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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 java.util.Map;

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

/**
 * A {@link org.directwebremoting.extend.Handler} that allows some very simple
 * search and replace templating. The general recommended syntax is to use
 * ${search} as the string to be searched for, to allow future expansion to a
 * more EL-like syntax.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public abstract class TemplateHandler extends CachingFileHandler
{
    /* (non-Javadoc)
     * @see org.directwebremoting.servlet.CachingFileHandler#generate(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse)
     */
    @Override
    protected String generateCachableContent(HttpServletRequest request, HttpServletResponse response) throws IOException
    {
        String template = generateTemplate(request, response);

        Map replace = getSearchReplacePairs();
        if (replace != null)
        {
            for (Map.Entry entry : replace.entrySet())
            {
                String search = entry.getKey();
                if (template.contains(search))
                {
                    template = template.replace(search, entry.getValue());
                }
            }
        }

        return template;
    }

    /**
     * Generate a template to undergo search and replace processing according to
     * the search and replace pairs from {@link #getSearchReplacePairs()}.
     * @param request The HTTP request data
     * @param response Where we write the HTTP response data
     * @return A template string containing ${} sections to be replaced
     */
    protected abstract String generateTemplate(HttpServletRequest request, HttpServletResponse response) throws IOException;

    /**
     * Mostly when we send a file out, we don't change anything so the default
     * set of search and replaces is empty.
     * Engine.js can override this with strings to customize the output
     * @return a map of search (key) and replace (value) strings
     */
    public Map getSearchReplacePairs()
    {
        return null;
    }
}