13.各种锁的理解

包括synchronized和ReetrantLock

可重入就是说某个线程已经获得某个锁，可以再次获取锁而不会出现死锁

 synchronized版本

public class demo1 {
public static void main(String[] args) {
Phone phOne=new Phone();
new Thread(()->{ phone.sms(); },"A").start();
new Thread(()->{ phone.call(); },"B").start();
}
}
class Phone{
public synchronized void sms(){
System.out.println(Thread.currentThread().getName()+" sms");
call();//这里再次获得了对象锁，而没有出现死锁
}
public synchronized void call(){
System.out.println(Thread.currentThread().getName()+" call");
}
}

ReentrantLock版本

public class demo2 {
public static void main(String[] args) {
Phone2 phOne=new Phone2();
new Thread(()->{ phone.sms(); },"A").start();
new Thread(()->{ phone.call(); },"B").start();
}
}
class Phone2{
Lock lock=new ReentrantLock();
public synchronized void sms(){
lock.lock();
try {
System.out.println(Thread.currentThread().getName()+" sms");
call();//这里再次获得了对象锁，而没有出现死锁
} catch (Exception e) {
e.printStackTrace();
}finally {
lock.unlock();
}
}
public synchronized void call(){
lock.lock();//锁必须配对，加了锁就要解锁
try {
System.out.println(Thread.currentThread().getName()+" call");
} catch (Exception e) {
e.printStackTrace();
}finally {
lock.unlock();
}
}
}

自旋锁

/**
* @author wuyimin
* @create 2021-07-11-13:40
* @description
* T1先进来拿到锁，期望是null，变成了thread，这时候T2拿到锁后为thread 开始自旋(阻塞)，
* 等待T1变会null，T2走出自旋
*/
public class test {
public static void main(String[] args) throws InterruptedException {
demo3 demo=new demo3();
new Thread(()->{
demo.myLock();
try {
TimeUnit.SECONDS.sleep(2);
} catch (Exception e) {
e.printStackTrace();
}finally {
demo.myUnlock();
}
},"T1").start();
TimeUnit.SECONDS.sleep(1);//让T1一定能先抢到锁
new Thread(()->{
demo.myLock();
try {
TimeUnit.SECONDS.sleep(1);
} catch (Exception e) {
e.printStackTrace();
}finally {
demo.myUnlock();
}
},"T2").start();
}
}

 死锁

public class deadLock {
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 lock1;
private String lock2;
public MyThread(String lockA,String lockB){
this.lock1=lockA;
this.lock2=lockB;
}
@SneakyThrows
@Override
public void run() {
synchronized (lock1){
//线程进来了拿到了1锁，然后又想要去拿2锁
System.out.println(Thread.currentThread().getName()+" lock:"+lock1+"want get=>"+lock2);
TimeUnit.SECONDS.sleep(2);
synchronized (lock2){
System.out.println("造成死锁");
}
}
}

如何排查送死锁

使用java bin目录下的工具

首先是查看进程号7112

 通过此命令查看死锁

 发现死锁