ReentrantLock学习

ReentrantLock

ReentrantLock功能

ReentrantLock和synchronized一样是可重入的

可重入即当线程拥有了锁时，当该线程再次请求锁资源的时候，线程是可以再次成功获得的。

static ReentrantLock lock = new ReentrantLock();

public static void main(String[] args) {
    new Thread(() ->{
        lock.lock();
        try{
            log.debug("获得了锁，再次请求锁");
            lock.lock();
            try{
                log.debug("再次获得了锁");
            }finally {
                log.debug("释放了锁");
                lock.unlock();
            }
        }finally {
            log.debug("释放了锁");
            lock.unlock();
        }
    },"t1").start();
    
}

ReentrantLock是可打断的

即请求锁资源的时候，当请求不到锁资源的时候，可以被interrupt方法打断。

static ReentrantLock lock = new ReentrantLock();

public static void main(String[] args) {
    Thread t1 = new Thread(() -> {
        try {
            // 如果当lock请求锁资源的同时被其他其他线程调用了interrupt方法
            // 就会进入异常状态
            lock.lockInterruptibly();
        } catch (InterruptedException e) {
            e.printStackTrace();
            log.debug("请求锁被打断");
            return; // 当请求锁被打断后没有获得锁，不应该在进入下面的语句
        }
        try {
            log.debug("获得到了锁资源");
        } finally {
            lock.unlock();
        }
    }, "t1");
    lock.lock();
    t1.start();
    try {
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    t1.interrupt();
}

ReentrantLock可以锁超时

synchronized请求锁会一直地等待锁资源，而ReentrantLock请求锁不会无限制的进行下去，使用tryLock()方法，可以在一次请求锁资源或请求一段时间的锁资源后结束请求锁。

    static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            log.debug("开始运行了");
            // 该情况下只会请求一次锁资源，请求到锁资源返回true，否则返回false
            // 加上时间限制的trylock是一样的。
            try {
                if (lock.tryLock(2, TimeUnit.SECONDS)) {
                    try {
                        log.debug("请求到了锁资源");
                    } finally {
                        lock.unlock();
                    }
                } else {
                    log.debug("没请求到锁资源");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "t1");
        lock.lock();
        log.debug("上锁了");
        t1.start();
        Thread.sleep(1000);
        log.debug("解锁了");
        lock.unlock();
    }

当一些线程一直无法获得锁资源时，使用公平锁就可以时获得锁变成先进先获得。
- ```
static ReentrantLock lock = new ReentrantLock(true);
```
条件变量，即ReentrantLock支持多条件的进入不同的WaitSet进行等待，synchronized就只有一个WaitSet队列。
- 当出现需要不同条件进入等待就可以使用该条件变量。
- ```
static ReentrantLock lock = new ReentrantLock();
static Condition waitCondition1Set = lock.newCondition();
// 然后调用condition的await方法即可进入等待状态。
```

设计模式—顺序执行

保证线程执行的一致性，因为这里是同一个锁对象所以不能用join

wait-notify

@Slf4j
public class Test5 {

    static boolean already = false;

    public static void main(String[] args) {
        final Object o = new Object();

        Thread t1 = new Thread(() -> {
            synchronized (o){
                while (!already){
                    try {
                        o.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("1");
            }
        }, "t1");
        Thread t2 = new Thread(() -> {
            synchronized (o){
                log.debug("2");
                already = true;
                o.notify();
            }
        }, "t2");
        t1.start();
        t2.start();
    }
}

park-unpark

@Slf4j
public class Test6 {
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            LockSupport.park();
            log.debug("1");
        }, "t1");
        Thread t2 = new Thread(() -> {
            LockSupport.park();
            log.debug("2");
            LockSupport.unpark(t1);
        }, "t2");
        t1.start();
        t2.start();
        Thread.sleep(2000);
        LockSupport.unpark(t2);
    }
}

设计模式—交替输出

三个线程交替输出

wait-notify

public class Test7 {
    public static void main(String[] args) {
        PrintObject object = new PrintObject(5,2);

        Thread t1 = new Thread(() -> {
            object.print(2,1);
        }, "t1");

        Thread t2 = new Thread(() -> {
            object.print(3,2);
        }, "t2");

        Thread t3 = new Thread(() -> {
            object.print(1,3);
        }, "t3");
        t1.start();
        t2.start();
        t3.start();
    }
}
class PrintObject{
    private int loopNum;
    private int flag;

    public PrintObject(int loopNum,int flag) {
        this.loopNum = loopNum;
        this.flag = flag;
    }

    public synchronized void print(int next,int now){
        for(int i = 0;i < loopNum;i++){
            while (flag != now){
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.println(now);
            flag = next;
            notifyAll();
        }
    }
}

await-signal

class PrintObject{
    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    private int loopNum;
    private int flag;

    public PrintObject(int loopNum,int flag) {
        this.loopNum = loopNum;
        this.flag = flag;
    }

    public void print(int next,int now){
        lock.lock();
        try{
            for(int i = 0;i < loopNum;i++){
                while (flag != now){
                    try {
                        condition.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(flag);
                flag = next;
                condition.signalAll();
            }
        }finally {
            lock.unlock();
        }
    }
}

park-unpark

@Slf4j
public class Test8 {
    static Thread t1;
    static Thread t2;
    static Thread t3;
    public static void main(String[] args) throws InterruptedException {
        t1 = new Thread(() -> {
            LockSupport.park();
            log.debug("1");
            LockSupport.unpark(t2);
        }, "t1");

        t2 = new Thread(() -> {
            LockSupport.park();
            log.debug("2");
            LockSupport.unpark(t3);
        }, "t2");

        t3 = new Thread(() -> {
            LockSupport.park();
            log.debug("3");
            LockSupport.unpark(t1);
        }, "t3");
        t1.start();
        t2.start();
        t3.start();
        Thread.sleep(1000);
        LockSupport.unpark(t1);
    }
}

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