手把手教你构建源码级组件——Java指定共享线程数目的共享锁
Posted 杨 戬
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文章目录
构造同步组件的步骤
之前的学习中我们学习了AQS的原理，其中有许多构建锁与同步器的相关概念我们需要了解到：
- 首先同步器是实现锁的关键，在锁的实现中聚合同步器，利用同步器实现锁的语义；
- 锁是面向使用者的，提供锁交互的实现；
- 同步器是面向锁的实现者，简化了锁的实现方式，屏蔽了同步状态管理、线程排队、等待/唤醒等底层操作。
从代码层面，同步器是基于模板模式实现的，可以通过AQS可重写的方法进行子类具体功能实现：
例如下面是AQS中tryAcquire模板方法的源码（如果子类没实现会怕抛出异常）
/**
* 模板方法：
* protected关键字
* 没有任何实现
* @param arg
* @return
*/
protected boolean tryAcquire(int arg)
throw new UnsupportedOperationException();
那么我们在构建同步组件的时候也就是需要实现以下几步：
1. 定义内部类Syn
随后将同步器组合在自定义同步组件的实现中，即定义内部类Syn继承AQS
public class XXX implements Lock
public class Sync extends AbstractQueuedSynchronizer
2. 继承同步器，重写指定方法
之后在Syn中重写AQS方法，根据同步器需求如下挑选实现不同方法
- tryAcquire(int arg)：独占式获取同步状态；
- tryRelease(int arg)：独占式释放同步状态；
- tryAcquireShared(int arg)：共享式获取同步状态，返回大于0的值表示获取成功，否则失败
- tryReleaseShared(int arg)：共享式释放锁
- isHeldExclusively()：当前线程是否在独占模式下被线程占用，一般该方法表示是否被当前线程占用
例如不可重入同步器：
public class XXX implements Lock
public class Sync extends AbstractQueuedSynchronizer
@Override
protected boolean tryAcquire(int arg)
final Thread current = Thread.currentThread();
if (compareAndSetState(0, 1))
// 获取成功之后，当前线程是该锁的持有者，不需要再可重入数
setExclusiveOwnerThread(current);
return true;
return false;
@Override
protected boolean tryRelease(int arg)
if (getState() == 0)
throw new IllegalMonitorStateException();
setExclusiveOwnerThread(null);
setState(0);
return true;
@Override
protected boolean isHeldExclusively()
return getState() == 1;
// 返回Condition，每个Condition都包含了一个队列
Condition newCondition()
return new ConditionObject();
3. 调用同步器方法
最后调用同步器提供的模板方法，即同步组件类实现Lock方法之后，在lock/unlock方法中调用内部类Syn的方法acquire(int arg)等方法
public class XXX implements Lock
........
private final Sync sync = new Sync();
@Override
public void lock()
sync.acquire(1);
@Override
public void unlock()
sync.release(1);
........
具体请看下面的实验部分
具体请看下面的实验部分
指定共享线程数目的共享锁实现
代码实现
package com.yyl.threadtest.utils;
import java.util.Collection;
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 MyShareLock implements Lock
// 可以看到共享等待队列中的线程
public Collection<Thread> getSharedQueuedThreads()
return syn.getSharedQueuedThreads();
private final Syn syn = new Syn(3);
private static final class Syn extends AbstractQueuedSynchronizer
int newShareCount=0;
Syn(int shareCount)
if (shareCount <= 0)
throw new IllegalArgumentException("share count must large than zero");
// 设置初始共享同步状态
setState(shareCount);
/**
* 共享锁指定数目
* @param reduceShareCount
* @return
*/
@Override
protected int tryAcquireShared(int reduceShareCount)
for (;;)
int currentShareCount = getState();
newShareCount = currentShareCount- reduceShareCount;
if (newShareCount < 0 ||
compareAndSetState(currentShareCount,newShareCount))
// newShareCount大于等于０才说明获取锁成功
if (newShareCount >= 0)
// System.out.println(Thread.currentThread().getName()+" hold lock, current share count is "+newShareCount+", "+new Date());
// newShareCount小于0表示获取失败所以需要返回
// compareAndSetState(currentShareCount,newShareCount)为true自然表示成功需要返回
return newShareCount;
@Override
protected boolean tryReleaseShared(int returnShareCount)
for (;;)
int currentShareCount = getState();
newShareCount = currentShareCount + returnShareCount;
if (compareAndSetState(currentShareCount,newShareCount))
// System.out.println(Thread.currentThread().getName() +" release lock, current share count is "+newShareCount+", "+new Date());
return true;
protected int getShareCount()
return getState();
/**
* 调用内部同步器Syn的acquireShare方法
*/
@Override
public void lock()
syn.acquireShared(1);
/**
* 调用内部同步器Syn的releaseShared方法
*/
@Override
public void unlock()
syn.releaseShared(1);
@Override
public void lockInterruptibly() throws InterruptedException
