juneau-rest/juneau-rest-server/src/main/java/org/apache/juneau/rest/ResponseHandler.java

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package org.apache.juneau.rest;

import java.io.*;

import javax.servlet.http.*;

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

/**
 * 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()}).
 * </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 <c>Foo</c> objects outside the
 * normal Juneau architecture.
 * <p class='bcode w800'>
 * 	<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>
 *
 * <ul class='seealso'>
 * 	<li class='link'>{@doc juneau-rest-server.RestMethod.MethodReturnTypes}
 * </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;
	 * @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) throws IOException, RestException;
}