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Update Java 并发.md #1

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Expand Up @@ -726,7 +726,7 @@ after

[Java SE 9 Enum Thread.State](https://docs.oracle.com/javase/9/docs/api/java/lang/Thread.State.html)

## 七、J.U.C - AQS
## 七、J.U.C - AQS(AbstractQueuedSynchornizer)

java.util.concurrent（J.U.C）大大提高了并发性能，AQS 被认为是 J.U.C 的核心。

Expand Down Expand Up @@ -814,6 +814,13 @@ public class CyclicBarrierExample {
before..before..before..before..before..before..before..before..before..before..after..after..after..after..after..after..after..after..after..after..
```

### Exchanger
Java中的Exchanger类是一个同步工具类，用于在两个线程之间交换数据。它提供了一个exchange()方法，当两个线程都调用该方法时，它们会被阻塞，直到彼此都调用了该方法，然后交换数据并返回。

Exchanger类的实现原理是基于AQS（AbstractQueuedSynchronizer）同步器。当一个线程调用exchange()方法时，它会尝试获取同步器的锁，如果同步器的状态为0，则表示没有其他线程在等待交换数据，该线程会将自己加入到同步器的等待队列中，并阻塞自己。当另一个线程也调用了exchange()方法时，它会尝试获取同步器的锁，如果同步器的状态为1，则表示有一个线程在等待交换数据，该线程会将自己从等待队列中取出，并与另一个线程交换数据，然后释放同步器的锁，使得两个线程都可以继续执行。

需要注意的是，Exchanger类只能用于两个线程之间交换数据，如果需要多个线程之间交换数据，可以使用CyclicBarrier或CountDownLatch等其他同步工具类。

### Semaphore

Semaphore 类似于操作系统中的信号量，可以控制对互斥资源的访问线程数。
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