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
