各种锁的理解

各种锁的理解

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）

public ReentrantLock() {
	sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

2、可重入锁

可重入锁（递归锁）

Synchronized

// 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版

public class Demo02 {
    public static void main(String[] args) {
        Phone2 phone = new Phone2();

        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 锁必须配对，否则就会死在里面
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + "sms");
            call(); // 这里也有锁
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
            lock.unlock();
        }

    }

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

3、自旋锁

spinlock

/**
 * 自旋锁
 */
public class SpinlockDemo {

    // int   0
    // Thread  null
    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
    // 加锁
    public void myUnLock(){
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName() + "==> myUnlock");
        atomicReference.compareAndSet(thread,null);
    }



}

测试

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

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


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

            try {
                TimeUnit.SECONDS.sleep(5);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.myUnLock();
            }

        },"T1").start();

        TimeUnit.SECONDS.sleep(1);

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

            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                lock.myUnLock();
            }

        },"T2").start();

    }
}

4、死锁

死锁测试-怎么排除死锁

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+"=>get"+lockB);

            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            synchronized (lockB){
                System.out.println(Thread.currentThread().getName() + "lock:"+lockB+"=>get"+lockA);
            }

        }
    }
}

解决方案

1、使用 jps -l 定位进程号

2、使用 jstack 进程号 找到死锁问题

面试，工作中！ 排查问题：
1、日志
2、堆栈信息

视频参考https://www.bilibili.com/video/BV1B7411L7tE
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