【java.util.concurrent】CyclicBarrier解析

一、概念

有点类似CountDownLatch类，只是CountDownLatch只触发一次事件，而CyclicBarrier可以多次重用。如果你希望多线程并行执行任务，需等待所有线程均已完成现步骤之后再进入下一步骤，直至最后所有任务完成。

二、代码示例

本人觉得《Thinking in Java》中的赛马例子非常好，能够一目了然。所以，在此就借用该书例子，如下：

class Horse implements Runnable {
private static int counter = 0;
private final int id = counter++;
private int strides = 0;
private static Random rand = new Random(47);
private static CyclicBarrier barrier;

public Horse(CyclicBarrier b) {
barrier = b;
}

public synchronized int getStrides() {
return strides;
}

public void run() {
try {
while (!Thread.interrupted()) {
synchronized (this) {
strides += rand.nextInt(3); // Produces 0, 1 or 2
}
barrier.await();
}
} catch (InterruptedException e) {
// A legitimate way to exit
} catch (BrokenBarrierException e) {
// This one we want to know about
throw new RuntimeException(e);
}
}

public String toString() {
return "Horse " + id + " ";
}

public String tracks() {
StringBuilder s = new StringBuilder();
for (int i = 0; i < getStrides(); i++)
s.append("*");
s.append(id);
return s.toString();
}
}

public class HorseRace {
static final int FINISH_LINE = 75;
private List<Horse> horses = new ArrayList<Horse>();
private ExecutorService exec = Executors.newCachedThreadPool();
private CyclicBarrier barrier;

public HorseRace(int nHorses, final int pause) {
barrier = new CyclicBarrier(nHorses, new Runnable() {
public void run() {
StringBuilder s = new StringBuilder();
for (int i = 0; i < FINISH_LINE; i++)
s.append("="); // The fence on the racetrack
System.out.println(s);
for (Horse horse : horses)
System.out.println(horse.tracks());
for (Horse horse : horses)
if (horse.getStrides() >= FINISH_LINE) {
System.out.println(horse + "won!");
exec.shutdownNow();
return;
}
try {
TimeUnit.MILLISECONDS.sleep(pause);
} catch (InterruptedException e) {
print("barrier-action sleep interrupted");
}
}
});
for (int i = 0; i < nHorses; i++) {
Horse horse = new Horse(barrier);
horses.add(horse);
exec.execute(horse);
}
}

public static void main(String[] args) {
int nHorses = 7;
int pause = 200;
if (args.length > 0) { // Optional argument
int n = new Integer(args[0]);
nHorses = n > 0 ? n : nHorses;
}
if (args.length > 1) { // Optional argument
int p = new Integer(args[1]);
pause = p > -1 ? p : pause;
}
new HorseRace(nHorses, pause);
}
}

最初结果展示：



最终结果展示：



可以明显看出，在所有的马都向前移动时，CyclicBarrier将自动调用Runnable栅栏动作，必须在栅栏处等待所有的马都完成本回合比赛。