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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.impl;

import java.util.HashMap;
import java.util.Map;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.Container;
import org.directwebremoting.ServerContext;
import org.directwebremoting.ServerContextFactory;
import org.directwebremoting.extend.Builder;

/**
 * A Builder that creates DefaultHubs.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DefaultBuilder implements Builder
{
    /**
     * This method calls created.getConstructor(constructorParameters) in order
     * to find a constructor which the Builder can use
     * @param created The type we wish to create
     */
    public DefaultBuilder(Class created)
    {
        this.created = created;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Builder#get()
     */
    public T get()
    {
        ServerContext serverContext = ServerContextFactory.get();
        return get(serverContext);
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Builder#get()
     */
    @SuppressWarnings("unchecked")
    public T get(ServerContext serverContext)
    {
        Container container = serverContext.getContainer();
        Map, Object> contextObjects = lookup.get(container);
        if (contextObjects == null)
        {
            throw new IllegalStateException("The passed ServerContext in unknown to the DefaultBuilder");
        }

        T t = (T) contextObjects.get(created);
        if (t == null)
        {
            throw new IllegalStateException("The DefaultBuilder knows nothing about objects of type " + created.getName());
        }

        return t;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.extend.Builder#attach(org.directwebremoting.Container)
     */
    public T attach(Container container)
    {
        try
        {
            T t = container.newInstance(created);
            Map, Object> contextObjects = lookup.get(container);
            if (contextObjects == null)
            {
                contextObjects = new HashMap, Object>();
                lookup.put(container, contextObjects);
            }
            contextObjects.put(created, t);
            return t;
        }
        catch (Exception ex)
        {
            log.warn("Failed to create an instance of " + created.getName());
            throw new RuntimeException(ex);
        }
    }

    /**
     * Our cache of created objects.
     */
    private static final Map, Object>> lookup = new HashMap, Object>>();

    /**
     * How we create objects of the given type
     */
    private final Class created;

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