// SAX entity resolver.
// No warranty; no copyright -- use this as you will.
// $Id$

package org.xml.sax;

import java.io.IOException;


/**
 * Basic interface for resolving entities.
 *
 * <blockquote>
 * <em>This module, both source code and documentation, is in the
 * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>
 * </blockquote>
 *
 * <p>If a SAX application needs to implement customized handling
 * for external entities, it must implement this interface and
 * register an instance with the SAX driver using the
 * {@link org.xml.sax.XMLReader#setEntityResolver setEntityResolver}
 * method.</p>
 *
 * <p>The XML reader will then allow the application to intercept any
 * external entities (including the external DTD subset and external
 * parameter entities, if any) before including them.</p>
 *
 * <p>Many SAX applications will not need to implement this interface,
 * but it will be especially useful for applications that build
 * XML documents from databases or other specialised input sources,
 * or for applications that use URI types other than URLs.</p>
 *
 * <p>The following resolver would provide the application
 * with a special character stream for the entity with the system
 * identifier "http://www.myhost.com/today":</p>
 *
 * <pre>
 * import org.xml.sax.EntityResolver;
 * import org.xml.sax.InputSource;
 *
 * public class MyResolver implements EntityResolver {
 *   public InputSource resolveEntity (String publicId, String systemId)
 *   {
 *     if (systemId.equals("http://www.myhost.com/today")) {
 *              // return a special input source
 *       MyReader reader = new MyReader();
 *       return new InputSource(reader);
 *     } else {
 *              // use the default behaviour
 *       return null;
 *     }
 *   }
 * }
 * </pre>
 *
 * <p>The application can also use this interface to redirect system
 * identifiers to local URIs or to look up replacements in a catalog
 * (possibly by using the public identifier).</p>
 *
 * @since SAX 1.0
 * @author David Megginson, 
 *         <a href="mailto:sax@megginson.com">sax@megginson.com</a>
 * @version 2.0
 * @see org.xml.sax.Parser#setEntityResolver
 * @see org.xml.sax.InputSource
 */
public interface EntityResolver {
    
    
    /**
     * Allow the application to resolve external entities.
     *
     * <p>The Parser will call this method before opening any external
     * entity except the top-level document entity (including the
     * external DTD subset, external entities referenced within the
     * DTD, and external entities referenced within the document
     * element): the application may request that the parser resolve
     * the entity itself, that it use an alternative URI, or that it
     * use an entirely different input source.</p>
     *
     * <p>Application writers can use this method to redirect external
     * system identifiers to secure and/or local URIs, to look up
     * public identifiers in a catalogue, or to read an entity from a
     * database or other input source (including, for example, a dialog
     * box).</p>
     *
     * <p>If the system identifier is a URL, the SAX parser must
     * resolve it fully before reporting it to the application.</p>
     *
     * @param publicId The public identifier of the external entity
     *        being referenced, or null if none was supplied.
     * @param systemId The system identifier of the external entity
     *        being referenced.
     * @return An InputSource object describing the new input source,
     *         or null to request that the parser open a regular
     *         URI connection to the system identifier.
     * @exception org.xml.sax.SAXException Any SAX exception, possibly
     *            wrapping another exception.
     * @exception java.io.IOException A Java-specific IO exception,
     *            possibly the result of creating a new InputStream
     *            or Reader for the InputSource.
     * @see org.xml.sax.InputSource
     */
    public abstract InputSource resolveEntity (String publicId,
					       String systemId)
	throws SAXException, IOException;
    
}

// end of EntityResolver.java