如何在Java中使用线程安全信号来暂停线程

Question

我有一堆并发运行的线程。有时，一个线程需要通知其他线程等待它完成一个作业，并再次向它们发出恢复的信号。因为我是Java同步的新手，所以我想知道做这件事的正确方法是什么。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，我编写的代码位于一个类中，该类将由每个线程执行。我无法访问这些线程或它们是如何运行的。我只是在线程内部。

更新2：

我的代码来自一个爬虫类。crawler类(crawler4j)知道如何处理并发。我唯一需要做的就是在运行函数之前暂停其他爬虫，之后再恢复它们。这段代码是我的爬虫的基础：

   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

下面是一个简短的示例，说明如何使用很酷的java并发工具来实现这一点：

对于Pause类，截取旧代码不再重要。

编辑：

下面是新的Test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);
    }

}

和实用工具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()。如果在调用pause(false)之前暂停，则会阻塞。

您的类将如下所示：

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.
        }
     }
}

调用_resume和_suspend将使您能够恢复和暂停Thread。_stop将允许您优雅地停止线程。请注意，一旦停止Thread，就没有办法再次恢复它。Thread不再可用。

代码取自一个真实的开源项目：http://jstock.hg.sourceforge.net/hgweb/jstock/jstock/file/b17c0fbfe37c/src/org/yccheok/jstock/engine/RealTimeStockMonitor.java#l247

Answer 3

可以使用wait()和notify()

线程正在等待：

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

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

通知线程继续

   synchronized (mutex) {
        notify();
   }