等待某（N）个线程执行完再执行某个线程的几种方法（Thread.join(),CountDownLatch,CyclicBarrier,Semaphore）

1.main线程中先调用threadA.join() ，再调用threadB.join()实现A->B->main线程的执行顺序

调用threadA.join()时，main线程会挂起，等待threadA执行完毕返回后再执行，到执行threadB.join()时再挂起，待threadB执行完毕返回继续执行main

使用场景：线程B依赖线程A的计算结果的场景

package concurrency;

public class JoinTest {
    public static void main(String[] args) throws InterruptedException{
        Thread threadA = new Thread(new JoinJob(),"thread-A");
        Thread threadB = new Thread(new JoinJob(),"thread-B");
        threadB.start();
        threadA.start();
        threadA.join();
        threadB.join();

        System.out.println("main ending...");
    }
}

class JoinJob implements Runnable{

    @Override
    public void run() {
        System.err.println(Thread.currentThread().getName() + " starting...");
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " ending...");

    }

}

运行结果

2.CountDownLatch

使用场景：多线程数据计算时需要先分几个线程计算第一步骤的数据，再合并一个线程计算第二步

 package concurrency;

 import java.util.concurrent.CountDownLatch;

 public class CountDownLatchTest {
     static CountDownLatch c = new CountDownLatch(6);//如果count大于实际线程数，c.await()会一直等待

     public static void main(String[] args) throws InterruptedException{
         for (int i = 0; i < c.getCount()/2; i++) {
             new Thread(new CountDownJob(c)).start();
             new Thread(new CountDownLatchJob2(c)).start();

         }

         System.out.println("waiting " + c.getCount() + " subthread doing");
         c.await();//await()会等待c的count值为0，才继续往下执行
         System.out.println("all subthread done!");
         System.out.println(Thread.currentThread().getName() + " done!");
     }
 }

 class CountDownJob implements Runnable{
     CountDownLatch cDownLatch;

     public CountDownJob(CountDownLatch cDownLatch) {
         super();
         this.cDownLatch = cDownLatch;
     }

     @Override
     public void run() {
         try {
             System.out.println(Thread.currentThread().getName() + " is doing something...");
             Thread.sleep(3000);
             cDownLatch.countDown();//线程结束时cDownLatch-1
         } catch (InterruptedException e) {
             e.printStackTrace();
         }

     }

 }
 class CountDownLatchJob2 implements Runnable{
     CountDownLatch cDownLatch;

     public CountDownLatchJob2(CountDownLatch cDownLatch) {
         super();
         this.cDownLatch = cDownLatch;
     }
     @Override
     public void run() {
         try {
             Thread.sleep(1000);
         } catch (InterruptedException e) {
             System.err.println("thread sleep exception");
         }
         System.out.println(Thread.currentThread().getName() + " is doing something");
         cDownLatch.countDown();//线程结束时cDownLatch-1
     }

 }

运行结果：

3.可循环使用的同步屏障CyclicBarrier

使用场景：多线程计算数据，等待全部线程执行完阶段一的工作之后，再执行后面的阶段工作

 package concurrency;

 import java.util.concurrent.BrokenBarrierException;
 import java.util.concurrent.CyclicBarrier;

 public class CyclicBarrierTest {
     public static void main(String[] args) throws InterruptedException, BrokenBarrierException{
         CyclicBarrier cBarrier = new CyclicBarrier(6);
         //CyclicBarrier 提供另一个构造函数传入所有线程执行完后需要执行的runnable
 //        CyclicBarrier cBarrier = new CyclicBarrier(6, new Runnable() {
 //
 //            @Override
 //            public void run() {
 //                System.out.println("doing something after all threads done.");
 //            }
 //        });
         for (int i = 0; i < cBarrier.getParties()/2; i++) {
             new Thread(new CyclicBarrierJob1(cBarrier)).start();
             new Thread(new CyclicBarrierJob2(cBarrier)).start();
         }
         //CyclicBarrier 可以重置再次使用，如计算发生错误时可以重置计数器，并让线程重新执行一次。
         //而CountDownLatch只可以使用一次
         //cBarrier.reset();
     }
 }

 class CyclicBarrierJob1 implements Runnable {
     CyclicBarrier cBarrier;

     public CyclicBarrierJob1(CyclicBarrier cBarrier) {
         super();
         this.cBarrier = cBarrier;
     }

     public void run() {
         System.out.println(Thread.currentThread().getName() + " finished phase1");
         try {
             cBarrier.await();//执行完阶段1的工作之后，等待其他线程全部执行完阶段1，才会继续执行阶段二
         } catch (InterruptedException e) {
             System.err.println("interrupted!");
         } catch (BrokenBarrierException e) {
             System.err.println("broken barrier!");
         }

         System.out.println(Thread.currentThread().getName() + " start phase2");
     }
 }
 class CyclicBarrierJob2 implements Runnable {
     CyclicBarrier cBarrier;

     public CyclicBarrierJob2(CyclicBarrier cBarrier) {
         super();
         this.cBarrier = cBarrier;
     }

     public void run() {
         System.out.println(Thread.currentThread().getName() + " finished phase1");
         try {
             cBarrier.await();//执行完阶段1的工作之后，等待其他线程全部执行完阶段1，才会继续执行阶段二
         } catch (InterruptedException e) {
             System.err.println("interrupted!");
         } catch (BrokenBarrierException e) {
             System.err.println("broken barrier!");
         }
         System.out.println(Thread.currentThread().getName() + " start phase2");

     }
 }

执行结果：

4.可控制并发线程数的信号量Semaphore

使用场景：控制同时访问特定资源的线程数量

 package concurrency;

 import java.util.concurrent.Semaphore;

 public class SemaphoreTest {
     public static void  main(String[] args) {
         final Semaphore semaphore = new Semaphore(3);//限制可以同时访问资源的线程数是10个

         for (int i = 0; i < 12; i++) {//新建30个线程
             new Thread(new Runnable() {

                 @Override
                 public void run() {
                     try {
                         semaphore.acquire();//获取到许可证才可以访问资源
                         System.out.println("accessing the database ..");
                         Thread.sleep(3000);
                         semaphore.release();//资源访问结束之后释放许可证
                     } catch (InterruptedException e) {
                         System.out.println("interrupted!");
                     }

                 }
             }).start();
         }

     }
 }