JDK并发包[同步控制]

Posted 2021-03-02 yuanjiehe

重入锁ReentrantLock[Re-entrant Lock]

1. ReentrantLock与Synchronized区别：

   1. 重入锁可以反复进入

      lock.lock();
      lock.lock();
      try{
      	i++;
      }finally{
      	lock.unlock();
      	lock.unlock();
      }
      

          //RentrantLock.java JDK8
      	/**
           * Acquires the lock.
           *
           * <p>Acquires the lock if it is not held by another thread and returns
           * immediately, setting the lock hold count to one.
           *
           * <p>If the current thread already holds the lock then the hold
           * count is incremented by one and the method returns immediately.
           *
           * <p>If the lock is held by another thread then the
           * current thread becomes disabled for thread scheduling
           * purposes and lies dormant until the lock has been acquired,
           * at which time the lock hold count is set to one.
           */
          public void lock() {
              sync.lock();
          }
      
          /**
           * Attempts to release this lock.
           *
           * <p>If the current thread is the holder of this lock then the hold
           * count is decremented.  If the hold count is now zero then the lock
           * is released.  If the current thread is not the holder of this
           * lock then {@link IllegalMonitorStateException} is thrown.
           *
           * @throws IllegalMonitorStateException if the current thread does not
           *         hold this lock
           */
          public void unlock() {
              sync.release(1);
          }
      

   2. 如果同一个线程多次获得锁，那么在释放锁的时候，也需要释放相同次数

   3. 若释放锁的次数过多，将会得到 java.lang.IllegalMonitorStateException异常

2. 高级功能

   1. 中断响应[lockInterruptibly()]：

      如果一个线程在等待锁，那么它依然可以收到一个通知，被告知无需再等待，可以停止工作了。

      public class IntLock implements Runnable {
          public static ReentrantLock lock1 = new ReentrantLock();
          public static ReentrantLock lock2 = new ReentrantLock();
          int lock;
      
          public IntLock(int lock) {
              this.lock = lock;
          }
      
          @Override
          public void run() {
              try {
                  if (lock == 1) {
                      lock1.lockInterruptibly();
                      try {
                          Thread.sleep(500);
                      } catch (InterruptedException e) {
                      }
                      lock2.lockInterruptibly();
                  } else {
                      lock2.lockInterruptibly();
                      try {
                          Thread.sleep(500);
                      } catch (InterruptedException e) {
                      }
                      lock1.lockInterruptibly();
                  }
              } catch (InterruptedException e) {
                  e.printStackTrace();
              } finally {
                  if (lock1.isHeldByCurrentThread()) {
                      lock1.unlock();
                  }
                  if (lock2.isHeldByCurrentThread()) {
                      lock2.unlock();
                  }
                  System.out.println(Thread.currentThread().getId() + ":线程退出");
              }
          }
      
          public static void main(String[] args) throws InterruptedException {
              IntLock r1 = new IntLock(1);
              IntLock r2 = new IntLock(2);
              Thread t1 = new Thread(r1);
              Thread t2 = new Thread(r2);
              t1.start();
              t2.start();
              Thread.sleep(1000);
              t2.interrupt();
          }
      }
      

   2. 锁申请等待限时[tryLock()]：

      public class TimeLock implements Runnable{
          public static ReentrantLock lock = new ReentrantLock();
          @Override
          public void run(){
              try{
                  if(lock.tryLock(5, TimeUnit.SECONDS)){
                      Thread.sleep(6000);
                  }else{
                      System.out.println("get lock failed");
                  }
              }catch (InterruptedException e){
                  e.printStackTrace();
              }finally {
                  if(lock.isHeldByCurrentThread()){
                      lock.unlock();
                  }
              }
          }
      
          public static void main(String[] args) {
              TimeLock tl = new TimeLock();
              Thread t1 = new Thread(tl);
              Thread t2 = new Thread(tl);
              t1.start();
              t2.start();
          }
      }
      

   3. 公平锁[public ReentrantLock(boolean fair)]：

      1. 大多数情况下，锁的申请都是非公平的。系统每次从锁的等待队列中随机挑选一个。
      2. 当ReentrantLock 的fair参数为true时，锁是公平的。
      3. 公平锁需要系统维护一个有序队列，实现成本高，性能相对低下。

3. 方法总结

   ReentrantLock重要方法	作用
   lock()	获得锁，若锁以及被占用，则等待
   lockInterruptibly()	获得锁，但是优先响应中断
   tryLock()	尝试获得锁
   tryLock(long time, TimeUnit unit)	在time时间内尝试获得锁
   unLock()	释放锁

重入锁的搭档：Condition条件

1. await() 方法会使当前线程等待，同时释放锁，当其他线程使用signal()或者signalAll()方法时，线程会重新获得锁并重新执行。当线程中断时，也能挑出等待。
2. awautUninterruptibly() 方法与await() 方法基本相同，但是并不会在等待过程中响应中断。
3. signal() 方法用于唤醒一个在等待中的线程。
4. signalAll() 唤醒所有在等待中的线程。

允许多个线程同时访问：Semaphore信号量

1. 信号量演示例子

   public class SempDemo implements Runnable{
       final Semaphore semp = new Semaphore(5);
       @Override
       public void run(){
           try{
               semp.acquire();
               Thread.sleep(2000);
            	System.out.println(Thread.currentThread().getId()+":done!");
               semp.release();
           }catch (InterruptedException e){
               e.printStackTrace();
           }
       }
   
       public static void main(String[] args) {
           ExecutorService exec = Executors.newFixedThreadPool(20);
           final SempDemo demo = new SempDemo();
           for(int i=0;i<20;i++){
               exec.submit(demo);
           }
       }
   }
   

2. 关键方法

   Semaphore方法	方法描述
   acquire()	从此信号量获取许可证，阻塞直到可用
   acquireUninterruptibly()	同acquire(), 但是不响应中断
   release()	释放一个许可证

ReadWriteLock读写锁

1. 特点：
   • 读-读不互斥；读-读之间不阻塞
   • 读-写互斥；读阻塞写，写阻塞读
   • 写-写互斥；写-写阻塞

倒计时器：CountDownLatch

1. 作用：允许一个或多个线程等待直到其他线程完成的同步辅助
2. 使用给定的计数初始化，await方法阻塞，直到由于countDown方法的调用导致当前计数器计数清零，之后所有等待线程被释放

循环栅栏：CyclicBarrier

1. 作用：允许一组线程全部等待彼此达到共同屏蔽点的同步辅助，可重复使用

线程阻塞工具类：LockSupport

1. 作用：可以在线程内任意位置让线程阻塞