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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.servlet;

import java.io.IOException;

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

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.extend.Handler;

/**
 * Handles an exception occurring during the request dispatching.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class ExceptionHandler implements Handler
{
    /* (non-Javadoc)
     * @see org.directwebremoting.Handler#handle(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
     */
    public void handle(HttpServletRequest request, HttpServletResponse response) throws IOException
    {
        log.warn("Error: " + cause);
        log.debug("Debug for stack trace:", cause);

        try
        {
            // We are going to act on this in engine.js so we are hoping that
            // that SC_NOT_IMPLEMENTED (501) is not something that the servers
            // use that much. I would have used something unassigned like 506+
            // But that could cause future problems and might not get through
            // proxies and the like
            response.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, "Error. Details logged to the console");
        }
        catch (Exception ex)
        {
            // If the browser has gone away we expect to fail, and may not be
            // able to recover
            // Technically an IOException should work here, but Jetty appears to
            // throw an ArrayIndexOutOfBoundsException sometimes, if the browser
            // has gone away.
            log.debug("Error in error handler, the browser probably went away: " + ex);
        }
    }

    /**
     * @param cause The cause of the failure
     */
    public void setException(Exception cause)
    {
        this.cause = cause;
    }

    /**
     * The cause of the failure
     */
    private Exception cause;

    /**
     * The log stream
     */
    private static final Log log = LogFactory.getLog(ExceptionHandler.class);
}