Quelle TaskQueue.java Sprache: JAVA

/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements.  See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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.apache.tomcat.util.threads;

import java.util.Collection;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.TimeUnit;

import org.apache.tomcat.util.res.StringManager;

/**
* As task queue specifically designed to run with a thread pool executor. The
* task queue is optimised to properly utilize threads within a thread pool
* executor. If you use a normal queue, the executor will spawn threads when
* there are idle threads and you won't be able to force items onto the queue
* itself.
*/
public class TaskQueue extends LinkedBlockingQueue<Runnable> {

    private static final long serialVersionUID = 1L;
    protected static final StringManager sm = StringManager.getManager(TaskQueue.class);

    private transient volatile ThreadPoolExecutor parent = null;

    public TaskQueue() {
        super();
    }

    public TaskQueue(int capacity) {
        super(capacity);
    }

    public TaskQueue(Collection<? extends Runnable> c) {
        super(c);
    }

    public void setParent(ThreadPoolExecutor tp) {
        parent = tp;
    }

    /**
     * Used to add a task to the queue if the task has been rejected by the Executor.
     *
     * @param o         The task to add to the queue
     *
     * @return          {@code true} if the task was added to the queue,
     *                      otherwise {@code false}
     */
    public boolean force(Runnable o) {
        if (parent == null || parent.isShutdown()) {
            throw new RejectedExecutionException(sm.getString("taskQueue.notRunning"));
        }
        return super.offer(o); //forces the item onto the queue, to be used if the task is rejected
    }

    @Override
    public boolean offer(Runnable o) {
      //we can't do any checks
        if (parent==null) {
            return super.offer(o);
        }
        //we are maxed out on threads, simply queue the object
        if (parent.getPoolSizeNoLock() == parent.getMaximumPoolSize()) {
            return super.offer(o);
        }
        //we have idle threads, just add it to the queue
        if (parent.getSubmittedCount() <= parent.getPoolSizeNoLock()) {
            return super.offer(o);
        }
        //if we have less threads than maximum force creation of a new thread
        if (parent.getPoolSizeNoLock() < parent.getMaximumPoolSize()) {
            return false;
        }
        //if we reached here, we need to add it to the queue
        return super.offer(o);
    }

    @Override
    public Runnable poll(long timeout, TimeUnit unit)
            throws InterruptedException {
        Runnable runnable = super.poll(timeout, unit);
        if (runnable == null && parent != null) {
            // the poll timed out, it gives an opportunity to stop the current
            // thread if needed to avoid memory leaks.
            parent.stopCurrentThreadIfNeeded();
        }
        return runnable;
    }

    @Override
    public Runnable take() throws InterruptedException {
        if (parent != null && parent.currentThreadShouldBeStopped()) {
            return poll(parent.getKeepAliveTime(TimeUnit.MILLISECONDS),
                    TimeUnit.MILLISECONDS);
            // yes, this may return null (in case of timeout) which normally
            // does not occur with take()
            // but the ThreadPoolExecutor implementation allows this
        }
        return super.take();
    }
}

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