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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.util.ArrayList;
import java.util.List;

import org.directwebremoting.ScriptBuffer;

/**
 * A simple utility class to extract a {@link String} from a {@link ScriptBuffer}.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class ScriptBufferUtil
{
    /**
     * Ensure we can't be created
     */
    private ScriptBufferUtil()
    {
    }

    /**
     * Return a string ready for output.
     * @param buffer The source of the script data
     * @param converterManager How we convert script variable to Javascript
     * @return Some Javascript to be eval()ed by a browser.
     * @throws MarshallException If an error happens during parameter marshalling
     */
    public static String createOutput(ScriptBuffer buffer, ConverterManager converterManager) throws MarshallException
    {
        return createOutput(buffer, converterManager, false);
    }

    /**
     * Return a string ready for output.
     * @param buffer The source of the script data
     * @param converterManager How we convert script variable to Javascript
     * @param jsonOutput Are we doing strict JSON output?
     * @return Some Javascript to be eval()ed by a browser.
     * @throws MarshallException If an error happens during parameter marshalling
     */
    public static String createOutput(ScriptBuffer buffer, ConverterManager converterManager, boolean jsonOutput) throws MarshallException
    {
        OutboundContext context = new OutboundContext(jsonOutput);
        List scriptParts = new ArrayList();

        // First convert everything into OutboundVariables
        for (Object part : buffer.getParts())
        {
            OutboundVariable ov = converterManager.convertOutbound(part, context);
            scriptParts.add(ov);
        }

        StringBuffer output = new StringBuffer();
        context.prepareForOutput();

        // First we look for the declaration code
        for (OutboundVariable ov : scriptParts)
        {
            output.append(ov.getDeclareCode());
        }

        // Then we look for the construction code
        for (OutboundVariable ov : scriptParts)
        {
            output.append(ov.getBuildCode());
        }

        // Then we output everything else
        for (OutboundVariable ov : scriptParts)
        {
            String assignCode = ov.getAssignCode();
            if (assignCode == null)
            {
                throw new NullPointerException();
            }
            output.append(assignCode);
        }

        return output.toString();
    }
}