JAVA 多线程同步工具类总结

这两天看了《JAVA 并发编程实战》一书，内容很不错，关于同步常用的几个工具类总结如下：

CountDownLatch 闭锁：可以延迟线程的进度，直到锁到达终止状态。闭锁的作用相当于一扇门，在锁到达终止状态之前这扇门一直是关闭的。当锁到达终止状态时，允许所有线程通过。CountDownLatch 有一个初始值，通过调用 countDown 可以减少该值，一直到 0 时到达终止状态。

FutureTask 用于执行一个可返回结果的长任务，任务在单独的线程中执行，其他线程可以用 get 方法取任务结果，如果任务尚未完成，线程在 get 上阻塞。

Semaphore 用于控制同时访问某资源，或同时执行某操作的线程数目。信号量有一个初始值即可以分配的信号量总数目。线程任务开始前先调用 acquire 取得信号量，任务结束后调用 release 释放信号量。在 acquire 是如果没有可用信号量，线程将阻塞在 acquire 上，直到其他线程释放一个信号量。

CyclicBarrier 栅栏用于多个线程多次迭代时进行同步，在一轮任务中，任何线程完成任务后都在 barrier 上等待，直到所有其他线程也完成任务，然后一起释放，同时进入下一轮迭代。

CountDownLatch 的例子：

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;

public class DemoOfLatch {

// 利用闭锁 CountDownLatch 控制主线程和子线程的同步
public static void main(String[] args) {

int numberOfThread = 5;
final CountDownLatch startLatch = new CountDownLatch(1);				// 用于控制子线程开始
final CountDownLatch stopLatch = new CountDownLatch(numberOfThread);	// 用于子线程计数
final AtomicInteger count = new AtomicInteger(0);						// 用于分配子线程唯一标识

System.out.println("Main thread start...");

for ( int i=0; i<numberOfThread; i++ ) {
Thread thread = new Thread( new Runnable() {

@Override
public void run() {
int tid = count.getAndIncrement();
try {
// 等代主线程打开启动信号
startLatch.await();
System.out.printf("Thread %d started...%n", tid);
int duration = (int)(Math.random() * 5000);
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
Thread.currentThread().interrupt();
} finally {
System.out.printf("Thread %d stoped...%n", tid);
// 线程终止前减少线程计数
stopLatch.countDown();
}
}

});
thread.start();
}

// 在放行子线程之前做点什么别的事情
System.out.println("Main thread do preparation work for child threads...");
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}

// 打开闭锁放行所有子线程
System.out.println("Main thread let child threads go...");
startLatch.countDown();

try {
// 等待子线程计数降为 0 即所有子线程执行完毕
System.out.println("Main thread wait for all child threads...");
stopLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}

System.out.println("Main thread exit...");
}

}

FutureTask 的例子：

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

public class DemoOfFutureTask {

public static void main(String[] args) {

// 创建一个 Future Task 用于并发执行长任务
final FutureTask<Movie> future = new FutureTask<Movie>( new Callable<Movie>() {

@Override
public Movie call() throws Exception {
System.out.println("Future task started...");
Thread.sleep(5000);
System.out.println("Future task finished...");
return new Movie("2012","Unknown");
}

});

// 在子线程中启动任务
Thread thread = new Thread(future);
thread.start();

// 主线程干点别的事情
System.out.println("Now let's do sth eles...");
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}

// 主线程开始取结果
System.out.println("Now wait for result of future task...");
try {
Movie res = future.get();
System.out.printf("Result from task is name=%s, actor=%s", res.name, res.actor);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}

}

public static class Movie {
final public String name;
final public String actor;
public Movie(String name, String actor) {
this.name = name;
this.actor = actor;
}
}

}

Semaphore 的例子：

import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicInteger;

public class DemoOfSemaphore {

/**
* @param args
*/
public static void main(String[] args) {

final int numOfThread = 5;
final AtomicInteger count = new AtomicInteger(0);		// 用于分配唯一线程标识
final Semaphore semaphore = new Semaphore(numOfThread); // 用于控制并发线程数目

for (int i = 0; i < 10; i++) {
Thread thread = new Thread(new Runnable() {

@Override
public void run() {
int tid = count.getAndIncrement();
try {
// 等待直到取得信号量
System.out.printf("Thread %d wait on semaphore...%n", tid);
semaphore.acquire();
// 取得信号量之后做点事情
System.out.printf("Thread %d get semaphore...%n", tid);
int duration = (int)(Math.random() * 5000);
Thread.sleep(duration);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 做完后释放信号量
System.out.printf("Thread %d release semaphore...%n", tid);
semaphore.release();
}

}

});
thread.start();
}

}

}

CyclicBarrier 的例子：

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class DemoOfBarrier {

public static void main(String[] args) {

final int numOfThread = 2;
final int numOfIteration = 2;

// 创建一个用于线程同步的 Barrier 对象
final CyclicBarrier barrier = new CyclicBarrier(numOfThread,
new Runnable() {

// 当所有线程到达 Barrier 后会执行这个任务
// 任务在第一个 到达 Barrier 的线程中执行
@Override
public void run() {
long tid = Thread.currentThread().getId();
// 当所有线程完成一轮迭代之后做点清除/准备/提交工作
System.out.printf("[%d] - All threads arrived barrier...%n", tid);
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.printf("[%d] - Clear work done...%n", tid);
}

});

// 创建并启动多个线程，他们在 Barrier 上同步
for (int i = 0; i < numOfThread; i++) {
Thread thread = new Thread(new Runnable() {

@Override
public void run() {
long tid = Thread.currentThread().getId();
for ( int k=0; k<numOfIteration; k++ ) {
try {
// 线程进行一轮迭代，做点事情
System.out.printf("Thread %d start its work...%n", tid);
long duration = (int)(Math.random()*5000);
Thread.sleep(duration);
// 做完迭代后等待其他线程完成迭代
System.out.printf("Thread %d wait on barrier...%n", tid);
int num = barrier.await();
// 显示完成的顺序
System.out.printf("Thread %d pass barrier with order=%d...%n", tid, num);
} catch (InterruptedException e) {
e.printStackTrace();
Thread.currentThread().interrupt();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}

});
thread.start();
}

}

}