1. CyclicBarrier 定义
CyclicBarrier 内部是通过 ReeantrantLock, Condition 以及计数器count, 来控制线程的执行; 当所有线程都到达统一的地方再执行接下来的代码.
特点:
1. CyclicBarrier 区别于 CountDownLatch 是可以重用
2. 用于CyclicBarrier的线程其中有一个被中断或等待超时, 都会造成, barrier broken, 并且重置初始的值 generation
先看一个简单的 demo
import org.apache.log4j.Logger;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
/**
* Created by xjk on 2016/5/9.
*/
public class TestCyclicBarrier {
private static final Logger logger = Logger.getLogger(TestCyclicBarrier.class);
private static final int THREAD_NUM = 5;
public static void main(String[] args) {
CyclicBarrier cb = new CyclicBarrier(THREAD_NUM, new Runnable() {
public void run() {
logger.info("Inside Barrier");
}
});
List<Thread> threads = new ArrayList<>();
for(int i = 0; i < THREAD_NUM; i++){
Thread thread = new Thread(new WorkerThread(cb));
threads.add(thread);
thread.start();
}
// wait until done
for(Thread thread : threads){
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
logger.info("All Thread done()");
}
public static class WorkerThread implements Runnable{
CyclicBarrier barrier;
public WorkerThread(CyclicBarrier barrier) {
this.barrier = barrier;
}
public void run() {
try {
logger.info("Working's waiting");
// 线程在这里等待, 直到所有线程都到达barrier
barrier.await();
logger.info("Thread ID:" + Thread.currentThread().getId() + " Working");
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
}
执行结果:
[2017-02-15 14:12:39,506] INFO Thread-0 (TestCyclicBarrier.java:57) - Working's waiting
[2017-02-15 14:12:39,506] INFO Thread-3 (TestCyclicBarrier.java:57) - Working's waiting
[2017-02-15 14:12:39,506] INFO Thread-1 (TestCyclicBarrier.java:57) - Working's waiting
[2017-02-15 14:12:39,506] INFO Thread-2 (TestCyclicBarrier.java:57) - Working's waiting
[2017-02-15 14:12:39,506] INFO Thread-4 (TestCyclicBarrier.java:57) - Working's waiting
[2017-02-15 14:12:39,509] INFO Thread-4 (TestCyclicBarrier.java:23) - Inside Barrier
[2017-02-15 14:12:39,510] INFO Thread-4 (TestCyclicBarrier.java:60) - Thread ID:15 Working
[2017-02-15 14:12:39,510] INFO Thread-0 (TestCyclicBarrier.java:60) - Thread ID:11 Working
[2017-02-15 14:12:39,510] INFO Thread-3 (TestCyclicBarrier.java:60) - Thread ID:14 Working
[2017-02-15 14:12:39,511] INFO Thread-2 (TestCyclicBarrier.java:60) - Thread ID:13 Working
[2017-02-15 14:12:39,510] INFO Thread-1 (TestCyclicBarrier.java:60) - Thread ID:12 Working
[2017-02-15 14:12:39,512] INFO main (TestCyclicBarrier.java:42) - All Thread done()
执行步骤:
(1) 一共有5个线程要求它们都到达 barrier.await() 才能继续向下执行
(2) 前4个线程调用 barrier.await() 时其实时在内部统一调用 Reeantrant.lock()获取 lock, 然后再调用 Condition.await() 将lock释放, 等待唤醒
(3) 第五个线程到达 barrier.await() 处, 先调用 Reeantrant.lock() 然后发现自己是最后一个线程, 则直接调用 Condition.signalAll() 唤醒其他线程, 最后自己释放 lock
(4) 其他4个线程被 signal 了都争相获取 lock, 最后又释放
2. CyclicBarrier 构造函数
下面两个构造函数的主要区别在于 command, 用于当所有线程都到达 barrier 时执行的
/**
* 指定 barrierCommand 的构造 CyclicBarrier
*/
public CyclicBarrier(int parties, Runnable barrierCommand) {
if(parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierCommand;
}
/**
* 构造 CyclicBarrier
*/
public CyclicBarrier(int parties){
this(parties, null);
}
3. CyclicBarrier 主要属性
private static class Generation{
boolean broken = false;
}
/** The lock for guarding barrier entry */
/** 全局的重入 lock */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
/** 控制线程等待 */
private final Condition trip = lock.newCondition();
/** The number of parties */
/** 参与到这次 barrier 的参与者个数 */
private final int parties;
/** The command to run when tripped */
/** 到达 barrier 时执行的command */
private final Runnable barrierCommand;
/** The current generation */
/** 初始化 generation */
private Generation generation = new Generation();
4. CyclicBarrier 生成 generation 方法
这是在 一个 barrier 完成后, 重新初始化值
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
/** 生成下一个 generation */
private void nextGeneration(){
