JUC之可重入锁（递归锁）

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