JUC(11)各种锁的理解（公平锁、可重入锁、自旋锁、死锁）

文章目录

• ​​1、公平锁、非公平锁​​
• ​​2、可重入锁​​
• ​​3、自旋锁​​
• ​​4、死锁​​

1、公平锁、非公平锁

公平锁：非常公平、不能插队、必须先来后到

• 非公平锁：非常不公平、可以插队、（默认非公平）



• 可以修改为公平锁

2、可重入锁

拿到外边的锁、就会自动拿到里边的锁。

package com.lock;

/**
 * synchronized
 */
public class Demo1 {
    public static void main(String[] args) {
        Phone phone = new Phone();

        new Thread(()->{
            phone.sms();
        },"A").start();

        new Thread(()->{
            phone.sms();
        },"B").start();
    }
}

class Phone{

    public synchronized void sms(){
        System.out.println(Thread.currentThread().getName()+"sms");
        call();
    }

    public synchronized  void call(){
        System.out.println(Thread.currentThread().getName()+"call");

    }
}

使用lock锁

package com.lock;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * synchronized
 */
public class Demo1 {
    public static void main(String[] args) {
        Phone phone = new Phone();

        new Thread(()->{
            phone.sms();
        },"A").start();

        new Thread(()->{
            phone.sms();
        },"B").start();
    }
}

class Phone{

    Lock lock = new ReentrantLock();

    public  void sms(){
        lock.lock();

        try {
            System.out.println(Thread.currentThread().getName()+"sms");
            call();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public   void call(){
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"call");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }

    }
}

3、自旋锁

spinlock

自己写自旋锁

package com.lock;

import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicStampedReference;

public class SpinLockDemo {

   AtomicReference<Thread> atomicReference =  new AtomicReference<>();

   //加锁
    public void mylock(){
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName()+"mylock");
        while (!atomicReference.compareAndSet(null,thread)){

        }
    }

    //解锁
    public void  myunlock(){
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName()+"myunlock");
        atomicReference.compareAndSet(thread,null);

    }


}

package com.lock;

import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) {
        SpinLockDemo spinLockDemo = new SpinLockDemo();

        new Thread(()->{
            spinLockDemo.mylock();

            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                spinLockDemo.myunlock();
            }

        },"A").start();



        new Thread(()->{
            spinLockDemo.mylock();
            try {
                System.out.println();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                spinLockDemo.myunlock();
            }

        },"B").start();
    }
}

4、死锁

package com.lock;

import java.util.concurrent.TimeUnit;

public class DeadLockDemo {
    public static void main(String[] args) {
        String lockA="lockA";
        String lockB ="lockB";

        new Thread(new MyThread(lockA,lockB),"T1").start();
        new Thread(new MyThread(lockB,lockA),"T2").start();
    }

}

class MyThread implements Runnable{
    private String lockA;
    private String lockB;

    public MyThread(String lockA,String lockB){
        this.lockA = lockA;
        this.lockB =lockB;
    }


    @Override
    public void run() {
        synchronized (lockA){
            System.out.println(Thread.currentThread().getName()+"lock:"+lockA+"==>"+lockB);
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (lockB){
                System.out.println(Thread.currentThread().getName()+"lock:"+lockB+"==>"+lockA);

            }
        }
    }
}

测试

1、使用jps定位进程号​​jps -l​​

2、使用​​jstack 进程号​​