本文从以下几个方面介绍Lock接口,分别从lock锁的介绍、为什么要用lock锁、方法介绍、可见性保证
Lock接口简介
Lock锁是一种工具,用来控制对共享资源的访问。
Lock锁和Synchronized锁两者各个有各自的使用场景,lock并不是来替换Synchronized锁的。
Lock接口最常见的实现类是ReentrantLock
Lock接口方法
lock接口中包含如下四个方法,分别是:lock()、tryLock()、tryLock(long time,TimeUnit unit)和lockInterruptibly(),接下来会详细介绍这四种方法是如何使用的,见下所示。
lock method
lock方法是普通的获取锁的方法,lock方法不会像synchronized方法一样在异常发生时自动释放锁,lock方法需要手动释放锁。所以在使用lock锁时一定要手动释放锁。
见下面的实现,如下
package com.yang.lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* lock不会自动释放锁,我们采用try -finally的方式实现,必须手动释放锁
*/
public class MustUnlock {
private static Lock lock=new ReentrantLock();
public static void main(String[] args) {
lock.tryLock();
try{
//制定锁定的资源,我们可以采用模拟的方式实现
System.out.println(Thread.currentThread().getName());
}finally {
lock.unlock();
}
}
}
使用lock方法时,当产生死锁时,会永久等待,即为lock锁的弊端。所有有个tryLock的方法,接下来我们一起看下tryLock方法
tryLock method
tryLock此方法用来尝试获取锁,如果当前锁没有被其他线程占用的时候,才是可以获取成功的,该方法的返回值为True,若返回False则说明锁没有获取成功
tryLock(long time,TimeUnit unit) method
此方法相比上面方法多了尝试锁的等待时间,下实例模拟两个线程获取两把锁,见下代码。
package com.yang.lock;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* 尝试获取锁,若能获取到锁,则返回true,否则返回false
* 用来避免死锁问题
* 某次的运行结果如下:
* 线程2取到了锁1
* 线程1取到了锁1
* 线程1获取锁2失败了,已经重试了
* 线程2获取到了锁2
* 线程2成功获取了两把锁
* 线程1取到了锁1
* 线程1获取到了锁2
* 线程1成功获取了两把锁
*
* Process finished with exit code 0
*/
public class TryDeadLock implements Runnable {
private static Lock lock1 = new ReentrantLock();
private static Lock lock2 = new ReentrantLock();
int flag = 1;
public static void main(String[] args) {
TryDeadLock tryDeadLock1=new TryDeadLock();
tryDeadLock1.flag=0;
TryDeadLock tryDeadLock2=new TryDeadLock();
tryDeadLock2.flag=1;
new Thread(tryDeadLock1).start();
new Thread(tryDeadLock2).start();
}
@Override
public void run() {
//模拟实现死锁
for (int i = 0; i < 100; i++) {
if (flag == 1) {
try {
if (lock1.tryLock(800, TimeUnit.MILLISECONDS)) {
try {
System.out.println("线程1取到了锁1");
Thread.sleep(new Random().nextInt(1000));
if (lock2.tryLock(800, TimeUnit.MILLISECONDS)) {
try {
System.out.println("线程1获取到了锁2");
System.out.println("线程1成功获取了两把锁");
break;
} finally {
lock2.unlock();
}
} else {
System.out.println("线程1获取锁2失败了,已经重试了");
}
} finally {
lock1.unlock();
Thread.sleep(new Random().nextInt(1000));
}
} else {
System.out.println("线程1获取锁1失败了,已经重试了");
}
} catch (Exception e) {
} finally {
}
} else {
try {
if (lock2.tryLock(3000, TimeUnit.MILLISECONDS)) {
try {
System.out.println("线程2取到了锁1");
Thread.sleep(new Random().nextInt(1000));
if (lock1.tryLock(800, TimeUnit.MILLISECONDS)) {
try {
System.out.println("线程2获取到了锁2");
System.out.println("线程2成功获取了两把锁");
break;
} finally {
lock1.unlock();
}
} else {
System.out.println("线程2获取锁2失败了,已经重试了");
}
} finally {
lock2.unlock();
Thread.sleep(new Random().nextInt(1000));
}
} else {
System.out.println("线程2获取锁1失败了,已经重试了");
}
} catch (Exception e) {
} finally {
}
}
}
}
}
lockInterruptibly method
相当于把try的等待时间设置为无限时间,但是可以被中断,下文通过代码的方式去演示。如下所示。
package com.yang.lock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* 演示加锁的等待时间为无限制,但是是可以被中断的
*/
public class LockInterruptiblyTest implements Runnable {
private Lock lock = new ReentrantLock();
public static void main(String[] args) {
LockInterruptiblyTest lockInterruptiblyTest=new LockInterruptiblyTest();
Thread thread0 =new Thread(lockInterruptiblyTest);
Thread thread1 =new Thread(lockInterruptiblyTest);
thread0.start();
thread1.start();
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
thread1.interrupt();
}
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + "尝试获取锁");
try {
lock.lockInterruptibly();
try {
System.out.println(Thread.currentThread().getName() + "拿到了锁");
Thread.sleep(5000);
} catch (InterruptedException e) {
System.out.println(Thread.currentThread().getName() + "睡眠期间中断了");
} finally {
System.out.println(Thread.currentThread().getName() + "释放了锁");
}
} catch (InterruptedException e) {
System.out.println(Thread.currentThread().getName()+"等锁期间被中断了");
} finally {
}
}
}