【多线程】工具类汇总

不同工具对比

CyclicBarrier vs CountdownLatch：CyclicBarrier可以通过reset()实现重置门限值，CountDownLatch为一次性。

一、CountDownLatch

注意：new CountDownLatch(5)中数字必须严格跟期望等待数字保持一致，否则可能出现意外的结果（因为超出5的线程会不被进行等待）

1、代码使用样例

　　通过CountDownLatch实现："主线程"等待"5个子线程"全部都完成"指定的工作(休眠1000ms)"之后，再继续运行。

 1 import java.util.concurrent.CountDownLatch;
 2 
 3 public class CountDownLatchTest {
 4 
 5     private static CountDownLatch latch = new CountDownLatch(5);
 6 
 7     public static void main(String[] args) {
 8 
 9         System.out.println("Main Thread start....");
10         System.out.println();
11 
12         for (int i = 0; i < 5; i++) {
13             new InnerThread().start();
14         }
15 
16         try {
17             latch.await();
18             System.out.println();
19             System.out.println("Main Thread latch.getCount = " + latch.getCount());
20         } catch (InterruptedException e) {
21             System.out.println("Exception happened in latch await: " + e);
22         }
23 
24         System.out.println("Main Thread end....");
25     }
26 
27     static class InnerThread extends Thread {
28         public void run() {
29             synchronized (InnerThread.class) {
30                 try {
31                     Thread.sleep(1000);
32                     System.out.println(Thread.currentThread().getName() + " sleep 1000 ms.");
33                     // 将CountDownLatch的数值减1
34                     latch.countDown();
35                     System.out.println(Thread.currentThread().getName() + " count number: " + latch.getCount());
36                 } catch (InterruptedException e) {
37                     e.printStackTrace();
38                 }
39             }
40         }
41     }
42 }

View Code

　　执行结果：主线程等待，直至 latch 通过 countDown 减为0后，主线程才继续执行。

 1 Main Thread start....
 2 
 3 Thread-1 sleep 1000 ms.
 4 Thread-1 count number: 4
 5 Thread-4 sleep 1000 ms.
 6 Thread-4 count number: 3
 7 Thread-0 sleep 1000 ms.
 8 Thread-0 count number: 2
 9 Thread-3 sleep 1000 ms.
10 Thread-3 count number: 1
11 Thread-2 sleep 1000 ms.
12 Thread-2 count number: 0
13 
14 Main Thread latch.getCount = 0
15 Main Thread end....

View Code

2、常用方法解读

CountDownLatch(int count) // 构造一个用给定计数初始化的 CountDownLatch。
// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断。
void await()      // 调用sync.acquireSharedInterruptibly(1)实现AQS中state=1
// 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断或超出了指定的等待时间。 
boolean await(long timeout, TimeUnit unit) 
// 递减锁存器的计数，如果计数到达零，则释放所有等待的线程。 
void countDown() // 调用sync.releaseShared(1) 实现AQS中state-1
long getCount() // 返回当前计数。