if (Thread.interrupted())
throw new IllegalStateException();
syn.acquireInterruptibly(1);
@Override
public boolean tryLock()
return false;
@Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException
return false;
@Override
public Condition newCondition()
return null;
测试Demo
package com.yyl.threadtest.utils;
import java.util.Date;
public class ShareLockTest
public static void main(String[] args)
final MyShareLock lock = new MyShareLock();
class Worker extends Thread
@Override
public void run()
// 一直不停在获取锁
while (true)
lock.lock();
try
System.out.println(Thread.currentThread().getName() +" hold lock, "+new Date());
// System.out.println(lock.getSharedQueuedThreads());
Thread.sleep(1000);
catch (InterruptedException e)
e.printStackTrace();
finally
lock.unlock();
System.out.println(Thread.currentThread().getName() +" release lock, "+new Date());
for (int i = 0; i < 10; i++)
Worker worker = new Worker();
// 以守护进程运行，VM退出不影响运行，这里只是为了一个打印效果，去掉注释一直打印
worker.setDaemon(true);
worker.start();
// 每隔一秒换行
for (int j = 0; j < 10; j++)
try
Thread.sleep(1000);
catch (InterruptedException e)
e.printStackTrace();
System.out.println();
运行结果
Thread-0 hold lock, Thu Feb 23 12:55:28 CST 2023
Thread-1 hold lock, Thu Feb 23 12:55:28 CST 2023
Thread-2 hold lock, Thu Feb 23 12:55:28 CST 2023
Thread-0 release lock, Thu Feb 23 12:55:29 CST 2023
Thread-4 hold lock, Thu Feb 23 12:55:29 CST 2023
Thread-5 hold lock, Thu Feb 23 12:55:29 CST 2023
Thread-1 release lock, Thu Feb 23 12:55:29 CST 2023
Thread-3 hold lock, Thu Feb 23 12:55:29 CST 2023
Thread-2 release lock, Thu Feb 23 12:55:29 CST 2023
Thread-5 release lock, Thu Feb 23 12:55:31 CST 2023
Thread-7 hold lock, Thu Feb 23 12:55:31 CST 2023
Thread-3 release lock, Thu Feb 23 12:55:31 CST 2023
Thread-9 hold lock, Thu Feb 23 12:55:31 CST 2023
Thread-4 release lock, Thu Feb 23 12:55:31 CST 2023
Thread-6 hold lock, Thu Feb 23 12:55:31 CST 2023
Thread-8 hold lock, Thu Feb 23 12:55:32 CST 2023
Thread-6 release lock, Thu Feb 23 12:55:32 CST 2023
Thread-9 release lock, Thu Feb 23 12:55:32 CST 2023
Thread-7 release lock, Thu Feb 23 12:55:32 CST 2023
Thread-1 hold lock, Thu Feb 23 12:55:32 CST 2023
Thread-0 hold lock, Thu Feb 23 12:55:32 CST 2023
Thread-8 release lock, Thu Feb 23 12:55:33 CST 2023
Thread-3 hold lock, Thu Feb 23 12:55:33 CST 2023
Thread-2 hold lock, Thu Feb 23 12:55:33 CST 2023
Thread-1 release lock, Thu Feb 23 12:55:33 CST 2023
Thread-5 hold lock, Thu Feb 23 12:55:33 CST 2023
Thread-0 release lock, Thu Feb 23 12:55:33 CST 2023
Thread-5 release lock, Thu Feb 23 12:55:34 CST 2023
Thread-6 hold lock, Thu Feb 23 12:55:34 CST 2023
Thread-9 hold lock, Thu Feb 23 12:55:34 CST 2023
Thread-4 hold lock, Thu Feb 23 12:55:34 CST 2023
Thread-3 release lock, Thu Feb 23 12:55:34 CST 2023
Thread-2 release lock, Thu Feb 23 12:55:34 CST 2023
Thread-4 release lock, Thu Feb 23 12:55:35 CST 2023
Thread-7 hold lock, Thu Feb 23 12:55:35 CST 2023
Thread-8 hold lock, Thu Feb 23 12:55:35 CST 2023
Thread-9 release lock, Thu Feb 23 12:55:35 CST 2023
Thread-1 hold lock, Thu Feb 23 12:55:35 CST 2023
Thread-6 release lock, Thu Feb 23 12:55:35 CST 2023
Thread-1 release lock, Thu Feb 23 12:55:36 CST 2023
Thread-8 release lock, Thu Feb 23 12:55:36 CST 2023
Thread-3 hold lock, Thu Feb 23 12:55:36 CST 2023
Thread-7 release lock, Thu Feb 23 12:55:36 CST 2023
Thread-5 hold lock, Thu Feb 23 12:55:36 CST 2023
Thread-0 hold lock, Thu Feb 23 12:55:36 CST 2023
Thread-4 hold lock, Thu Feb 23 12:55:37 CST 2023
Thread-5 release lock, Thu Feb 23 12:55:37 CST 2023
Thread-3 release lock, Thu Feb 23 12:55:37 CST 2023
Thread-0 release lock, Thu Feb 23 12:55:37 CST 2023
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