Java 同步工具类

循环屏障 CyclicBarrier 精确控制多个线程同一时刻并发执行。

static CyclicBarrier barrier = new CyclicBarrier(4, new Runnable() {
 public void run() {
 System.out.println("跑...");
 }
});
static AtomicInteger number = new AtomicInteger();
static Runnable runner = new Runnable() {
 public void run() {
 int num = number.addAndGet(1);
 System.out.println(num+" 号选手准备完毕");
 try {barrier.await();} catch (Exception e) {}
 System.out.println(num+" 号选手奔跑...");
 }
};
public static void main( String[] args ) throws Exception{
 System.out.println("各就各位，预备...");
 for(int i = 0; i<3; i++)
 new Thread(runner).start();
 barrier.await();
}

倒数闸门 CountDownLatch

在多线程环境下，它允许一个或多个线程一直等待，直到其他线程的操作执行完后再执行。通过CountDownLatch 可以控制某个线程的执行时机。

Main 线程等待5个线程执行完毕后，最后打印

static CountDownLatch latch = new CountDownLatch(3);
public static void main( String[] args ) throws Exception{
 for(int i = 0; i<3;i++)
 new Thread(new Runnable() {
 public void run() {
 try {Thread.sleep(1000);} catch (Exception e) {}
 System.out.println(Thread.currentThread().getName()+" 计算完毕");
 latch.countDown();
 }
 }).start();
 latch.await();
 System.out.println("main 统计结果");
}

信号量 Semaphore

限制并发量，保护一个重要的（代码）部分防止一次超过 N 个线程进入。

public Semaphore(int permits):设置最大阈值
public void acquire() : 获取一个值
public void release() : 释放一个值
public void acquire(int permits):获取permits个值
public void release(int permits):释放permits个值

// 只允许一个线程同时进入
Semaphore semaphore = new Semaphore(1);
//critical section
semaphore.acquire();
...
semaphore.release();


// 允许3个线程同时进入
Semaphore semaphore = new Semaphore(10);
//critical section
semaphore.acquire(3);
...
semaphore.release(3);

对象交换器 Exchanger

Exchanger 可以让两个线程互相交换对象。

static Exchanger exchanger = new Exchanger();
public static void main(String[] arg) throws Exception {
new Thread(new Runnable() {
 public void run() {
try {
 String receive =(String)exchanger.exchange("A");
 printf(System.currentTimeMillis()+":"+Thread.currentThread().getName()+" send: A receive:"+receive);
 } catch (Exception e) {}
} }).start();
 Thread.sleep(5000);
 String r = (String) exchanger.exchange("B");
 printf(System.currentTimeMillis()+":"+Thread.currentThread().getName()+" send: B receive:"+r);
}