在面试时,经常会有面试官问道,一个主线程有多个子线程,如何能使子线程的业务执行完成之后,再执行主线程业务逻辑。对于这个问题,本人能够想到的有五种方法,详细请移步源码
1、使用线程类自带的join方法,将子线程加入到主线程,在子线程执行完之后,在执行主线程逻辑。
例如
- public static void joinDemo()
- throws InterruptedException
- {
- System.out.println("=========Test with join=====");
- JoinWorker worker1 = new JoinWorker("worker1");
- JoinWorker worker2 = new JoinWorker("worker2");
- worker1.start();
- worker2.start();
- worker1.join();
- worker2.join();
- doSuperWork();
- }
2、使用JDK的并发包中的CountDownLatch类, 使用CountDownLatch,每个线程调用其countDown方法使计数器-1,主线程调用await方法阻塞等待,直到CountDownLatch计数器为0时继续执行,例如
首先,定义子线程
- static class CountDownLatchWorker extends Thread
- {
- String workerName;
- CountDownLatch latch;
- public CountDownLatchWorker(String workerName, CountDownLatch latch)
- {
- this.workerName = workerName;
- this.latch = latch;
- }
- public void run()
- {
- System.out.println("Sub Worker " + workerName + " do work begin at "
- + sdf.format(new Date()));
- new ThreadWaitDemo().doSomeWork();// 做实际工作
- System.out.println("Sub Worker " + workerName + " do work complete at "
- + sdf.format(new Date()));
- latch.countDown();// 完成之后,计数器减一
- }
- }
主线程中调研await方法阻塞等待,直到所有线程完成
- public static void countDownLatchDemo()
- throws InterruptedException
- {
- System.out.println("=========Test with CountDownLatch=====");
- CountDownLatch latch = new CountDownLatch(2);
- CountDownLatchWorker worker1 = new CountDownLatchWorker("worker1", latch);
- CountDownLatchWorker worker2 = new CountDownLatchWorker("worker2", latch);
- worker1.start();
- worker2.start();
- //主线程阻塞等待
- latch.await();
- doSuperWork();
- }
3、使用JDK并发包CyclicBarrier,CyclicBarrier类似于CountDownLatch也是个计数器, 不同的是CyclicBarrier的await()方法没被调用一次,计数便会减少1,并阻塞住当前线程。当计数减至0时,阻塞解除,所有在此 CyclicBarrier 上面阻塞的线程开始运行。 在这之后,如果再次调用 await()方法,计数就又会变成 N-1,新一轮重新开始CyclicBarrier初始时还可带一个Runnable的参数,此Runnable任务在CyclicBarrier的数目达到后,所有其它线程被唤醒前被执行。
示例如下
- public static void cyclicBarrierDemo()
- throws InterruptedException, BrokenBarrierException
- {
- System.out.println("=========Test with CyclicBarrier=====");
- CyclicBarrier cb = new CyclicBarrier(2, new Runnable()
- {
- // 将主线程业务放到CyclicBarrier构造方法中,所有线程都到达Barrier时执行
- @SuppressWarnings("static-access")
- public void run()
- {
- new ThreadWaitDemo().doSuperWork();
- }
- });// 设定需要等待两个线程
- ExecutorService executor = Executors.newFixedThreadPool(2);
- CyclicBarrierWorker worker1 = new CyclicBarrierWorker("worker1", cb);
- CyclicBarrierWorker worker2 = new CyclicBarrierWorker("worker2", cb);
- executor.execute(worker1);
- executor.execute(worker2);
- executor.shutdown();
- }
4、使用JDK并发包中的Executors框架,ExecutorService的的invokeAll方法调研callable集合,批量执行多个线程,在invokeAll方法结束之后,再执行主线程其他业务逻辑
示例如下
- public static void callableDemo()
- throws InterruptedException
- {
- System.out.println("=========Test with Callable=====");
- List<Callable<Integer>> callList = new ArrayList<Callable<Integer>>();
- ExecutorService exec = Executors.newFixedThreadPool(2);
- // 采用匿名内部类实现
- callList.add(new Callable<Integer>()
- {
- public Integer call()
- throws Exception
- {
- System.out.println("Sub Worker worker1 do work begin at " + sdf.format(new Date()));
- new ThreadWaitDemo().doSomeWork();// 做实际工作
- System.out.println("Sub Worker worker1 do work complete at "
- + sdf.format(new Date()));
- return 0;
- }
- });
- callList.add(new Callable<Integer>()
- {
- public Integer call()
- throws Exception
- {
- System.out.println("Sub Worker worker2 do work begin at " + sdf.format(new Date()));
- new ThreadWaitDemo().doSomeWork();// 做实际工作
- System.out.println("Sub Worker worker2 do work complete at "
- + sdf.format(new Date()));
- return 0;
- }
- });
- exec.invokeAll(callList);
- exec.shutdown();
- doSuperWork();
- }