几天前我在思考这样一个问题，如果用java的多线程来做一个合力完成的事，例如合并请求，假设前端来了3个请求，到java这一层需要合成一个请求并返回，那么需要启用3个线程去请求，并等所有的结果都返回了再合并一下返回给前端，这该如何去做？
最开始我找到的是Thread.join这个方法。
Thread.join这个方法在源码中的描述是
Waits for this thread to die.
意思是等这个线程执行完了再去执行主线程。
那么我们针对上述问题可以实现一个这样的版本
package thread;
/**
* @author lkxiaolou
*/
public class ThreadPrint implements Runnable {
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + " is running");
try {
Thread.sleep(5000L);
} catch (Exception e) {
}
}
public static void main(String[] args) {
Thread thread1 = new Thread(new ThreadPrint());
Thread thread2 = new Thread(new ThreadPrint());
Thread thread3 = new Thread(new ThreadPrint());
thread1.start();
thread2.start();
try {
// 等待子线程完成后再执行主线程
thread1.join();
thread2.join();
} catch (Exception e) {
}
thread3.start();
}
}
先启动线程1和2，再等1、2执行完再启动第3个线程。
运行结果是这样的
Thread-0 is running
Thread-1 is running
Thread-1 done
Thread-0 done
Thread-2 is running
Thread-2 done
线程0和1不一定谁先执行完，但一定是在线程2执行前执行完成。
后来我又发现了java对多线程提供了一组并发包来做这些事情，其中有个叫CountDownLatch的类。这个类用起来简单也更加好理解，CountDownLatch提供了一个计数器，每当一个线程执行完计数器减1，直到减成0，主线程才可以开始执行。
package thread;
import java.util.concurrent.CountDownLatch;
/**
* @author lkxiaolou
*/
public class ThreadPrintCountDownLatch implements Runnable {
public static CountDownLatch countDownLatch = new CountDownLatch(2);
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + " is running");
try {
Thread.sleep(1000L);
} catch (Exception e) {
}
System.out.println(Thread.currentThread().getName() + " done");
countDownLatch.countDown();
}
public static void main(String[] args) {
Thread thread1 = new Thread(new ThreadPrintCountDownLatch());
Thread thread2 = new Thread(new ThreadPrintCountDownLatch());
Thread thread3 = new Thread(new ThreadPrintCountDownLatch());
thread1.start();
thread2.start();
try {
countDownLatch.await();
} catch (Exception e) {
}
thread3.start();
}
}
输出也和上面一样
Thread-0 is running
Thread-1 is running
Thread-0 done
Thread-1 done
Thread-2 is running
Thread-2 done
要注意减1的操作 countDownLatch.countDown(); 一定要放在关键部分最后，关键部分是你想等的操作已经完成了才行。countDownLatch有个缺点是计数器只能减不能加，一旦减到了0就不能再用了。
看了countDownLatch之后又发现了两个类似的类：CyclicBarrier和Semaphore。
CyclicBarrier，通过它可以实现让一组线程等待至某个状态之后再全部同时执行。
package thread;
import java.util.concurrent.CyclicBarrier;
/**
* @author lkxiaolou
*/
public class ThreadPrintCyclicBarrier implements Runnable {
public ThreadPrintCyclicBarrier(CyclicBarrier cyclicBarrier) {
this.cyclicBarrier = cyclicBarrier;
}
private CyclicBarrier cyclicBarrier;
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + " is running");
try {
Thread.sleep(1000L);
} catch (Exception e) {
}
try {
cyclicBarrier.await();
} catch (Exception e) {
}
System.out.println(Thread.currentThread().getName() + " done");
}
public static void main(String[] args) {
CyclicBarrier cyclicBarrier = new CyclicBarrier(2, new ThreadPrint()); // 这个线程输出带括号
Thread thread1 = new Thread(new ThreadPrintCyclicBarrier(cyclicBarrier));
Thread thread2 = new Thread(new ThreadPrintCyclicBarrier(cyclicBarrier));
thread1.start();
thread2.start();
}
}
输出是这样的
Thread-0 is running
Thread-1 is running
[Thread-0] is running
[Thread-0] done
Thread-0 done
Thread-1 done
可以看出CyclicBarrier可以让所有线程先执行，执行到cyclicBarrier.await再等待，等所有线程都执行到cyclicBarrier.await时执行我们指定的线程（也可以不指定），指定线程执行完了再把之前暂停的线程都起起来同时执行。似乎能做的事情更多。
最后我们看一下 Semaphore类，这个翻译成信号量，他能控制对临界资源的访问。
假若一个工厂有5台机器，但是有8个工人，一台机器同时只能被一个工人使用，只有使用完了，其他工人才能继续使用。那么我们就可以通过Semaphore来实现。
package thread;
import java.util.concurrent.Semaphore;
/**
* @author lkxiaolou
*/
public class ThreadPrintSemaphore implements Runnable {
public static Semaphore semaphore = new Semaphore(2);
@Override
public void run() {
try {
semaphore.acquire();
} catch (Exception e) {
}
System.out.println(Thread.currentThread().getName() + " is running");
try {
Thread.sleep(1000L);
} catch (Exception e) {
}
try {
semaphore.release();
} catch (Exception e) {
}
System.out.println(Thread.currentThread().getName() + " done");
}
public static void main(String[] args) {
Thread thread1 = new Thread(new ThreadPrintSemaphore());
Thread thread2 = new Thread(new ThreadPrintSemaphore());
Thread thread3 = new Thread(new ThreadPrintSemaphore());
thread1.start();
thread2.start();
thread3.start();
}
}
输出结果：
Thread-0 is running
Thread-1 is running
Thread-1 done
Thread-2 is running
Thread-0 done
Thread-2 done
多次运行可以看出0和1线程必须有一个释放了，2才能开始运行。
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