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012// ***************************************************************************************************************************
013package org.apache.juneau.rest;
014
015import org.apache.juneau.http.exception.*;
016import org.apache.juneau.rest.annotation.*;
017import org.apache.juneau.rest.converters.*;
018import org.apache.juneau.serializer.*;
019
020/**
021 * REST method response converter.
022 *
023 * <p>
024 * Implements a filter mechanism for REST method calls that allows response objects to be converted to some other POJO
025 * after invocation of the REST method.
026 *
027 * <p>
028 * Converters are associated with REST methods through the following:
029 * <ul class='javatree'>
030 *    <li class='ja'>{@link Rest#converters()}
031 *    <li class='ja'>{@link RestMethod#converters()}
032 *    <li class='jf'>{@link RestContext#REST_converters}
033 *    <li class='jm'>{@link RestContextBuilder#converters(Class...)}
034 *    <li class='jm'>{@link RestContextBuilder#converters(RestConverter...)}
035 * </ul>
036 *
037 * <h5 class='section'>Example:</h5>
038 * <p class='bcode w800'>
039 *    <jk>public class</jk> RequestEchoResource <jk>extends</jk> RestServlet {
040 *
041 *       <jc>// GET request handler</jc>
042 *       <ja>@RestMethod</ja>(name=<jsf>GET</jsf>, path=<js>"/*"</js>, converters={Queryable.<jk>class</jk>,Traversable.<jk>class</jk>})
043 *       <jk>public</jk> HttpServletRequest doGet(RestRequest req) {
044 *          res.setTitle(<js>"Contents of HttpServletRequest object"</js>);
045 *          <jk>return</jk> req;
046 *       }
047 *    }
048 * </p>
049 *
050 * <p>
051 * Converters can also be associated at the servlet level using the {@link Rest#converters() @Rest(converters)} annotation.
052 * <br>Applying converters at the resource level is equivalent to applying converters to each resource method individually.
053 *
054 * <h5 class='topic'>How to implement</h5>
055 *
056 * Implementers should simply implement the {@link #convert(RestRequest, Object)} and return back a 'converted' object.
057 * <br>It's up to the implementer to decide what this means.
058 *
059 * <p>
060 * Subclasses must implement one of the following constructors:
061 * <ul>
062 *    <li><jk>public</jk> T();  <jc>// No-arg constructor</jc>
063 *    <li><jk>public</jk> T(PropertyStore);  <jc>// Property store of the RestContext</jc>
064 * </ul>
065 *
066 * <p>
067 * Subclasses can also be defined as inner classes of the resource class.
068 *
069 * <h5 class='topic'>Predefined converters</h5>
070 * <ul class='javatree'>
071 *    <li class='jc'>{@link Traversable} - Allows URL additional path info to address individual elements in a POJO tree.
072 *    <li class='jc'>{@link Queryable} - Allows query/view/sort functions to be performed on POJOs.
073 *    <li class='jc'>{@link Introspectable} - Allows Java public methods to be invoked on the returned POJOs.
074 * </ul>
075 *
076 * <ul class='seealso'>
077 *    <li class='jf'>{@link RestContext#REST_converters} - Registering converters with REST resources.
078 *    <li class='link'>{@doc juneau-rest-server.Converters}
079 * </ul>
080 */
081public interface RestConverter {
082
083   /**
084    * Performs post-call conversion on the specified response object.
085    *
086    * @param req The servlet request.
087    * @param res The response object set by the REST method through the {@link RestResponse#setOutput(Object)} method.
088    * @return The converted object.
089    * @throws HttpException Thrown if any errors occur during conversion.
090    * @throws SerializeException Generic serialization error occurred.
091    */
092   public Object convert(RestRequest req, Object res) throws HttpException, SerializeException;
093}