柵欄不同於倒計時器的一點是倒計時器是一個或N個線程等待其他線程調用countdown()到指定次數後再繼續執行,而柵欄是N個線程之間互相等待,當調用await()到達指定次數後就會喚醒所有等待線程,同時還可以在到達指定數量時觸發一個定製的動作(Runnable,由最後一個調用await()方法並喚醒所有線程的那個線程執行)。另外柵欄是可以循環使用的。
柵欄的實現方式是獨佔鎖+Condition條件來實現的。
先看個Demo:
package com.pzx.test005;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierDemo {
static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:SS ");
static CyclicBarrier cyclicBarrier;
public static void main(String[] args) {
cyclicBarrier = new CyclicBarrier(5, new Runnable() {
@Override
public void run() {
System.out.println(sdf.format(new Date()) + Thread.currentThread().getName() + " do something...");
}
});
ThreadA threadA = new ThreadA();
for(int i=0; i<5; i++) {
Thread thread = new Thread(threadA, "t"+i);
thread.start();
}
}
static class ThreadA implements Runnable{
@Override
public void run() {
System.out.println(sdf.format(new Date()) + Thread.currentThread().getName() + " begin work...");
try {
System.out.println(sdf.format(new Date()) + Thread.currentThread().getName() + " await...");
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(sdf.format(new Date()) + Thread.currentThread().getName() + " continue work...");
}
}
}
執行結果:
2018-09-11 13:54:27 t0 begin work...
2018-09-11 13:54:27 t0 await...
2018-09-11 13:54:27 t4 begin work...
2018-09-11 13:54:27 t4 await...
2018-09-11 13:54:27 t1 begin work...
2018-09-11 13:54:27 t1 await...
2018-09-11 13:54:27 t3 begin work...
2018-09-11 13:54:27 t3 await...
2018-09-11 13:54:27 t2 begin work...
2018-09-11 13:54:27 t2 await...
2018-09-11 13:54:27 t2 do something...
2018-09-11 13:54:27 t2 continue work...
2018-09-11 13:54:27 t0 continue work...
2018-09-11 13:54:27 t4 continue work...
2018-09-11 13:54:27 t1 continue work...
2018-09-11 13:54:27 t3 continue work...
public class CyclicBarrier {
// 內部類,因爲柵欄可以循環使用,每使用一次都是一代,這個類可判斷是否處於同一代
private static class Generation {
boolean broken = false;
}
// 可重入鎖
private final ReentrantLock lock = new ReentrantLock();
// 創建Condition,釋放所有線程的條件
private final Condition trip = lock.newCondition();
// 柵欄中的線程總個數
private final int parties;
// 當要釋放時觸發的動作
private final Runnable barrierCommand;
// 創建當前代
private Generation generation = new Generation();
// 還剩多少線程可以達到條件,進來一個線程就-1
private int count;
// 構造函數
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
// 調用await方法阻塞,最後一個線程會喚醒所有線程
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
// 開啓獨佔鎖
lock.lock();
try {
// 獲取當前代數
final Generation g = generation;
// 如果當前代的柵欄被破壞了就拋異常
if (g.broken)
throw new BrokenBarrierException();
// 當前線程如果處於中斷狀態就清除中斷標誌,破壞柵欄,拋出被中斷的異常
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
// 一個線程進來後,count-1
int index = --count;
// 如果count已經減到0了,說明是時候喚醒所有線程往下運行了
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
// 如果構造函數中有傳入Runnable對象,就在喚醒所有線程之前執行Runnable裏面
// 的run方法
if (command != null)
command.run();
ranAction = true;
// 重置參數,進入下一代
nextGeneration();
return 0;
} finally {
// 如果上述喚醒的過程(command.run();)發生了什麼意外導致異常了,就破壞當前
// 的柵欄
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
// 循環,一直到要被喚醒了,或者柵欄被破壞了,或者線程被中斷了,或者超時了
for (;;) {
try {
// 如果沒有設置超時參數,就執行await(即當前線程阻塞,釋放上面的獨佔鎖,
// 等待被喚醒)
if (!timed)
trip.await();
// 如果設置了超時參數,就阻塞超時參數那麼長的時間
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
// 如果發生了中斷異常
// 如果還是在當前代並且當前代沒有被破壞
if (g == generation && ! g.broken) {
// 破壞柵欄,拋出異常
breakBarrier();
throw ie;
} else {
// 發生中斷
Thread.currentThread().interrupt();
}
}
// 如果當前代被破壞了,就拋出異常
if (g.broken)
throw new BrokenBarrierException();
// 如果在這個執行過程中換代了就返回index(即當前count-1值)
if (g != generation)
return index;
// 輸入的超時參數的合法性判斷,不合法就破壞當前代的柵欄
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
// 開啓下一代
private void nextGeneration() {
// 喚醒所有的等待的線程,相當於把柵欄拉開
trip.signalAll();
// 重置count和generation
count = parties;
generation = new Generation();
}
// 破壞柵欄
private void breakBarrier() {
// 當前代被破壞置true,重置count,喚醒所有的線程
generation.broken = true;
count = parties;
trip.signalAll();
}
// 重置柵欄
public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}