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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boiler plate code between the web browser and your Java code.

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/*
 * Copyright 2005 Joe Walker
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.directwebremoting.extend;

import java.io.IOException;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * A Marshaller is responsible for all the on-the-wire communication between
 * DWR on the server and the HTTP channel. engine.js does the corresponding
 * work on the Javascript side.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public interface Marshaller
{
    /**
     * Marshall an incomming HttpRequest into an abstract Calls POJO that
     * defines the functions that we need to call.
     * @param request The incoming Http request
     * @param response An Ajax response, XML, JSON, Javascript, etc.
     * @return Data specifying the methods to call
     * @throws IOException If the connection breaks
     * @throws ServerException If an error occurs during parsing
     */
    Calls marshallInbound(HttpServletRequest request, HttpServletResponse response) throws IOException, ServerException;

    /**
     * Marshall the return values from executing this batch of requests.
     * @param replies The objects to convert into a reply
     * @param request The incoming Http request
     * @param response An Ajax response, XML, JSON, Javascript, etc.
     * @throws IOException If the connection breaks
     */
    void marshallOutbound(Replies replies, HttpServletRequest request, HttpServletResponse response) throws IOException;

    /**
     * Try to find a batchId to send to the client so it knows what broke
     * @param request The incoming Http request
     * @param response An Ajax response, XML, JSON, Javascript, etc.
     * @param ex The exception that we wish to propogate to the client
     * @throws IOException If writing to the output stream fails
     */
    void marshallException(HttpServletRequest request, HttpServletResponse response, Exception ex) throws IOException;

    /**
     * Check if we can coerce the given type
     * @param paramType The type to check
     * @return true iff paramType is coercable
     */
    boolean isConvertable(Class paramType);
}