各種鎖的理解

公平鎖、非公平鎖

公平鎖：先到先得（不可插隊）

非公平鎖：達者為先（可插隊）---------->默認

public ReentrantLock() {
    //默認非公平鎖
    sync = new NonfairSync();
}

//重載的構造方法，通過fair控制是否公平
public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

可重入鎖（遞歸鎖）

所有的鎖都是可重入鎖

Synchronized版

package org.example.lock;


public class Demo01 {
    public static void main(String[] args) {
        phone1 p1 = new phone1();
        new Thread(()->{
            p1.ems();
        },"A").start();
        new Thread(()->{
            p1.ems();
        },"B").start();
    }
}
class phone1{
    public synchronized void ems(){
        System.out.println(Thread.currentThread().getName()+"---------->ems");
        call();
    }
    public synchronized void call(){
        System.out.println(Thread.currentThread().getName()+"---------->call");
    }
}

ems方法中包含了call方法，所以當我們調用ems方法獲取到鎖時，也把call方法的synchronized鎖獲取到了。

錯誤理論

• 當線程A運行ems方法后運行call方法時ems鎖釋放，線程B可以獲取到ems方法

正確理論

• 當線程A運行ems方法后運行call方法時ems方法的鎖還未釋放時就拿到了call方法中的鎖，當call方法的鎖釋放后ems方法的鎖才會釋放。線程B此時就可以運行ems方法了

Lock版

package org.example.lock;

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

public class Demo02 {
    public static void main(String[] args) {
        phone2 p2 = new phone2();
        new Thread(()->{
            p2.ems();
        },"A").start();
        new Thread(()->{
            p2.ems();
        },"B").start();
    }
}
class phone2{
    Lock lock = new ReentrantLock();
    public  void ems(){
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"---------->ems");
            call();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            //等待call方法鎖解鎖后再解鎖
            lock.unlock();
        }

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

    }
}

自旋鎖

spinlock(不斷嘗試直至成功)

已經見過了，就是unsafe中的自增getAndAddInt方法中的do-while循環就是一把自旋鎖

自己寫一把鎖

package org.example.lock;

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

public class SpinLockDemo {
    //int 0
    //Thread null
    public static AtomicReference<Thread> atomic = new AtomicReference<>();

    public static void lock(){
        Thread thread = Thread.currentThread();
        System.out.println("===============>"+thread.getName()+"===========>lock");
        //自旋鎖,若線程等于null，則compareAndSet為true，加！就為false，就會一直循環
        while (!atomic.compareAndSet(null,thread)){

        }
    }
    public static void unlock(){
        Thread thread = Thread.currentThread();
        System.out.println("===============>"+thread.getName()+"===========>unlock");
        //自旋鎖
        atomic.compareAndSet(thread,null);
    }

    public static void main(String[] args) throws InterruptedException {
        new Thread(()->{
            try {
                lock();
                TimeUnit.SECONDS.sleep(10);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                unlock();
            }
        },"A").start();
        TimeUnit.SECONDS.sleep(1);
        new Thread(()->{
            try {
                lock();
                TimeUnit.SECONDS.sleep(2);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                unlock();
            }
        },"B").start();
    }
}

死鎖

死鎖是什么

死鎖測試

package org.example.lock;

import java.util.concurrent.TimeUnit;

public class DeadLockDemo {
    public static void main(String[] args) {
        String a = "A";
        String b = "B";
        new Thread(()->{new MyThread(a, b).run();},"A").start();
        new Thread(()->{new MyThread(b, a).run();},"B").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) {
                throw new RuntimeException(e);
            }
            synchronized (lockB){
                System.out.println(Thread.currentThread().getName()+"lock:"+lockB+"=>get"+lockA);
            }
        }
    }
}

程序突然卡住死鎖如何排查？

1、使用jps-l定位進程號

查看當前java活著的進程

2、使用jstack 進程號查看死鎖問題

查找到一個死鎖問題！

面試或者工作中排查問題：

1、查看異常

2、查看日志

3、查看堆棧信息