   * may not be null
   * @param property the name of the property to be set
   * @param values the value(s) for the property; may be empty if 
   * any existing property is to be removed
   */
  void setProperty( String nodePath, String property, Object... values );
  void setProperty( Path nodePath, Name property, Object... values );

  /**
   * Set the supplied reference on the supplied node.
   *
   * @param nodePath the path to the node containing the property; 
   * may not be null
   * @param property the name of the property to be set
   * @param paths the paths to the referenced property, which may be
   * absolute paths or relative to the sequencer output node;
   * may be empty if any existing property is to be removed
   */
  void setReference( String nodePath, String property, String... paths );
}
		

Each sequencer must be configured to describe the areas or types of content that the sequencer is capable of handling. This is done by specifying these patterns using path expressions that identify the nodes (or node patterns) that should be sequenced and where to store the output generated by the sequencer. We'll see how to fully configure a sequencer in the next chapter, but before then let's dive into path expressions in more detail.

A path expression consist of two parts: a selection criteria (or an input path) and an output path:

  inputPath => outputPath 

The inputPath part defines an expression for the path of a node that is to be sequenced. Input paths consist of '/' separated segments, where each segment represents a pattern for a single node's name (including the same-name-sibling indexes) and '@' signifies a property name.

Let's first look at some simple examples:

  //(*.(jpg|jpeg|gif|bmp|pcx|png)[*])/jcr:content[@jcr:data] => /images/$1 

A number of sequencers are already available in JBoss DNA, and are outlined in detail later in the document. Note that we do want to build more sequencers in the upcoming releases.

The current release of JBoss DNA comes with six sequencers. However, it's very easy to create your own sequencers and to then configure JBoss DNA to use them in your own application.

Creating a custom sequencer involves the following steps:

It's that simple.

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-graph</artifactId>

  <version>0.5</version>

</dependency>

     

<!-- DNA-related unit testing utilities and classes -->

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-graph</artifactId>

  <version>0.5</version>

  <type>test-jar</type>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-common</artifactId>

  <version>0.5</version>

  <type>test-jar</type>

  <scope>test</scope>

</dependency>

<!-- Unit testing -->

<dependency>

  <groupId>junit</groupId>

  <artifactId>junit</artifactId>

  <version>4.4</version>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>org.hamcrest</groupId>

  <artifactId>hamcrest-library</artifactId>

  <version>1.1</version>

  <scope>test</scope>

</dependency>

<!-- Logging with Log4J -->

<dependency>

  <groupId>org.slf4j</groupId>

  <artifactId>slf4j-log4j12</artifactId>

  <version>1.4.3</version>

  <scope>test</scope>

</dependency>

<dependency>

  <groupId>log4j</groupId>

  <artifactId>log4j</artifactId>

  <version>1.2.14</version>

  <scope>test</scope>

</dependency>    

<!-- JBoss DNA JCR Repository -->

<dependency>

  <groupId>org.jboss.dna</groupId>

  <artifactId>dna-jcr</artifactId>

  <version>0.5</version>

  <scope>test</scope>

</dependency>

<!-- Java Content Repository API -->

<dependency>

  <groupId>javax.jcr</groupId>

  <artifactId>jcr</artifactId>

  <version>1.0.1</version>

  <scope>test</scope>

</dependency>

     

public class ImageMetadataSequencer implements StreamSequencer {

    public static final String METADATA_NODE = "image:metadata";
    public static final String IMAGE_PRIMARY_TYPE = "jcr:primaryType";
    public static final String IMAGE_MIXINS = "jcr:mixinTypes";
    public static final String IMAGE_MIME_TYPE = "jcr:mimeType";
    public static final String IMAGE_ENCODING = "jcr:encoding";
    public static final String IMAGE_FORMAT_NAME = "image:formatName";
    public static final String IMAGE_WIDTH = "image:width";
    public static final String IMAGE_HEIGHT = "image:height";
    public static final String IMAGE_BITS_PER_PIXEL = "image:bitsPerPixel";
    public static final String IMAGE_PROGRESSIVE = "image:progressive";
    public static final String IMAGE_NUMBER_OF_IMAGES = "image:numberOfImages";
    public static final String IMAGE_PHYSICAL_WIDTH_DPI = "image:physicalWidthDpi";
    public static final String IMAGE_PHYSICAL_HEIGHT_DPI = "image:physicalHeightDpi";
    public static final String IMAGE_PHYSICAL_WIDTH_INCHES = "image:physicalWidthInches";
    public static final String IMAGE_PHYSICAL_HEIGHT_INCHES = "image:physicalHeightInches";

