可重入锁

可重入锁

可重入锁就是一个线程已经获得了一把锁，可以重复获取而不会导致死锁

synchronized 和Lock都是可重入锁

synchronized 可重入锁示例

package com.example.juc;

public class TestReentrantLock {
    public static void main(String[] args) {
        Phone1 phone1 = new Phone1();
        new Thread(() -> {
            phone1.sendSms();
        }, "A").start();
        new Thread(() -> {
            phone1.sendSms();
        }, "B").start();

    }
}

class Phone1 {
    public synchronized void sendSms() {
        System.out.println(Thread.currentThread().getName() + "sms");
        call();
    }

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

}

Lock 可重入锁示例

package com.example.juc;

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

public class TestReentrantLock2 {
    public static void main(String[] args) {
        Phone2 phone1 = new Phone2();
        new Thread(() -> {
            phone1.sendSms();
        }, "A").start();
        new Thread(() -> {
            phone1.sendSms();
        }, "B").start();

    }
}

class Phone2 {
    private Lock lock = new ReentrantLock();

    public void sendSms() {
        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();
        }

    }


}

synchronized vs. Lock

1. synchronized是一个java关键字，Lock是一个类
2. synchronized可以修饰方法和代码块，Lock只能修饰代码块
3. 使用方式也不一样，synchronized自动加锁解锁，Lock需要手动加锁解锁
4. Lock加锁和解锁是配对的，否则会出现死锁的情况

死锁情况

package com.example.juc;

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

public class TestReentrantLock2 {
    public static void main(String[] args) {
        Phone2 phone1 = new Phone2();
        new Thread(() -> {
            phone1.sendSms();
        }, "A").start();
        new Thread(() -> {
            phone1.sendSms();
        }, "B").start();

    }
}

class Phone2 {
    private Lock lock = new ReentrantLock();

    public void sendSms() {
        lock.lock();
        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();
        }

    }


}

Asms
Acall
...

这样也能化解死锁

package com.example.juc;

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

public class TestReentrantLock2 {
    public static void main(String[] args) {
        Phone2 phone1 = new Phone2();
        new Thread(() -> {
            phone1.sendSms();
        }, "A").start();
        new Thread(() -> {
            phone1.sendSms();
        }, "B").start();

    }
}

class Phone2 {
    private Lock lock = new ReentrantLock();

    public void sendSms() {
        lock.lock();
        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();
            lock.unlock();
        }

    }


}

Asms
Acall
Bsms
Bcall

但是如果线程B改成phone1.call(),代码会报错

Asms
Acall
Bcall
Exception in thread "B" java.lang.IllegalMonitorStateException
	at java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:151)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1263)
	at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:457)
	at com.example.juc.Phone2.call(TestReentrantLock2.java:44)
	at com.example.juc.TestReentrantLock2.lambda$main$1(TestReentrantLock2.java:13)
	at java.lang.Thread.run(Thread.java:748)