可重入锁和非可重入锁

1 可重入锁 （ReentrantLock和synchronized）
可重入锁指的是可重复可递归调用的锁，在外层使用锁之后，在内层仍然可以使用，并且不发生死锁，这样的锁就叫做可重入锁
如ReentrantLock和synchronized都是可重入锁的

如：

public class ReentrantTest1 implements Runnable {

    public synchronized void get() {
        System.out.println(Thread.currentThread().getName());
        set();
    }

    public synchronized void set() {
        System.out.println(Thread.currentThread().getName());
    }

    public void run() {
        get();
    }

    public static void main(String[] args) {
        ReentrantTest1 rt = new ReentrantTest1();
        for(;;){
            new Thread(rt).start();
        }
    }
}

输出结果，没有死锁

Thread-8492
Thread-8492
Thread-8494
Thread-8494
Thread-8495
Thread-8495
Thread-8493
Thread-8493

2 不可重入锁
简单说就是不可重入锁，与可重入锁相反，不可递归调用，否则递归调用就发生死锁

import java.util.concurrent.atomic.AtomicReference;

public class UnreentrantLock {

    private AtomicReference<Thread> owner = new AtomicReference<Thread>();

    public void lock() {
        Thread current = Thread.currentThread();
        //这句是很经典的“自旋”语法，AtomicInteger中也有
        for (;;) {
            if (!owner.compareAndSet(null, current)) {
                return;
            }
        }
    }

    public void unlock() {
        Thread current = Thread.currentThread();
        owner.compareAndSet(current, null);
    }
}

上面代码，如果第一次lock了，在第一次unlock前，又进行第二次lock，则发生死循环，即死锁

改造一下就变成可重入锁了

import java.util.concurrent.atomic.AtomicReference;

public class UnreentrantLock {

    private AtomicReference<Thread> owner = new AtomicReference<Thread>();
    private int state = 0;

    public void lock() {
        Thread current = Thread.currentThread();
        if (current == owner.get()) {
            state++;
            return;
        }
        //这句是很经典的“自旋”式语法，AtomicInteger中也有
        for (;;) {
            if (!owner.compareAndSet(null, current)) {
                return;
            }
        }
    }

    public void unlock() {
        Thread current = Thread.currentThread();
        if (current == owner.get()) {
            if (state != 0) {
                state--;
            } else {
                owner.compareAndSet(current, null);
            }
        }
    }
}