    /**
     * {@inheritDoc}
     */
    public void sequence( InputStream stream, SequencerOutput output, 
                           context ) {
        ImageMetadata metadata = new ImageMetadata();
        metadata.setInput(stream);
        metadata.setDetermineImageNumber(true);
        metadata.setCollectComments(true);

        // Process the image stream and extract the metadata ...
        if (!metadata.check()) {
            metadata = null;
        }
        // Generate the output graph if we found useful metadata ...
        if (metadata != null) {
            // Place the image metadata into the output map ...
            output.setProperty(METADATA_NODE, IMAGE_PRIMARY_TYPE, "image:metadata");
            // output.psetProperty(METADATA_NODE, IMAGE_MIXINS, "");
            output.setProperty(METADATA_NODE, IMAGE_MIME_TYPE, metadata.getMimeType());
            // output.setProperty(METADATA_NODE, IMAGE_ENCODING, "");
            output.setProperty(METADATA_NODE, IMAGE_FORMAT_NAME, metadata.getFormatName());
            output.setProperty(METADATA_NODE, IMAGE_WIDTH, metadata.getWidth());
            output.setProperty(METADATA_NODE, IMAGE_HEIGHT, metadata.getHeight());
            output.setProperty(METADATA_NODE, IMAGE_BITS_PER_PIXEL, metadata.getBitsPerPixel());
            output.setProperty(METADATA_NODE, IMAGE_PROGRESSIVE, metadata.isProgressive());
            output.setProperty(METADATA_NODE, IMAGE_NUMBER_OF_IMAGES, 
                               metadata.getNumberOfImages());
            output.setProperty(METADATA_NODE, IMAGE_PHYSICAL_WIDTH_DPI,  
						metadata.getPhysicalWidthDpi());
            output.setProperty(METADATA_NODE, IMAGE_PHYSICAL_HEIGHT_DPI,  
						metadata.getPhysicalHeightDpi());
            output.setProperty(METADATA_NODE, IMAGE_PHYSICAL_WIDTH_INCHES,  
						metadata.getPhysicalWidthInch());
            output.setProperty(METADATA_NODE, IMAGE_PHYSICAL_HEIGHT_INCHES,  
						metadata.getPhysicalHeightInch());
        }
    }
}

StreamSequencer sequencer = new ImageMetadataSequencer();
MockSequencerOutput output = new MockSequencerOutput();
MockSequencerContext context = new MockSequencerContext();
InputStream stream = null;
try {
    stream = this.getClass().getClassLoader().getResource("caution.gif").openStream();
    sequencer.sequence(stream,output,context);   // writes to 'output'
    assertThat(output.getPropertyValues("image:metadata", "jcr:primaryType"), 
               is(new Object[] {"image:metadata"}));
    assertThat(output.getPropertyValues("image:metadata", "jcr:mimeType"), 
               is(new Object[] {"image/gif"}));
    // ... make more assertions here
    assertThat(output.hasReferences(), is(false));
} finally {
    stream.close();
}

 sequencer = new ImageMetadataSequencer();
MockSequencerOutput output = new MockSequencerOutput();
MockSequencerContext context = new MockSequencerContext();
InputStream stream = null;
try {
    stream = this.getClass().getClassLoader().getResource("caution.pict").openStream();
    sequencer.sequence(stream,output,context);   // writes to 'output'
    assertThat(output.hasProperties(), is(false));
    assertThat(output.hasReferences(), is(false));
} finally {
    stream.close();
}

In this chapter, we described how JBoss DNA sequences files as they're uploaded into a repository. We've also learned in previous chapters about the JBoss DNA execution contexts, graph model, and connectors. In the next part we'll put all these pieces together to learn how to set up a JBoss DNA repository and access it using the JCR API.