• 可重入锁：又称递归锁。在外层使用锁后，内层仍然可以使用，并不发生死锁，这样的锁就叫可重入锁。synchronized默认是一个可重入锁。

public class Demo01 {
    public synchronized void m1() {
        System.out.println(Thread.currentThread().getName() + " come in m1..");
        m2();
        System.out.println(Thread.currentThread().getName() + " end m1...");
    }
    public synchronized void m2() {
        System.out.println(Thread.currentThread().getName() + " come in m2..");
        m3();
        System.out.println(Thread.currentThread().getName() + " end m2...");
    }
    public synchronized void m3() {
        System.out.println(Thread.currentThread().getName() + " end m3...");
    }
    public static void main(String[] args) {
        Demo01 demo01 = new Demo01();
        new Thread(()->{
            demo01.m1();
        },"t1").start();
    }
    private static void reEntryLockMethod() {
        final Object o = new Object();
        new Thread(()->{
            // synchronized是可重入锁
            synchronized (o) {
                System.out.println(Thread.currentThread().getName() + " 一层");
                synchronized (o) {
                    System.out.println(Thread.currentThread().getName() + " 二层");
                    synchronized (o) {
                        System.out.println(Thread.currentThread().getName() + " 三层");
                    }
                }
            }
        }, "t1").start();
    }
}

public class Demo01 {
    // 显式锁
    static Lock lock = new ReentrantLock();
    public static void main(String[] args) {
        new Thread(()->{
            lock.lock();
            try {
                lock.lock();
                System.out.println(Thread.currentThread().getName());
            } catch (Exception e) {
                throw new RuntimeException(e);
            } finally {
                lock.unlock();
            }
            lock.unlock();
            }, "t1").start();
        new Thread(()->{
            lock.lock();
            System.out.println(Thread.currentThread().getName());
            lock.unlock();
            }, "t2").start();
    }
}