Question

我有一堆并发运行的线程。有时线程需要通知其他线程等待它完成一个作业并再次发出信号以便恢复。由于我对Java的同步不熟悉，我想知道做这种事情的正确方法是什么。我的代码是这样的：

private void Concurrent() {
    if (shouldRun()) {
        // notify threads to pause and wait for them
        DoJob();
        // resume threads
    }

    // Normal job...
}

更新

请注意，我编写的代码位于将由每个线程执行的类中。我无法访问这些线程或它们如何运行。我只是在线程内部。

更新2：

我的代码来自一个抓取工具类。爬虫类（crawler4j）知道如何处理并发。我唯一需要的是在运行函数之前暂停其他爬虫并在之后恢复它们。此代码是我的抓取工具的基础：

   public class TestCrawler extends WebCrawler {
    private SingleThread()
    {
        //When this function is running, no other crawler should do anything
    }

    @Override
    public void visit(Page page) {
        if(SomeCriteria())
        {
            //make all other crawlers stop until I finish
            SingleThread();
            //let them resume
        }

        //Normal Stuff
    }
   }

Answer 1

这是一个简短的例子，介绍如何使用酷的java并发内容实现这一点：

使用Pause类

snip 旧代码无关紧要。

编辑：

这是新的测试类：

package de.hotware.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {

    private Pause mPause;

    public Test() {
        this.mPause = new Pause();
    }

    public void concurrent() throws InterruptedException {
        while(true) {
            this.mPause.probe();
            System.out.println("concurrent");
            Thread.sleep(100);
        }
    }

    public void crucial() throws InterruptedException {
        int i = 0;
        while (true) {
            if (i++ % 2 == 0) {
                this.mPause.pause(true);
                System.out.println("crucial: exclusive execution");
                this.mPause.pause(false);
            } else {
                System.out.println("crucial: normal execution");
                Thread.sleep(1000);
            }
        }
    }

    public static void main(String[] args) {
        final Test test = new Test();
        Runnable run = new Runnable() {

            @Override
            public void run() {
                try {
                    test.concurrent();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        };
        Runnable cruc = new Runnable() {

            @Override
            public void run() {
                try {
                    test.crucial();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        };
        ExecutorService serv = Executors.newCachedThreadPool();
        serv.execute(run);
        serv.execute(run);
        serv.execute(cruc);
    }

}

实用程序Pause类：

package de.hotware.test;

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Utility class to pause and unpause threads
 * with Java Concurrency
 * @author Martin Braun
 */
public class Pause {

    private Lock mLock;
    private Condition mCondition;
    private AtomicBoolean mAwait;

    public Pause() {
        this.mLock = new ReentrantLock();
        this.mCondition = this.mLock.newCondition();
        this.mAwait = new AtomicBoolean(false);
    }

    /**
     * waits until the threads until this.mAwait is set to true
     * @throws InterruptedException
     */
    public void probe() throws InterruptedException {
        while(this.mAwait.get()) {
            this.mLock.lock();
            try {
                this.mCondition.await();
            } finally {
                this.mLock.unlock();
            }
        }
    }

    /**
     * pauses or unpauses
     */
    public void pause(boolean pValue) {
        if(!pValue){
            this.mLock.lock();
            try {
                this.mCondition.signalAll();
            } finally {
                this.mLock.unlock();
            }
        }
        this.mAwait.set(pValue);
    }

}

基本用法是在每次运行之前调用probe（）。如果它被暂停直到调用pause（false），它将被阻止。

你的课程看起来像这样：

public class TestCrawler extends WebCrawler {

private Pause mPause;

public TestCrawler(Pause pPause) {
    this.mPause = pPause;
}

private SingleThread()
{
        //When this function is running, no other crawler should do anything
}

@Override
public void visit(Page page) {
    if(SomeCriteria())
    {
        //only enter the crucial part once if it has to be exclusive
        this.mPause.probe();
        //make all other crawlers stop until I finish
        this.mPause.pause(true);
        SingleThread();
        //let them resume
        this.mPause.pause(false);
    }
    this.mPause.probe();
    //Normal Stuff
}
}

Answer 2

public class StockMonitor extends Thread {

    private boolean suspend = false; 
    private volatile Thread thread;

    public StockMonitor() {
        thread = this;
    }

    // Use name with underscore, in order to avoid naming crashing with
    // Thread's.
    private synchronized void _wait() throws InterruptedException {
        while (suspend) {
            wait();
        }
    }

    // Use name with underscore, in order to avoid naming crashing with
    // Thread's.
    public synchronized void _resume() {
        suspend = false;
        notify();
    }

    // Use name with underscore, in order to avoid naming crashing with
    // Thread's.
    public synchronized void _suspend() {
        suspend = true;
    }  

     public void _stop() { 
        thread = null;
        // Wake up from sleep.
        interrupt();     
     }

     @Override
     public void run() {
        final Thread thisThread = Thread.currentThread();
        while (thisThread == thread) {
            _wait();
            // Do whatever you want right here.
        }
     }
}

通过拨打_resume和_suspend，您可以恢复并暂停Thread。 _stop会让你优雅地停止线程。请注意，一旦停止Thread，就无法再次恢复它。 Thread不再可用。

正在从现实世界的开源项目中挑选代码：http://jstock.hg.sourceforge.net/hgweb/jstock/jstock/file/b17c0fbfe37c/src/org/yccheok/jstock/engine/RealTimeStockMonitor.java#l247

Answer 3

您可以使用wait（）和notify（）

线程等待：

  // define mutex as field
  Object mutex = new Object();

   // later:
   synchronized(mutex) {
        wait();
   }

通知线程继续

   synchronized (mutex) {
        notify();
   }

如何将线程安全的信号线程在Java中暂停

更新

更新2：

3 个答案: