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package org.apache.juneau.rest;

import java.io.*;

import javax.servlet.http.*;

import org.apache.juneau.*;
import org.apache.juneau.rest.annotation.*;
import org.apache.juneau.rest.response.*;

/**
 * Defines the interface for handlers that convert POJOs to appropriate HTTP responses.
 * 
 * <p>
 * The REST Server API uses the concept of registered response handlers for converting objects returned by REST
 * methods or set through {@link RestResponse#setOutput(Object)} into appropriate HTTP responses.
 * 
 * <p>
 * Response handlers can be associated with REST resources via the following:
 * <ul>
 *    <li class='ja'>{@link RestResource#responseHandlers}
 *    <li class='jm'>{@link RestContextBuilder#responseHandlers(Class...)}
 *    <li class='jm'>{@link RestContextBuilder#responseHandlers(ResponseHandler...)}
 * </ul>
 * 
 * <p>
 * By default, REST resources are registered with the following response handlers:
 * <ul class='spaced-list'>
 *    <li class='jc'>
 *       {@link DefaultHandler} - Serializes POJOs using the Juneau serializer API.
 *    <li class='jc'>
 *       {@link ReaderHandler} - Pipes the output of {@link Reader Readers} to the response writer
 *       ({@link RestResponse#getWriter()}).
 *    <li class='jc'>
 *       {@link InputStreamHandler} - Pipes the output of {@link InputStream InputStreams} to the response output
 *       stream ({@link RestResponse#getOutputStream()}).
 *    <li class='jc'>
 *       {@link RedirectHandler} - Handles {@link Redirect} objects.
 *    <li class='jc'>
 *       {@link WritableHandler} - Handles {@link Writable} objects.
 *    <li class='jc'>
 *       {@link StreamableHandler} - Handles {@link Streamable} objects.
 * </ul>
 * 
 * <p>
 * Response handlers can be used to process POJOs that cannot normally be handled through Juneau serializers, or
 * because it's simply easier to define response handlers for special cases.
 * 
 * <p>
 * The following example shows how to create a response handler to handle special <code>Foo</code> objects outside the
 * normal Juneau architecture.
 * <p class='bcode'>
 *    <ja>@RestResource</ja>(
 *       path=<js>"/example"</js>,
 *       responseHandlers=FooHandler.<jk>class</jk>
 *    )
 *    <jk>public class</jk> Example <jk>extends</jk> RestServlet {
 * 
 *       <ja>@RestMethod</ja>(name=<jsf>GET</jsf>, path=<js>"/"</js>)
 *       <jk>public</jk> Foo test1() {
 *          <jk>return new</jk> Foo(<js>"123"</js>);
 *       }
 * 
 *       <jk>public static class</jk> FooHandler <jk>implements</jk> ResponseHandler {
 *          <ja>@Override</ja>
 *          <jk>public boolean</jk> handle(RestRequest req, RestResponse res, Object output) <jk>throws</jk> IOException, RestException {
 *             <jk>if</jk> (output <jk>instanceof</jk> Foo) {
 *                Foo foo = (Foo)output;
 *                <jc>// Set some headers and body content.</jc>
 *                res.setHeader(<js>"Foo-ID"</js>, foo.getId());
 *                res.getWriter().write(<js>"foo.id="</js> + foo.getId());
 *                <jk>return true</jk>;  <jc>// We handled it.</jc>
 *             }
 *             <jk>return false</jk>;  <jc>// We didn't handle it.</jc>
 *          }
 *       }
 *    }
 * </p>
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul>
 *    <li class='link'><a class="doclink" href="../../../../overview-summary.html#juneau-rest-server.MethodReturnTypes">Overview &gt; juneau-rest-server &gt; Method Return Types</a>
 * </ul>
 */
public interface ResponseHandler {

   /**
    * Process this response if possible.
    * This method should return <jk>false</jk> if it wasn't able to process the response.
    * 
    * @param req The HTTP servlet request.
    * @param res The HTTP servlet response;
    * @param output The POJO returned by the REST method that now needs to be sent to the response.
    * @return true If this handler handled the response.
    * @throws IOException
    *    If low-level exception occurred on output stream.
    *    Results in a {@link HttpServletResponse#SC_INTERNAL_SERVER_ERROR} error.
    * @throws RestException
    *    If some other exception occurred.
    *    Can be used to provide an appropriate HTTP response code and message.
    */
   boolean handle(RestRequest req, RestResponse res, Object output) throws IOException, RestException;
}