Java中的显示锁ReentrantLock使用与原理详解

//打印奇数的线程
private static class OldRunner implements Runnable{
  private MyNumber n;

  public OldRunner(MyNumber n) {
    this.n = n;
  }

  public void run() {
    while (true){
      n.waitToOld(); //等待数据变成奇数
      System.out.println("old:" + n.getVal());
      n.increase();
      if (n.getVal()>98){
        break;
      }
    }
  }
}
//打印偶数的线程
private static class EvenRunner implements Runnable{
  private MyNumber n;

  public EvenRunner(MyNumber n) {
    this.n = n;
  }

  public void run() {
    while (true){
      n.waitToEven();      //等待数据变成偶数
      System.out.println("even:"+n.getVal());
      n.increase(); 
      if (n.getVal()>99){
        break;
      }
    }
  }
}

private static class MyNumber{
  private int val;

  public MyNumber(int val) {
    this.val = val;
  }

  public int getVal() {
    return val;
  }
  public synchronized void increase(){
    val++;
    notify(); //数据变了，唤醒另外的线程
  }
  public synchronized void waitToOld(){
    while ((val % 2)==0){
      try {
        System.out.println("i am "+Thread.currentThread().getName()+" ,but now is even:"+val+",so wait");
        wait(); //只要是偶数，一直等待
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }
  public synchronized void waitToEven(){
    while ((val % 2)!=0){
      try {
        System.out.println("i am "+Thread.currentThread().getName()+" ,but now old:"+val+",so wait");
        wait(); //只要是奇数，一直等待
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }
}

MyNumber n = new MyNumber(0);
Thread old=new Thread(new OldRunner(n),"old-thread");
Thread even = new Thread(new EvenRunner(n),"even-thread");
old.start();
even.start();

private static class MyNumber{
  private Lock lock = new ReentrantLock();
  private Condition condition = lock.newCondition();
  private int val;

  public MyNumber(int val) {
    this.val = val;
  }

  public int getVal() {
    return val;
  }
  public void increase(){
    lock.lock();
    try {
      val++;
      condition.signalAll(); //通知线程
    }finally {
      lock.unlock();
    }

  }
  public void waitToOld(){
    lock.lock();
    try{
      while ((val % 2)==0){
        try {
          System.out.println("i am should print old ,but now is even:"+val+",so wait");
          condition.await();
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }finally {
      lock.unlock();
    }
  }
  public void waitToEven(){
    lock.lock(); //显示的锁定
    try{
      while ((val % 2)!=0){
        try {
          System.out.println("i am should print even ,but now old:"+val+",so wait");
          condition.await();//执行等待
        } catch (InterruptedException e) {
          e.printStackTrace();
        }
      }
    }finally {
      lock.unlock(); //显示的释放
    }

  }
}

public ReentrantLock() {
  sync = new NonfairSync();
}

public void lock() {
  sync.lock();
}

final void lock() {
  acquire(1);
}

final void lock() {
  if (compareAndSetState(0, 1)) //先看当前锁是不是已经被占有了，如果没有，就直接将当前线程设置为占有的线程
    setExclusiveOwnerThread(Thread.currentThread());
  else    
    acquire(1); //锁已经被占有的情况下，尝试获取
}

public final void acquire(int arg) {
  if (!tryAcquire(arg) &&
    acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) //一旦抢失败，就会进入队列，进入队列后则是依据FIFO的原则来执行唤醒
    selfInterrupt();
}

public final boolean release(int arg) {
  if (tryRelease(arg)) {
    Node h = head;
    if (h != null && h.waitStatus != 0)
      unparkSuccessor(h);
    return true;
  }
  return false;
}

protected final boolean tryAcquire(int acquires) {
  final Thread current = Thread.currentThread();
  int c = getState(); //获取当前的同步状态
  if (c == 0) {
    //等于0 表示没有被其它线程获取过锁
    if (!hasQueuedPredecessors() &&
      compareAndSetState(0, acquires)) {
      //hasQueuedPredecessors 判断在当前线程的前面是不是还有其它的线程，如果有，也就是锁sync上有一个等待的线程，那么它不能获取锁，这意味着，只有等待时间最长的线程能够获取锁，这就是是公平性的体现
      //compareAndSetState 看当前在内存中存储的值是不是真的是0，如果是0就设置成accquires的取值。对于JAVA，这种需要直接操作内存的操作是通过unsafe来完成，具体的实现机制则依赖于操作系统。
      //存储获取当前锁的线程
      setExclusiveOwnerThread(current);
      return true;
    }
  }
  else if (current == getExclusiveOwnerThread()) {
    //判断是不是当前线程获取的锁
    int nextc = c + acquires;
    if (nextc < 0)//一个线程能够获取同一个锁的次数是有限制的，就是int的最大值
      throw new Error("Maximum lock count exceeded");
    setState(nextc); //在当前的基础上再增加一次锁被持有的次数
    return true;
  }
  //锁被其它线程持有，获取失败
  return false;
}

final boolean nonfairTryAcquire(int acquires) {
  final Thread current = Thread.currentThread();
  int c = getState();
  if (c == 0) {
    //锁没有被持有
    //可以看到这里会无视sync queue中是否有其它线程，只要执行到了当前线程，就会去获取锁
    if (compareAndSetState(0, acquires)) { 
      setExclusiveOwnerThread(current); //在判断一次是不是锁没有被占有，没有就去标记当前线程拥有这个锁了
      return true;
    }
  }
  else if (current == getExclusiveOwnerThread()) {
    int nextc = c + acquires; 
    if (nextc < 0) // overflow      
      throw new Error("Maximum lock count exceeded");
    setState(nextc);//如果当前线程已经占有过，增加占有的次数
    return true;
  }
  return false;
}

protected final boolean tryRelease(int releases) {
  int c = getState() - releases;
  if (Thread.currentThread() != getExclusiveOwnerThread()) //只能是线程拥有这释放
    throw new IllegalMonitorStateException();
  boolean free = false;
  if (c == 0) {
    //当占有次数为0的时候，就认为所有的锁都释放完毕了
    free = true; 
    setExclusiveOwnerThread(null);
  }
  setState(c); //更新锁的状态
  return free;
}

public boolean tryLock() {
//本质上就是执行一次非公平的抢锁
return sync.nonfairTryAcquire(1); 
}