java并发编程 第四期 多线程中的锁

java中的锁有 公平锁和非公平锁 ，可重入锁，自旋锁，读写锁（独占锁/写锁，共享锁/读锁，互斥锁）

1.公平锁和非公平锁 ：公平锁就是线程先来先到，非公平锁可能出现某个线程长时间获取不到锁的情况，会出现线程饥饿。非公平锁优点是吞吐量大

常见的synchronized也是可重入锁，非公平锁

2.自旋锁：用while方式尝试获取锁，减少了线程切换的的消耗，缺点循环会消耗CPU，CAS就是使用自旋锁实现的

3.可重入锁：线程获取方法外部锁后，进入该方法内部可以自动获取该锁的代码，可以避免死锁

4.独占锁：一次只能被一个线程持有。ReentrantLock 和synchronized都是独占锁

5.共享锁：该线程可以被多个线程共同持有 R

ReentrantLock 可重入锁，又名递归锁 ReentrantLock 有公平锁和非公平锁两种构造

public ReentrantLock() {
sync = new NonfairSync();
}

public ReentrantLock(boolean fair) {

sync = fair ? new FairSync() : new NonfairSync();

}

可重入锁代码验证 synchronized版

/*输出如下 原因：t1 获取了sendSms方法 虽然内部t1sendMail加锁，
*t1 依然可以获取内部方法的锁 原因两个方法是同一把锁
*需要注意的是   reentrantLock.lock();需要和reentrantLock.unlock();成对使用
*否则第二个线程将无法进入get方法，输出如下
*t1sendSms
*t1sendMail
*t2sendSms
*t2sendMail
*---------以上是synchronized版，下面是reentrantLock版-------------
*Thread-0get
*Thread-0set
*
*/
/** 正常锁配对使用输出
 * t1sendSms
 * t1sendMail
 * t2sendSms
 * t2sendMail
 * ---------以上是synchronized版，下面是reentrantLock版-------------
 * Thread-0get
 * Thread-0set
 * Thread-1get
 * Thread-1set
 */
package com.lock;

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

class Phone implements Runnable {
    public synchronized void sendSms() {
        System.out.println(Thread.currentThread().getName() + "sendSms");
        sendMail();
    }

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

    ReentrantLock reentrantLock = new ReentrantLock();


    @Override
    public void run() {
        get();
    }

    public void get() {
        reentrantLock.lock();
        reentrantLock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + "get");
            set();
        } finally {
            reentrantLock.unlock();
            reentrantLock.unlock();
        }

    }

    public void set() {
        reentrantLock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + "set");
        } finally {
            reentrantLock.unlock();
        }

    }
}

/**
 * @author liuxu
 * @date 2021/11/13 15:48
 */

public class ReentrantLockDemo {


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

        new Thread(() -> {
            phone.sendSms();
        }, "t1").start();

        new Thread(() -> {
            phone.sendSms();
        }, "t2").start();

        TimeUnit.SECONDS.sleep(3);
System.out.println("---------以上是synchronized版，下面是reentrantLock版-------------");
        Thread t3 = new Thread(phone);
        Thread t4 = new Thread(phone);
        t3.start();
        t4.start();
    }
}

递归锁代码验证

package com.lock;

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

/**
 * 输出如下 锁生效，可以做到线程安全
 * 线程：AAA来了
 * 线程：AAA解锁
 * 线程：BBB来了
 * ------BBB----
 * 线程：BBB解锁
 * @author liuxu
 * @date 2021/11/13 16:44
 */
public class SpinLockDemo {
    AtomicReference<Thread> reference =new AtomicReference<>();
    public void lock(){
        Thread thread=Thread.currentThread();
        System.out.println("线程："+thread.getName()+"来了");
        while (!reference.compareAndSet(null,thread)){

        }
    }

    public void unlock(){
        Thread thread=Thread.currentThread();
        reference.compareAndSet(thread,null);
        System.out.println("线程："+thread.getName()+"解锁");

    }

    public static void main(String[] args) {
        SpinLockDemo spinLockDemo = new SpinLockDemo();
        new Thread(()->{
            spinLockDemo.lock();
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                spinLockDemo.unlock();
            }

        },"AAA").start();
        try {
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }


        new Thread(()->{
            spinLockDemo.lock();
            try {
               System.out.println("------BBB----");
            } catch (Exception e) {

                e.printStackTrace();
            }finally {
                spinLockDemo.unlock();
            }


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