实现reentrantlock和读写锁

1 可以手动实现一个类似reentrantlock的工具，首先要维护一个state的标志，代表当前是否有线程已经使用资源。线程lock的时候，

会用cas给state加1，其他线程检测状态。另外需要维护一个等待队列，争夺不到资源的线程统一挂起（park），等线程unlock的时候，

标志减为0，同时从队列里挑一个线程unpark唤醒，继续得到资源操作；如果想让队列线程竞争，就都唤醒，最终只有一个得到资源。

这是实现了基本的锁。

public class LkLock {

    private ReentrantLock lock;

    private Thread ownerThread;

    private LinkedBlockingQueue<Thread> waitList = new LinkedBlockingQueue<Thread>(10000);

    private static Unsafe unsafe =null;

    private int state=0;

    private static long stateOffset;

    public LkLock(){
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe"); //Internal reference
 f.setAccessible(true);
            unsafe = (Unsafe) f.get(null);
            stateOffset= unsafe.objectFieldOffset(LkLock.class.getDeclaredField("state"));
        } catch (Exception e) {
            System.out.println(e+" error");
        }
    }

    public void lock(){
        if(compareAndSetState(0,1)){
            ownerThread=Thread.currentThread();
        }else{
            acquire(1);
        }
    }

    public void unlock(){
        compareAndSetState(1,0);
        if(!CollectionUtils.isEmpty(waitList)){
            Thread t=waitList.poll();
            LockSupport.unpark(t);
        }

    }

    private void acquire(int i){
        try {
            //加入等待队列
 waitList.put(Thread.currentThread());
            //挂起
 LockSupport.park();
        } catch (InterruptedException e) {
            System.out.println(e);
        }
    }

    public int getState() {
        return state;
    }

    private boolean compareAndSetState(int src, int target){
        return unsafe.compareAndSwapInt(this, stateOffset, src, target);
    }

}

2 同样如果想要实现能够重入的读写锁，读可重入，写唯一，并且读写互斥，那么需要用两个资源分别给读写的线程竞争，

读的资源可累加，每次线程readlock的时候加一，不设上限；写的线程如上面只可唯一线程获得资源。并且无论读写线程竞争资源之前，

都要检测对方资源是否已经被占用，如果占用也要挂起，这样可以达到互斥。改动如下

public void readLock(){
    if(writeState!=0){
        acquire();
    }
    if(compareAndSetReadState(readState,readState+1)){
        System.out.println(readState+" 读 我看到的 "+Thread.currentThread());
    }else{
        acquire();
    }
}

public void readUnlock(){
    compareAndSetReadState(readState,readState-1);
    if(!CollectionUtils.isEmpty(waitList)){
        while (waitList.size()!=0){
            Thread t=waitList.poll();
            LockSupport.unpark(t);
        }
    }

}

public void writeLock(){
    if(readState!=0){
        acquire();
    }

    if(compareAndSetWriteState(0,1)){
        System.out.println(writeState+" 写 我看到的 "+Thread.currentThread());
    }else{
        acquire();
    }
}

public void writeUnlock(){
    compareAndSetWriteState(1,0);
    if(!CollectionUtils.isEmpty(waitList)){
        while (waitList.size()!=0){
            Thread t=waitList.poll();
            LockSupport.unpark(t);
        }
    }

}

巴特西

实现reentrantlock和读写锁

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