可重入锁，也叫做递归锁，指的是同一线程 外层函数获得锁之后 ，内层递归函数仍然有获取该锁的代码，但不受影响。比如ReentrantLock 和synchronized 都是 可重入锁

直接放代码

public class SubService {

ReentrantLock lock = new ReentrantLock();

public void subService1() {

    lock.lock();

    System.out.println("thread = " + Thread.currentThread().getId() + " --subService1");

    subService2();

    lock.unlock();

}

public synchronized void subService2() {

    System.out.println("thread = " + Thread.currentThread().getId() + " --subService2");

}

}

public class Service extends SubService {

public synchronized void service1(){

    System.out.println("thread = " + Thread.currentThread().getId() + " --service1");

    service2();

}

public synchronized void service2() {

    System.out.println("thread = " + Thread.currentThread().getId() + " --service2");

    service3();

}

public synchronized void service3() {

    System.out.println("thread = " + Thread.currentThread().getId() + " --service3");

    this.subService1();

}

}

public class MyThread extends Thread {

@Override

public void run() {

    Service service = new Service();

    service.service1();

}

public static void main(String[] args) throws Exception {

    MyThread thread1 = new MyThread();

    thread1.start();

    MyThread thread2 = new MyThread();

    thread2.start();

}

} 

运行结果

通过测试，可以得知 ReentrantLock 和synchronized锁是可以重入的，且在父子类继承环境中同样有效，如果该类型锁不可重入，则在调用service1()方法的时候，应该发生死锁，结果是同一线程的锁内部函数中锁可以重入。

来源 ：

java多线程编程核心技术

Java锁的种类以及辨析（四）：可重入锁 http://ifeve.com/java_lock_see4/