27.ReentrantLock

1.与synchronized不同点：

可中断
可以设置超时时间
可以设置公平锁，公平锁就是为了解决饥饿线程，让线程排队，先进先出，先来的线程先执行。
支持多个条件变量

2.与synchronized相同点都支持锁的可重入。

基本格式：

//获取锁
reentrantLock.lock();
try {} finally {//必须释放锁reentrantLock.unlock();
}

可重入

同一个线程如果首次获得了这把锁，因为它是这把锁的拥有者，因此有权利再次获取这把锁。

如果是不可重入锁，那么第二次获得锁时，自己也会被锁挡住。

@Slf4j
public class ReentrantLockDemo {private static ReentrantLock reentrantLock = new ReentrantLock();public static void main(String[] args) {//获取锁reentrantLock.lock();try {log.info("调用main方法...");method1();} finally {//必须释放锁reentrantLock.unlock();}}public static void method1() {reentrantLock.lock();try {log.info("调用method1方法...");method2();}finally {reentrantLock.unlock();}}public static void method2() {reentrantLock.lock();try {log.info("调用method2方法...");}finally {reentrantLock.unlock();}}
}

可被中断

lockInterruptibly方法表示可被打断的锁

lokc方法加锁表示不可被打断的锁

可打断的锁，可以防止死锁。

@Slf4j
public class Reen {private static ReentrantLock reentrantLock = new ReentrantLock();public static void main(String[] args) {Thread t1 = new Thread(() -> {try {log.info("尝试获取锁....");//lockInterruptibly可被打断的锁reentrantLock.lockInterruptibly();} catch (InterruptedException e) {e.printStackTrace();log.info("等待获取锁的线程被打断....返回");return;}try {log.info("获取到了锁...");}finally {reentrantLock.unlock();}}, "t1");//main线程获取锁reentrantLock.lock();t1.start();try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}log.info("打断线程t1....");t1.interrupt();}
}

可以设置超时时间

tryLock方法，设置超时时间，避免长时间等待，防止死锁。

tryLock方法不设置超时时间参数表示获取不到锁立刻返回。

@Slf4j
public class ReenDemo {private static ReentrantLock reentrantLock = new ReentrantLock();public static void main(String[] args) {Thread t1 = new Thread(() -> {//尝试获取锁，时间是立刻返回log.info("尝试获取锁...");try {if(!reentrantLock.tryLock(2, TimeUnit.SECONDS)) {log.info("尝试获取锁失败，返回...");return;}log.info("尝试获取锁成功...");} catch (InterruptedException e) {e.printStackTrace();log.info("尝试获取锁被打断，返回...");return;}try {}finally {reentrantLock.unlock();}}, "t1");reentrantLock.lock();t1.start();try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}try {}finally {reentrantLock.unlock();}}
}

解决哲学家吃饭的死锁问题：

public class DeadLockDemo {public static void main(String[] args) {Chopstick chopstick1 = new Chopstick("1");Chopstick chopstick2 = new Chopstick("2");Chopstick chopstick3 = new Chopstick("3");Chopstick chopstick4 = new Chopstick("4");Chopstick chopstick5 = new Chopstick("5");new Philosopher("哲学家1", chopstick1, chopstick2).start();new Philosopher("哲学家2", chopstick2, chopstick3).start();new Philosopher("哲学家3", chopstick3, chopstick4).start();new Philosopher("哲学家4", chopstick4, chopstick5).start();new Philosopher("哲学家5", chopstick5, chopstick1).start();}
}@Slf4j
class Philosopher extends Thread {//左边的筷子Chopstick left;//右边的筷子Chopstick right;public Philosopher(String name, Chopstick left, Chopstick right) {super(name);//设置线程名称this.left = left;this.right = right;}@Overridepublic void run() {while (true) {//这里意思是一位哲学家获取了两根筷子后吃完饭，又重新开始下一轮...//解决死锁问题，使用ReentrantLock.tryLock方法，避免长时间等待不释放锁的情况if(left.tryLock()) {try {if(right.tryLock()) {try {eat();}finally {right.unlock();}}}finally {left.unlock();}}//            synchronized (left) {
//                synchronized (right) {
//                    eat();
//                }
//            }}}private void eat() {log.info("eat...");try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}
}/*** 筷子类*/
class Chopstick extends ReentrantLock{//名称private String name;public Chopstick(String name) {this.name = name;}@Overridepublic String toString() {return "Chopstick{" +"name='" + name + '\'' +'}';}
}