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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 org.directwebremoting.Container;
import org.directwebremoting.ServerContext;
import org.directwebremoting.ui.Callback;

/**
 * A class to help with the use of {@link Callback}s
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class CallbackHelperFactory
{
    /**
     * Accessor for the current CallbackHelper.
     * @return The current CallbackHelper.
     */
    public static CallbackHelper get()
    {
        return factory.get();
    }

    /**
     * Accessor for the current CallbackHelper in more complex setups.
     * For some setups DWR may not be able to discover the correct environment
     * (i.e. ServletContext), so we need to tell it. This generally happens if
     * you have DWR configured twice in a single context. Unless you are writing
     * code that someone else will configure, it is probably safe to use the
     * simpler {@link #get()} method.
     * @param ctx The servlet context to allow us to bootstrap
     * @return The current CallbackHelper.
     */
    public static CallbackHelper get(ServerContext ctx)
    {
        return factory.get(ctx);
    }

    /**
     * Internal method to allow us to get the Builder from which we
     * will get CallbackHelper objects.
     * Do NOT call this method from outside of DWR.
     */
    public static CallbackHelper attach(Container container)
    {
        return factory.attach(container);
    }

    private static Factory factory = Factory.create(CallbackHelperBuilder.class);

    /**
     * Hack to get around Generics not being implemented by erasure
     */
    public interface CallbackHelperBuilder extends Builder
    {
    }
}