18、JUC：各种锁的理解

18.1、公平锁、非公平锁

公平锁：非常公平，不能够插队，必须先来后到！
非公平锁：非常不公平，可以插队(默认都是非公平锁)

public ReentrantLock() {
    sync = new NonfairSync();
}
// 可以通过传递参数true将它变成公平锁
public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

18.2、可重入锁（递归锁）

Synchtonized

package com.chen.lock;

// Synchronized
public class Demo01 {

    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(); // 这里也有锁
    }

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

}

Lock

package com.chen.lock;

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

public class Demo02 {

    public static void main(String[] args) {
        Phone phone = new Phone();

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

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

}

class Phone2{

    Lock lock = new ReentrantLock();

    public void sms(){
        lock.lock(); // 细节问题 lock.lock();lock.unlock(); lock 必须要配对，否则就会死在里面
        try {
            System.out.println(Thread.currentThread().getName()+"sms");
            call(); // 这里也有锁
        } 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();
        }
    }

}

18.3、自旋锁

spinlock

我们自定义一个锁来测试

package com.chen.lock;

import java.util.concurrent.atomic.AtomicReference;

/**
 * 自旋锁
 */
public class SpinLockDemo {

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

    // 加锁
    public void myLock(){
        Thread thread = Thread.currentThread();
        System.out.println(thread.currentThread().getName()+"==>myLock");
        // 自旋锁
        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);
    }

}

测试

package com.chen.lock;

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

public class TestSpinLock {
    public static void main(String[] args) throws InterruptedException {
//        Lock lock = new ReentrantLock();
        lock.lock();
        lock.unlock();

        // 底层使用的自旋锁CAS
        SpinLockDemo lock = new SpinLockDemo();

        new Thread(()->{
            lock.myLock();

            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.myUnLock();
            }
        },"T1").start();

        TimeUnit.SECONDS.sleep(1);

        new Thread(()->{

            lock.myLock();

            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.myUnLock();
            }
        },"T2").start();
    }
}

18.4、死锁

**死锁是什么 **

死锁测试，怎么排除死锁：

package com.chen.lock;

import lombok.SneakyThrows;

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;
    }

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

解决问题

1、使用JPS -l定位进程号

2、使用jstack 进程号查看信息

学习视频链接：https://www.bilibili.com/video/BV1B7411L7tE?p=38