1 可重入鎖 （ReentrantLock和synchronized）
可重入鎖指的是可重復可遞歸調(diào)用的鎖，在外層使用鎖之后，在內(nèi)層仍然可以使用，并且不發(fā)生死鎖，這樣的鎖就叫做可重入鎖
如ReentrantLock和synchronized都是可重入鎖的

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();
        }
    }
}

輸出結(jié)果，沒有死鎖

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

2 不可重入鎖
簡單說就是不可重入鎖，與可重入鎖相反，不可遞歸調(diào)用，否則遞歸調(diào)用就發(fā)生死鎖

import java.util.concurrent.atomic.AtomicReference;

public class UnreentrantLock {

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

    public void lock() {
        Thread current = Thread.currentThread();
        //這句是很經(jīng)典的“自旋”語法，AtomicInteger中也有
        for (;;) {
            if (!owner.compareAndSet(null, current)) {
                return;
            }
        }
    }

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

上面代碼，如果第一次lock了，在第一次unlock前，又進行第二次lock，則發(fā)生死循環(huán)，即死鎖

改造一下就變成可重入鎖了

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;
        }
        //這句是很經(jīng)典的“自旋”式語法，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);
            }
        }
    }
}