ThreadPoolExecutor 中的同步队列

Question

我正在尝试了解 ThreadPoolExecutor 中队列的行为。在下面的程序中，当我使用 LinkedBlockingQueue 时，我一次只能向线程池提交一个任务。但是，如果我将 LinkedBlockingQueue 替换为 SynchronousQueue，我可以立即将所有 5 个任务提交到池中。在这种情况下，SynchronousQueue 与 LinkedBlockingQueue 有何不同？

Java 程序：

import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Sample {
    public static void main(String[] args) throws InterruptedException {
        LinkedBlockingQueue<Runnable> threadPoolQueue = new LinkedBlockingQueue<>();
//      SynchronousQueue<Runnable> threadPoolQueue = new SynchronousQueue<>();
        ThreadFactory threadFactory = Executors.defaultThreadFactory();
        ThreadPoolExecutor tpe = new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, threadPoolQueue, threadFactory);
        Runnable np;

        for (int i = 1; i <= 5; i++) {
            np = new SampleWorker("ThreadPoolWorker " + i);
            tpe.submit(np);
        }

        System.out.println(tpe.getCorePoolSize());
        System.out.println(tpe.getPoolSize());
        System.out.println(tpe.getActiveCount());

        tpe.awaitTermination(Long.MAX_VALUE, TimeUnit.MILLISECONDS);
        tpe.shutdown();
        System.out.println("Main task finished");
    }
}

class SampleWorker implements Runnable {
    private String workerName;

    SampleWorker(String tName) {
        workerName = tName;
    }

    @Override
    public void run() {
        try {
            for (int i = 1; i <= 10; i++) {
                Thread.sleep(3000);
                System.out.println(this.workerName);
            }
            System.out.println(this.workerName + " finished");
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

Answer 1

当您向 ThreadPoolExecutor 提交任务时，它的工作方式如下：

if (numberOfWorkingThreads < corePoolSize) {
   startNewThreadAndRunTask();
} else if (workQueue.offer(task)) {
   if (numberOfWorkingThreads == 0) {
       startNewThreadAndRunTask();
   }
} else if (numberOfWorkingThreads < maxPoolSize)
    startNewThreadAndRunTask();
} else {
    rejectTask();
}

当使用没有初始值的LinkedBlockingQueue时， workQueue.offer(task)总会成功，结果只有一个线程已启动。
调用SynchronousQueue.offer(task)时，只有在另一个线程正在等待接收它。因为没有等待 thread, false 将被返回并创建新线程时间。

ThreadPoolExecutor 中的同步队列

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