16.17. Namespace Registry

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();
}

@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();
}

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