// signal completion of last generation
// 唤醒所有等待的线程来获取 AQS 的state的值
trip.signalAll();
// set up next generation
// 重新赋值计算器
count = parties;
// 重新初始化 generation
generation = new Generation();
}
5. CyclicBarrier breakBarrier 方法
breakBarrier 主要用于等待的线程当被中断, 或等待超时执行
/**
* Sets current barrier generation as broken and wakes up everyone
* Called only while holding lock
*/
/** 当某个线程被中断 / 等待超时 则将 broken = true, 并且唤醒所有等待中的线程 */
private void breakBarrier(){
generation.broken = true;
count = parties;
trip.signalAll();
}
6. CyclicBarrier 主方法 awaitXX
await 方法主要用于等待获取, 具体看下面的 comment
/**
* 进行等待所有线程到达 barrier
* 除非: 其中一个线程被 inetrrupt
*/
public int await() throws InterruptedException, BrokenBarrierException{
try{
return dowait(false, 0L);
}catch (TimeoutException toe){
throw new Error(toe); // cannot happen
}
}
/**
* 进行等待所有线程到达 barrier
* 除非: 等待超时
*/
public int await(long timeout, TimeUnit unit) throws Exception{
return dowait(true, unit.toNanos(timeout));
}
/**
* Main barrier code, covering the various policies
*/
/**
* CyclicBarrier 的核心方法, 主要是所有线程都获取一个 ReeantrantLock 来控制
*/
private int dowait(boolean timed, long nanos)throws InterruptedException, BrokenBarrierException, TimeoutException{
final ReentrantLock lock = this.lock;
lock.lock(); // 1. 获取 ReentrantLock
try{
final Generation g = generation;
if(g.broken){ // 2. 判断 generation 是否已经 broken
throw new BrokenBarrierException();
}
if(Thread.interrupted()){ // 3. 判断线程是否中断, 中断后就 breakBarrier
breakBarrier();
throw new InterruptedException();
}
int index = --count; // 4. 更新已经到达 barrier 的线程数
if(index == 0){ // triped // 5. index == 0 说明所有线程到达了 barrier
boolean ranAction = false;
try{
final Runnable command = barrierCommand;
if(command != null){ // 6. 最后一个线程到达 barrier, 执行 command
command.run();
}
ranAction = true;
nextGeneration(); // 7. 更新 generation
return 0;
}finally {
if(!ranAction){
breakBarrier();
}
}
}
// loop until tripped, broken, interrupted, or timed out
for(;;){
try{
if(!timed){
trip.await(); // 8. 没有进行 timeout 的 await
}else if(nanos > 0L){
nanos = trip.awaitNanos(nanos); // 9. 进行 timeout 方式的等待
}
}catch (InterruptedException e){
if(g == generation && !g.broken){ // 10. 等待的过程中线程被中断, 则直接唤醒所有等待的 线程, 重置 broken 的值
breakBarrier();
throw e;
}else{
/**
* We're about to finish waiting even if we had not
* been interrupted, so this interrupt is deemed to
* "belong" to subsequent execution
*/
/**
* 情况
* 1. await 抛 InterruptedException && g != generation
* 所有线程都到达 barrier(这是会更新 generation), 并且进行唤醒所有的线程; 但这时 当前线程被中断了
* 没关系, 当前线程还是能获取 lock, 但是为了让外面的程序知道自己被中断过, 所以自己中断一下
* 2. await 抛 InterruptedException && g == generation && g.broken = true
* 其他线程触发了 barrier broken, 导致 g.broken = true, 并且进行 signalALL(), 但就在这时
* 当前的线程也被 中断, 但是为了让外面的程序知道自己被中断过, 所以自己中断一下
*
*/
Thread.currentThread().interrupt();
}
}
if(g.broken){ // 11. barrier broken 直接抛异常
throw new BrokenBarrierException();
}
if(g != generation){ // 12. 所有线程到达 barrier 直接返回
return index;
}
if(timed && nanos <= 0L){ // 13. 等待超时直接抛异常, 重置 generation
breakBarrier();
throw new TimeoutException();
}
}
}finally {
lock.unlock(); // 14. 调用 awaitXX 获取lock后进行释放lock
}
}
7. CyclicBarrier 一般方法
/**
* 判断 barrier 是否 broken = true
*/
public boolean isBroken(){
final ReentrantLock lock = this.lock;
lock.lock();
try{
return generation.broken;
}finally {
lock.unlock();
}
}
// 重置 barrier
public void reset(){
final ReentrantLock lock = this.lock;
lock.lock();
try{
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
}finally {
lock.unlock();
}
}
/**
* 获取等待中的线程
*/
public int getNumberWaiting(){
final ReentrantLock lock = this.lock;
lock.lock();
try{
return parties - count;
}finally {
lock.unlock();
}
}
8. 总结
CyclicBarrier 主要用 ReeantrantLock 与 Condition 来控制线程资源的获取, 在理解 CyclicBarrier时, 首先需要理解 ReentrantLock, Condition.
参考:
Java 8 源码分析 Condition
Java 8 源码分析 ReentrantLock
Java多线程之JUC包：CyclicBarrier源码学习笔记
