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);
    }
}
posted @ 2022-05-06 19:58  大队长11  阅读(35)  评论(0编辑  收藏  举报