AQS（AbstractQueueSynchronizer），抽象队列同步器，是jdk提供的一种用来自定义实现同步组件的工具，java并发中的ReentrantLock/CountDownLatch/Semaphore也都是继承了它而实现。
现在陈达想要用它来实现一个自己的同步器，功能是限制同一时间内只有2个线程才能获取锁。
总的思路是同步组件QycLock实现Lock接口，实现加锁lock与解锁unlock。QycLock内部集成了一个继承自AQS的自定义同步器QycSync，主要的同步逻辑均在QynSync上，QycLcok仅是个包装。
QycSync继承自AQS，便是拥有了同步状态和同步队列，把初始状态值设为2，一个线程获取同步状态后，用CAS的方式让状态值-1，释放同步状态后，用CAS的方式让状态值+1。
QycSync代码如下：
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| package com.iloveqyc.service.test.lock;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
public class QycLock implements Lock {
private QycSync qycSync = new QycSync(2);
@Override
public void lock() {
qycSync.acquireShared(1);
}
@Override
public void lockInterruptibly() throws InterruptedException {
}
@Override
public boolean tryLock() {
return false;
}
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return false;
}
@Override
public void unlock() {
qycSync.releaseShared(1);
}
@Override
public Condition newCondition() {
return null;
}
static class QycSync extends AbstractQueuedSynchronizer {
QycSync(int count) {
if (count <= 0) {
throw new RuntimeException();
}
setState(count);
}
@Override
protected int tryAcquireShared(int arg) {
for (;;) {
int currentState = getState();
int newState = currentState - 1;
if (newState < 0) {
System.out.println(Thread.currentThread().getName() + "到达时同步状态已经小于0，将会在同步队列中调用sun.misc.Unsafe.park，进入等待状态");
return newState;
}
boolean isSuccess = compareAndSetState(currentState, newState);
if (!isSuccess) {
System.out.println(Thread.currentThread().getName() + "尝试获取锁失败, 同步状态currentState预期为" + currentState + "， 实际不是");
continue;
}
return newState;
}
}
@Override
protected boolean tryReleaseShared(int arg) {
int currentState = getState();
int newState = currentState + 1;
return compareAndSetState(currentState, newState);
}
}
}
|
QycThread代码如下：
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| package com.iloveqyc.service.test.lock;
public class QycThread implements Runnable {
private QycLock qycLock;
public QycThread(QycLock qycLock) {
this.qycLock = qycLock;
}
@Override
public void run() {
qycLock.lock();
System.out.println(Thread.currentThread().getName() + "成功获取了锁！即将带着锁睡眠60s");
try {
Thread.sleep(60000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "睡眠60s结束，即将释放锁");
qycLock.unlock();
}
}
|
main方法如下：
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| package com.iloveqyc.service.test.lock;
public class QycTest {
public static void main(String[] args) {
QycLock lock = new QycLock();
for (int i = 0; i < 10; i++) {
Thread t = new Thread(new QycThread(lock));
t.setName(String.valueOf(i));
t.start();
}
}
}
|