JUC之可重入锁（递归锁）

一时失言乱红尘 2023-06-21 03:52 55阅读 0赞

# 可重入锁（又名递归锁） #

### 理论 ###

指的是同一线程外层函数获得锁之后，内层递归函数仍然能够获得该锁的代码

在同一个线程在外层方法获取锁的时候，在进入内层方法会自动获取锁

也就是说，线程可以进入任何一个它已经拥有的锁所同步着的代码块

ReentrantLock/synchronized就是一个典型的可重入锁

作用：避免死锁

### 代码 ###

**synchronized就是一个典型的可重入锁案例**

package Lock;
    
    //1.创建资源类
    
    class Phone{
        public synchronized void sendSMS()throws Exception{
            System.out.println(Thread.currentThread().getName() + "\t invoked sendSMS()");
            sendEmail();
        }
        public synchronized void sendEmail()throws Exception{
            System.out.println(Thread.currentThread().getName() + "\t ########invoked  sendEmail()");
        }
    }
    
    public class ReenterLockDemo {
        //线程操作资源类
        public static void main(String[] args) throws Exception{
       Phone phone =new Phone();
            new Thread(() -> {
                try {
                    phone.sendSMS();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "t1").start();
            new Thread(() -> {
                try {
                    phone.sendSMS();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "t2").start();
        }
    }

t1	 invoked sendSMS()     //t1线程在外层方法获取锁的时候
    t1	 ########invoked  sendEmail()    //t1进入内层方法会自动获取锁
    t2	 invoked sendSMS()
    t2	 ########invoked  sendEmail()
    
    Process finished with exit code 0

**ReentrantLock就是一个典型的可重入锁 **

package Lock;
    
    //1.创建资源类
    
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.Lock;
    import java.util.concurrent.locks.ReentrantLock;
    
    class Phone implements Runnable{
        public synchronized void sendSMS()throws Exception{
            System.out.println(Thread.currentThread().getName() + "\t invoked sendSMS()");
            sendEmail();
        }
        public synchronized void sendEmail()throws Exception{
            System.out.println(Thread.currentThread().getName() + "\t ########invoked  sendEmail()");
        }
    
        Lock lock=new ReentrantLock();
        @Override
        public void run(){
            get();
        }
    
        public void get(){
            lock.lock();//与下面的unlock对应
            lock.lock();
            try{
                System.out.println(Thread.currentThread().getName() + "\t ########invoked get()");
                set();
            }finally {
                lock.unlock();
                lock.unlock();
            }
        }
        public void set(){
            lock.lock();
            try {
                System.out.println(Thread.currentThread().getName() + "\t ########invoked set()");
            } finally {
                lock.unlock();
            }
        }
    }
    
    public class ReenterLockDemo {
        //线程操作资源类
        public static void main(String[] args) throws Exception{
       Phone phone =new Phone();
            new Thread(() -> {
                try {
                    phone.sendSMS();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "t1").start();
            new Thread(() -> {
                try {
                    phone.sendSMS();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "t2").start();
    
            //暂停一会儿线程
            try {
                TimeUnit.SECONDS.sleep(4); }catch (InterruptedException e){e.printStackTrace();}
            System.out.println();
            System.out.println();
            System.out.println();
            System.out.println();
    
    
          Thread t3 =new Thread(phone,"t3");
          Thread t4 =new Thread(phone,"t4");
    
    
          t3.start();
          t4.start();
        }
    }

t1	 invoked sendSMS()
    t1	 ########invoked  sendEmail()
    t2	 invoked sendSMS()
    t2	 ########invoked  sendEmail()
    
    
    
    
    t3	 ########invoked get()
    t3	 ########invoked set()
    t4	 ########invoked get()
    t4	 ########invoked set()
    
    Process finished with exit code 0