Java多執行緒等待所有執行緒結束(CountDownLatch/CyclicBarrier)
阿新 • • 發佈:2019-01-22
本文主要是參考官方文件做一學習用途。
官方連結:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CountDownLatch.html
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/CyclicBarrier.html
多執行緒設計過程中,經常會遇到需要等待其它執行緒結束以後再做其他事情的情況,比如多執行緒下載檔案,每個執行緒都會下載檔案的一部分,在所有執行緒結束以後,需要將各部分再次拼接成一個完整的檔案。
有幾種方案:
1.在主執行緒中設定一自定義全域性計數標誌,在工作執行緒完成時,計數減一。主執行緒偵測該標誌是否為0,一旦為0,表示所有工作執行緒已經完成。
2.使用Java標準的類CountDownLatch來完成這項工作,原理是一樣的,計數。
CountDownLatch
CountDownLatch 初始化設定count,即等待(await)count個執行緒或一個執行緒count次計數,通過工作執行緒來countDown計數減一,直到計數為0,await阻塞結束。
設定的count不可更改,如需要動態設定計數的執行緒數,可以使用CyclicBarrier.
下面的例子,所有的工作執行緒中準備就緒以後,並不是直接執行,而是等待主執行緒的訊號後再執行具體的操作。
package com.example.multithread; import java.util.concurrent.CountDownLatch; class Driver { private static final int TOTAL_THREADS = 10; private final CountDownLatch mStartSignal = new CountDownLatch(1); private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS); void main() { for (int i = 0; i < TOTAL_THREADS; i++) { new Thread(new Worker(mStartSignal, mDoneSignal, i)).start(); } System.out.println("Main Thread Now:" + System.currentTimeMillis()); doPrepareWork();// 準備工作 mStartSignal.countDown();// 計數減一為0,工作執行緒真正啟動具體操作 doSomethingElse();//做點自己的事情 try { mDoneSignal.await();// 等待所有工作執行緒結束 } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } System.out.println("All workers have finished now."); System.out.println("Main Thread Now:" + System.currentTimeMillis()); } void doPrepareWork() { System.out.println("Ready,GO!"); } void doSomethingElse() { for (int i = 0; i < 100000; i++) { ;// delay } System.out.println("Main Thread Do something else."); } } class Worker implements Runnable { private final CountDownLatch mStartSignal; private final CountDownLatch mDoneSignal; private final int mThreadIndex; Worker(final CountDownLatch startSignal, final CountDownLatch doneSignal, final int threadIndex) { this.mDoneSignal = doneSignal; this.mStartSignal = startSignal; this.mThreadIndex = threadIndex; } @Override public void run() { // TODO Auto-generated method stub try { mStartSignal.await();// 阻塞,等待mStartSignal計數為0執行後面的程式碼 // 所有的工作執行緒都在等待同一個啟動的命令 doWork();// 具體操作 System.out.println("Thread " + mThreadIndex + " Done Now:" + System.currentTimeMillis()); mDoneSignal.countDown();// 完成以後計數減一 } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } public void doWork() { for (int i = 0; i < 1000000; i++) { ;// 耗時操作 } System.out.println("Thread " + mThreadIndex + ":do work"); } } public class CountDownLatchTest { public static void main(String[] args) { // TODO Auto-generated method stub new Driver().main(); } }
通過Executor啟動執行緒:
class CountDownLatchDriver2 { private static final int TOTAL_THREADS = 10; private final CountDownLatch mDoneSignal = new CountDownLatch(TOTAL_THREADS); void main() { System.out.println("Main Thread Now:" + System.currentTimeMillis()); doPrepareWork();// 準備工作 Executor executor = Executors.newFixedThreadPool(TOTAL_THREADS); for (int i = 0; i < TOTAL_THREADS; i++) { // 通過內建的執行緒池維護建立的執行緒 executor.execute(new RunnableWorker(mDoneSignal, i)); } doSomethingElse();// 做點自己的事情 try { mDoneSignal.await();// 等待所有工作執行緒結束 } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } System.out.println("All workers have finished now."); System.out.println("Main Thread Now:" + System.currentTimeMillis()); } void doPrepareWork() { System.out.println("Ready,GO!"); } void doSomethingElse() { for (int i = 0; i < 100000; i++) { ;// delay } System.out.println("Main Thread Do something else."); } } class RunnableWorker implements Runnable { private final CountDownLatch mDoneSignal; private final int mThreadIndex; RunnableWorker(final CountDownLatch doneSignal, final int threadIndex) { this.mDoneSignal = doneSignal; this.mThreadIndex = threadIndex; } @Override public void run() { // TODO Auto-generated method stub doWork();// 具體操作 System.out.println("Thread " + mThreadIndex + " Done Now:" + System.currentTimeMillis()); mDoneSignal.countDown();// 完成以後計數減一 // 計數為0時,主執行緒結束阻塞,繼續執行其他任務 try { // 可以繼續做點其他的事情,與主執行緒無關了 Thread.sleep(5000); System.out.println("Thread " + mThreadIndex + " Do something else after notifing main thread"); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } public void doWork() { for (int i = 0; i < 1000000; i++) { ;// 耗時操作 } System.out.println("Thread " + mThreadIndex + ":do work"); } }
輸出: