多线程-CountDownLatch，CyclicBarrier，Semaphore，Exchanger，Phaser
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CountDownLatch
一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。用给定的计数初始化CountDownLatch。调用countDown()计数减一，当计数到达零之前await()方法会一直阻塞，计数无法被重置。
public class CountDownLatch {
private final Sync sync;
public CountDownLatch(int count);
public void countDown() {
sync.releaseShared(1);
}
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
}
CountDownLatch中主要有countDown()和await()方法。
countDown()递减计数，如果计数达到零，则是否所有等待的线程。
1. 如果当前计数大于零，则计数减一；
2. 如果减一之后计数为零，则重新调度所有等待该计数为零的线程；
3. 如果计数已经为零，则不发生任何操作；
await()使当前线程在计数为零之前一直阻塞，除非线程被中断或超出指定的等待时间；
如果计数为零，则立刻返回true
在进入此方法时，当前线程已经设置了中断状态或在等待时被中断，则抛出InterruptedException异常，并且清除当前线程的中断状态。如果超出了指定等待时间，则返回false，如果该时间小于等于零，则此方法根本不会等待。
package org.github.lujiango;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class Test16 {
public static void main(String[] args) throws InterruptedException {
final CountDownLatch begin = new CountDownLatch(1);
final CountDownLatch end = new CountDownLatch(10);
final ExecutorService exec = Executors.newFixedThreadPool(10);
for (int i = 0; i < 10; i++) {
final int no = i + 1;
Runnable run = new Runnable() {
@Override
public void run() {
try {
begin.await();
TimeUnit.MILLISECONDS.sleep((long) (Math.random() * 10000));
System.out.println("No." + no + " arrived");
} catch (Exception e) {
} finally {
end.countDown();
}
}
};
exec.submit(run);
}
System.out.println("Game start");
begin.countDown();
end.await();
System.out.println("Game over");
exec.shutdown();
}
}
CyclicBarrier
一个同步辅助类，它允许一组线程互相等待，直到到达某个公共屏障点。在涉及一组固定大小的线程的程序中，这些线程必须不时的互相等待。
package org.github.lujiango;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class Test16 {
public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
final CyclicBarrier end = new CyclicBarrier(10);
final ExecutorService exec = Executors.newFixedThreadPool(10);
System.out.println("Game start");
for (int i = 0; i < 10; i++) {
final int no = i + 1;
Runnable run = new Runnable() {
@Override
public void run() {
try {
end.await();
TimeUnit.MILLISECONDS.sleep((long) (Math.random() * 10000));
System.out.println("No." + no + " arrived");
} catch (Exception e) {
} finally {
}
}
};
exec.submit(run);
}
System.out.println("Game over");
exec.shutdown();
}
}
需要所有的子任务都完成时，才执行主任务，这个时候可以选择使用CyclicBarrier。
Semaphore
一个计数信号量，信号量维护了一个许可集，在许可可用之前会阻塞每一个acquire()，然后获取该许可。每个release()释放许可，从而可能释放一个正在阻塞的获取者。
Semaphore只对可用许可的号码进行计数，并采取相应的行动，拿到信号的线程可以进入代码，否则就等待。
package org.github.lujiango;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
public class Test17 {
public static void main(String[] args) {
ExecutorService exec = Executors.newCachedThreadPool();
final Semaphore semp = new Semaphore(5);
for (int i = 0; i < 20; i++) {
final int no = i;
Runnable run = new Runnable() {
@Override
public void run() {
try {
semp.acquire();
System.out.println("Accessing: " + no);
TimeUnit.MILLISECONDS.sleep((long) (Math.random() * 10000));
} catch (Exception e) {
} finally {
semp.release();
}
}
};
exec.submit(run);
}
exec.shutdown();
}
}
Exchanger
Exchanger可以在两个线程之间交换数据，只能在两个线程，不支持更多的线程之间互换数据。
当线程A调用Exchanger对象的exchage()方法后，会阻塞；直到B线程也调用exchange()方法，然后线程以安全的方式交换数据，之后A和B线程继续执行。
package org.github.lujiango;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Exchanger;
public class Test18 {
public static void main(String[] args) {
Exchanger<List<Integer>> ex = new Exchanger<List<Integer>>();
new A(ex).start();
new B(ex).start();
}
}
class A extends Thread {
List<Integer> list = new ArrayList<Integer>();
Exchanger<List<Integer>> ex;
public A(Exchanger<List<Integer>> ex) {
this.ex = ex;
}
@Override
public void run() {
Random random = new Random();
for (int i = 0; i < 10; i++) {
list.clear();
list.add(random.nextInt(10));
list.add(random.nextInt(10));
list.add(random.nextInt(10));
try {
list = ex.exchange(list);
} catch (Exception e) {
}
}
}
}
class B extends Thread {
List<Integer> list = new ArrayList<Integer>();
Exchanger<List<Integer>> ex;
public B(Exchanger<List<Integer>> ex) {
this.ex = ex;
}
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
list = ex.exchange(list);
} catch (Exception e) {
}
System.out.println(list);
}
}
}
Phaser
Phaser是一个灵活的线程同步工具，它包含了CountDownLatch和CyclicBarrier的相关功能。
CountDownLatch的countDown()和await()可以通过Phaser的arrive()和awaitAdvance(int n)代替
而CyclicBarrier的await可以使用Phaser的arriveAndAwaitAdvance()方法代替
用Phaser代替CountDownLatch：
package org.github.lujiango;
import java.util.concurrent.Phaser;
import java.util.concurrent.TimeUnit;
public class Test19 {
public static void main(String[] args) throws InterruptedException {
final Phaser latch = new Phaser(10);
for (int i = 1; i <= 10; i++) {
final int id = i;
Thread t = new Thread(new Runnable() {
@Override
public void run() {
try {
TimeUnit.SECONDS.sleep((long) (Math.random() * 10));
System.out.println("thread: " + id + " is running");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
latch.arrive();
}
}
});
t.start();
}
latch.awaitAdvance(latch.getPhase());
System.out.println("all thread has run");
}
}
package org.github.lujiango;
import java.util.concurrent.Phaser;
import java.util.concurrent.TimeUnit;
public class Test19 {
public static void main(String[] args) throws InterruptedException {
final Phaser latch = new Phaser(10);
for (int i = 1; i <= 10; i++) {
final int id = i;
Thread t = new Thread(new Runnable() {
@Override
public void run() {
try {
TimeUnit.SECONDS.sleep((long) (Math.random() * 10));
latch.arriveAndAwaitAdvance(); // 所有线程都执行到这里，才会继续执行，否则全部阻塞
System.out.println("thread: " + id + " is running");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
latch.arrive();
}
}
});
t.start();
}
}
}
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