主线程等待几个子线程执行完成方案

 有时，为了程序的性能，我们有必要对程序中的for循环(含有sql/rpc操作)进行并发处理，要求是并发处理完之后才能继续执行主线程。现给出如下两种方案：

1. CountDownLatch

package com.itlong.whatsmars.base.sync;

import java.util.concurrent.CountDownLatch;

/**
 * Created by shenhongxi on 2016/8/12.
 */
public class CountDownLatchTest {

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(3);
        long start = System.currentTimeMillis();
        for (int i = 0; i < 3; i++) {
            new Thread(new SubRunnable(i, latch)).start();
        }
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(System.currentTimeMillis() - start);

        System.out.println("Main finished");
    }

    static class SubRunnable implements Runnable {
        private int id = -1;
        private CountDownLatch latch;

        SubRunnable(int id, CountDownLatch latch) {
            this.id = id;
            this.latch = latch;
        }

        @Override
        public void run() {
            try {
                Thread.sleep(3000);
                System.out.println(String
                        .format("Sub Thread %d finished", id));
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                latch.countDown();
            }
        }
    }
}

 CountDownLatch用队列来存放任务，主要是一个构造器和两个方法，相关代码这里不予赘述。CountDownLatch很贴合我们的要求，但没用到线程池，考虑到性能，我推荐下面的这种方案。

2. ExecutorService

package com.itlong.whatsmars.base.sync;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Created by shenhongxi on 2016/8/12.
 */
public class CallableTest {

    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(3);

        List<Callable<Void>> subs = new ArrayList<Callable<Void>>();
        for (int i = 0; i < 3; i++) {
            subs.add(new SubCallable(i));
        }

        long start = System.currentTimeMillis();
        try {
            pool.invokeAll(subs);
        } finally {
            pool.shutdown();
        }
        System.out.println(System.currentTimeMillis() - start);

        System.out.println("Main finished");
    }

    static class SubCallable implements Callable<Void> {
        private int id = -1;

        public SubCallable(int id) {
            this.id = id;
        }

        @Override
        public Void call() throws Exception {
            try {
                Thread.sleep(3000);
                System.out.println(String
                        .format("Child Thread %d finished", id));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return null;
        }
    }

}

 AbstractExecutorService

public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException {
        if (tasks == null)
            throw new NullPointerException();
        List<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
        boolean done = false;
        try {
            for (Callable<T> t : tasks) {
                RunnableFuture<T> f = newTaskFor(t);
                futures.add(f);
                execute(f);
            }
            for (Future<T> f : futures) {
                if (!f.isDone()) {
                    try {
                        f.get();
                    } catch (CancellationException ignore) {
                    } catch (ExecutionException ignore) {
                    }
                }
            }
            done = true;
            return futures;
        } finally {
            if (!done)
                for (Future<T> f : futures)
                    f.cancel(true);
        }
    }

接下来我做了个join的试验，发现同样可以达到目的，但不推荐此法。

package com.itlong.whatsmars.base.sync;

/**
 * Created by shenhongxi on 2016/8/12.
 * 子线程与主线程是顺序执行的，各子线程之间还是异步的
 */
public class JoinTest {

    public static void main(String[] args) throws Exception {
        Thread t1 = new Thread(new SubRunnable(0));
        Thread t2 = new Thread(new SubRunnable(1));
        Thread t3 = new Thread(new SubRunnable(2));

        long start = System.currentTimeMillis();
        t1.start();
        t2.start();
        t3.start();

        t1.join();
        t2.join();
        t3.join();
        System.out.println(System.currentTimeMillis() - start);

        System.out.println("Main finished");
    }

    static class SubRunnable implements Runnable {
        private int id = -1;

        SubRunnable(int id) {
            this.id = id;
        }

        @Override
        public void run() {
            try {
                System.out.println("hi, I'm id-" + id);
                Thread.sleep(9000);
                System.out.println(String
                        .format("Sub Thread %d finished", id));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

}

最后，我们顺便提下org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor

public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport implements SchedulingTaskExecutor {

	private final Object poolSizeMonitor = new Object();

	private int corePoolSize = 1;

	private int maxPoolSize = Integer.MAX_VALUE;

	private int keepAliveSeconds = 60;

	private boolean allowCoreThreadTimeOut = false;

	private int queueCapacity = Integer.MAX_VALUE;

	private ThreadPoolExecutor threadPoolExecutor;


	/**
	 * Set the ThreadPoolExecutor's core pool size.
	 * Default is 1.
	 * <p><b>This setting can be modified at runtime, for example through JMX.</b>
	 */
	public void setCorePoolSize(int corePoolSize) {
		synchronized (this.poolSizeMonitor) {
			this.corePoolSize = corePoolSize;
			if (this.threadPoolExecutor != null) {
				this.threadPoolExecutor.setCorePoolSize(corePoolSize);
			}
		}
	}

	/**
	 * Return the ThreadPoolExecutor's core pool size.
	 */
	public int getCorePoolSize() {
		synchronized (this.poolSizeMonitor) {
			return this.corePoolSize;
		}
	}

看到我们熟悉的ThreadPoolExecutor之后，我们瞬间明白了一切。

另外我们脑补下几个接口/类的关系

public interface ExecutorService extends Executor {
  <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException;
}

public interface Executor {
    void execute(Runnable command);
}

public abstract class AbstractExecutorService implements ExecutorService{
  public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
    // ...
  }
}

public class ThreadPoolExecutor extends AbstractExecutorService {
  public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }
}