06.20 如何学习多线程作加法计算

问题：多线程计算1+2+…+100，如：起四个线程，分别计算1+2+..25, 26+27+…+50, 51+52+…+75, 76+77+…100, 最后将总和相关，输出应为5050

解决方法：

• 依次调用thread.join()，主线程输出结果。注意：sum为共享变量，访问共享变量时，用synchronized同步
• 使用countDownLatch, 子线程执行完调用 countdownlatch.countdown()，主线程调用countdownlatc.await() 等待子线程执行完成，输出结果。 注意：sum为共享变量，访问共享变量时，用synchronized同步
• 使用cyclicbarrier, 子线程执行完调用 cyclicbarrier.await(), 最后都到达barrier时，输出结果。注意：sum为共享变量，访问共享变量时，用synchronized同步
• 通过线程池管理线程，用Future取得各子线程执行结果，最后将结果相加。

使用thread.join

依次调用thread.join()，主线程输出结果。注意：sum为共享变量，访问共享变量时，用synchronized同步。代码如下：

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

子线程执行完调用 countdownlatch.countdown()，主线程调用countdownlatc.await() 等待子线程执行完成，输出结果。 注意：sum为共享变量，访问共享变量时，用synchronized同步，代码如下：

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;
 }
 }
}