Question

I have a bunch of threads running concurrently. Sometimes a thread needs to notify other threads to wait for it to finish a job and signal them again to resume. Since I’m somehow new to Java’s synchronization, I wonder what is the right way to do such thing. My code is something like this:

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

    // Normal job...
}

Update:

Note that the code I wrote is inside a class which will be executed by each thread. I don’t have access to those threads or how they are running. I’m just inside threads.

Update 2:

My code is from a crawler class. The crawler class (crawler4j) knows how to handle concurrency. The only thing I need is to pause other crawlers before running a function and resume them afterwards. This code is the basics of my crawler:

   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

Here is a short example on how to achieve this with the cool java concurrency stuff:

snip old code doesn’t matter anymore with the Pause class.

EDIT:

Here is the new Test class:

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);
    }

}

And the utility Pause class:

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);
    }

}

The basic usage is to call probe() before each run. This will block if it is paused until pause(false) is called.

Your class would look like this:

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
}
}