如何學習多線程作加法計算

問題：多線程計算1+2+…+100，如：起四個線程，分別計算1+2+..25, 26+27+…+50, 51+52+…+75, 76+77+…100, 最後將總和相關，輸出應為5050

解決方法：

• 依次調用thread.join()，主線程輸出結果。注意：sum為共享變量，訪問共享變量時，用synchronized同步
• 使用countDownLatch, 子線程執行完調用 countdownlatch.countdown()，主線程調用countdownlatc.await() 等待子線程執行完成，輸出結果。 注意：sum為共享變量，訪問共享變量時，用synchronized同步
• 使用cyclicbarrier, 子線程執行完調用 cyclicbarrier.await(), 最後都到達barrier時，輸出結果。注意：sum為共享變量，訪問共享變量時，用synchronized同步
• 通過線程池管理線程，用Future取得各子線程執行結果，最後將結果相加。

使用thread.join

依次調用thread.join()，主線程輸出結果。注意：sum為共享變量，訪問共享變量時，用synchronized同步。代碼如下：

package thread;public class ThreadAdd { public static int sum = 0; public static Object LOCK = new Object(); public static void main(String[] args) throws InterruptedException { ThreadAdd add = new ThreadAdd(); ThreadTest thread1 = add.new ThreadTest(1, 25); ThreadTest thread2 = add.new ThreadTest(26, 50); ThreadTest thread3 = add.new ThreadTest(51, 75); ThreadTest thread4 = add.new ThreadTest(76, 100); thread1.start(); thread2.start(); thread3.start(); thread4.start(); thread1.join(); thread2.join(); thread3.join(); thread4.join(); System.out.println("total result: "+sum); } class ThreadTest extends Thread { private int begin; private int end; @Override public void run() { synchronized (LOCK) { for (int i = begin; i <= end; i++) { sum += i; } System.out.println("from "+Thread.currentThread().getName()+" sum="+sum); } } public ThreadTest(int begin, int end) { this.begin = begin; this.end = end; } }}

使用countDownLatch

子線程執行完調用 countdownlatch.countdown()，主線程調用countdownlatc.await() 等待子線程執行完成，輸出結果。 注意：sum為共享變量，訪問共享變量時，用synchronized同步，代碼如下：

package thread;import java.util.concurrent.CountDownLatch;public class ThreadAddLatch { public static int sum = 0; public static Object LOCK = new Object(); public static CountDownLatch countdown = new CountDownLatch(4); public static void main(String[] args) throws InterruptedException { ThreadAddLatch add = new ThreadAddLatch(); ThreadTest thread1 = add.new ThreadTest(1, 25); ThreadTest thread2 = add.new ThreadTest(26, 50); ThreadTest thread3 = add.new ThreadTest(51, 75); ThreadTest thread4 = add.new ThreadTest(76, 100); thread1.start(); thread2.start(); thread3.start(); thread4.start(); countdown.await(); System.out.println("total result: "+sum); } class ThreadTest extends Thread { private int begin; private int end; @Override public void run() { synchronized (LOCK) { for (int i = begin; i <= end; i++) { sum += i; } System.out.println("from "+Thread.currentThread().getName()+" sum="+sum); } countdown.countDown(); } public ThreadTest(int begin, int end) { this.begin = begin; this.end = end; } }}

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關鍵字: 線程 多線程 技術