001// SAX entity resolver.
002// http://www.saxproject.org
003// No warranty; no copyright -- use this as you will.
004// $Id: EntityResolver.java,v 1.1 2004/12/23 22:38:42 mark Exp $
005
006package org.xml.sax;
007
008import java.io.IOException;
009
010
011/**
012 * Basic interface for resolving entities.
013 *
014 * <blockquote>
015 * <em>This module, both source code and documentation, is in the
016 * Public Domain, and comes with <strong>NO WARRANTY</strong>.</em>
017 * See <a href='http://www.saxproject.org'>http://www.saxproject.org</a>
018 * for further information.
019 * </blockquote>
020 *
021 * <p>If a SAX application needs to implement customized handling
022 * for external entities, it must implement this interface and
023 * register an instance with the SAX driver using the
024 * {@link org.xml.sax.XMLReader#setEntityResolver setEntityResolver}
025 * method.</p>
026 *
027 * <p>The XML reader will then allow the application to intercept any
028 * external entities (including the external DTD subset and external
029 * parameter entities, if any) before including them.</p>
030 *
031 * <p>Many SAX applications will not need to implement this interface,
032 * but it will be especially useful for applications that build
033 * XML documents from databases or other specialised input sources,
034 * or for applications that use URI types other than URLs.</p>
035 *
036 * <p>The following resolver would provide the application
037 * with a special character stream for the entity with the system
038 * identifier "http://www.myhost.com/today":</p>
039 *
040 * <pre>
041 * import org.xml.sax.EntityResolver;
042 * import org.xml.sax.InputSource;
043 *
044 * public class MyResolver implements EntityResolver {
045 *   public InputSource resolveEntity (String publicId, String systemId)
046 *   {
047 *     if (systemId.equals("http://www.myhost.com/today")) {
048 *              // return a special input source
049 *       MyReader reader = new MyReader();
050 *       return new InputSource(reader);
051 *     } else {
052 *              // use the default behaviour
053 *       return null;
054 *     }
055 *   }
056 * }
057 * </pre>
058 *
059 * <p>The application can also use this interface to redirect system
060 * identifiers to local URIs or to look up replacements in a catalog
061 * (possibly by using the public identifier).</p>
062 *
063 * @since SAX 1.0
064 * @author David Megginson
065 * @version 2.0.1 (sax2r2)
066 * @see org.xml.sax.XMLReader#setEntityResolver
067 * @see org.xml.sax.InputSource
068 */
069public interface EntityResolver {
070
071
072    /**
073     * Allow the application to resolve external entities.
074     *
075     * <p>The parser will call this method before opening any external
076     * entity except the top-level document entity.  Such entities include
077     * the external DTD subset and external parameter entities referenced
078     * within the DTD (in either case, only if the parser reads external
079     * parameter entities), and external general entities referenced
080     * within the document element (if the parser reads external general
081     * entities).  The application may request that the parser locate
082     * the entity itself, that it use an alternative URI, or that it
083     * use data provided by the application (as a character or byte
084     * input stream).</p>
085     *
086     * <p>Application writers can use this method to redirect external
087     * system identifiers to secure and/or local URIs, to look up
088     * public identifiers in a catalogue, or to read an entity from a
089     * database or other input source (including, for example, a dialog
090     * box).  Neither XML nor SAX specifies a preferred policy for using
091     * public or system IDs to resolve resources.  However, SAX specifies
092     * how to interpret any InputSource returned by this method, and that
093     * if none is returned, then the system ID will be dereferenced as
094     * a URL.  </p>
095     *
096     * <p>If the system identifier is a URL, the SAX parser must
097     * resolve it fully before reporting it to the application.</p>
098     *
099     * @param publicId The public identifier of the external entity
100     *        being referenced, or null if none was supplied.
101     * @param systemId The system identifier of the external entity
102     *        being referenced.
103     * @return An InputSource object describing the new input source,
104     *         or null to request that the parser open a regular
105     *         URI connection to the system identifier.
106     * @exception org.xml.sax.SAXException Any SAX exception, possibly
107     *            wrapping another exception.
108     * @exception java.io.IOException A Java-specific IO exception,
109     *            possibly the result of creating a new InputStream
110     *            or Reader for the InputSource.
111     * @see org.xml.sax.InputSource
112     */
113    public abstract InputSource resolveEntity (String publicId,
114                                               String systemId)
115        throws SAXException, IOException;
116
117}
118
119// end of EntityResolver.java