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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.dwrp;

/**
 * A Sleeper allows the request to halt and cease execution for some time,
 * while still allowing output.
 * There are 3 envisaged implementations
 * 
 * Old servlet stacks where the only option is to call
 * {@link Object#wait(long)} and {@link Object#notify()} to resume.
 * Jetty, where an Ajax Continuation causes an exception
 * Newer async servlets where the implementation will probably continue
 * with the servlet engine able to detect that it should not complete the
 * request.
 * 
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
interface Sleeper
{
    /**
     * 'halt' the current execution in some way.
     * This method should be the last meaningful thing that is done on a
     * request, and work that needs to be done before completion should be
     * done in a {@link Runnable} so the system can schedule it at an
     * appropriate time.
     * @param onAwakening The action to take when {@link #wakeUp()} is called
     */
    void goToSleep(Runnable onAwakening);

    /**
     * This method should attempt to resume the execution.
     * It is possible that this method will be called more than once at the
     * same time so Sleepers should be prepared take steps to be woken only
     * once.
     */
    void wakeUp();
}