AQS机制详解

案例一

public class AqsThread extends Thread {private Lock lock;public AqsThread(String name, Lock lock) {super(name);this.lock = lock;}@Overridepublic void run() {lock.lock();try {System.out.println(Thread.currentThread().getName() + "running");} finally {lock.unlock();}}
}

public class AqsDemo {public static void main(String[] args) {ReentrantLock lock = new ReentrantLock();AqsThread t1 = new AqsThread("t1", lock);AqsThread t2 = new AqsThread("t2", lock);t1.start();t2.start();}
}

执行案例的运行结果.(其中之一)

线程t1先执行lock操作,获取锁.然后线程t2执行lock操作.然后t1进行unlock操作,然后t2获取锁成功,接着执行unlock操作.

t1线程调用lock.lock方法，方法调用顺序.

t2线程调用lock.lock方法，其方法调用顺序.

最后的结果是被阻塞.源码如下.

t1线程调用lock.unlock，其方法调用顺序.

t1线程中调用lock.unlock后，经过一系列的调用，最终的状态是释放了许可，因为调用了LockSupport.unpark。这时，t2线程就可以继续运行了。此时，会继续恢复t2线程运行环境，继续执行LockSupport.park后面的语句.

进一步调用.

t2线程调用lock.unlock，其方法调用顺序

t2线程执行lock.unlock后，最终达到的状态还是与之前的状态一样.

案例二

public class DepotDemo {private int size;private int capacity;private Lock lock;private Condition fullCondition;private Condition emptyCondition;public DepotDemo(int capacity) {this.capacity = capacity;lock = new ReentrantLock();fullCondition = lock.newCondition();emptyCondition = lock.newCondition();}public void produce(int no) {int left = no;lock.lock();try {while (left > 0) {while (size >= capacity)  {System.out.println(Thread.currentThread().getName() + " before await");fullCondition.await();System.out.println(Thread.currentThread().getName() + " after await");}int inc = (left + size) > capacity ? (capacity - size) : left;left -= inc;size += inc;System.out.println("produce = " + inc + ", size = " + size);emptyCondition.signal();}} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}public void consume(int no) {int left = no;lock.lock();try {while (left > 0) {while (size <= 0) {System.out.println(Thread.currentThread() + " before await");emptyCondition.await();System.out.println(Thread.currentThread() + " after await");}int dec = (size - left) > 0 ? left : size;left -= dec;size -= dec;System.out.println("consume = " + dec + ", size = " + size);fullCondition.signal();}} catch (InterruptedException e) {e.printStackTrace();} finally {lock.unlock();}}
}

public class ReentrantLockTest {static class Consumer {private DepotDemo depot;public Consumer(DepotDemo depot) {this.depot = depot;}public void consume(int no) {new Thread(new Runnable() {@Overridepublic void run() {depot.consume(no);}}, no + " consume thread").start();}}static class Producer {private DepotDemo depot;public Producer(DepotDemo depot) {this.depot = depot;}public void produce(int no) {new Thread(new Runnable() {@Overridepublic void run() {depot.produce(no);}}, no + " produce thread").start();}}}class ReentrantLockDemo {public static void main(String[] args) throws InterruptedException {DepotDemo depot = new DepotDemo(500);new ReentrantLockTest.Producer(depot).produce(500);new ReentrantLockTest.Producer(depot).produce(200);new ReentrantLockTest.Consumer(depot).consume(500);new ReentrantLockTest.Consumer(depot).consume(200);}
}