Java编程思想学习笔记-使用显式的Lock对象

若要保证后台线程在trylock()之前运行得到锁，可加“屏障”，如下1,2,3步，而trylock()不管设定时间与否都不会阻塞主线程而是立即返回：

//: concurrency/AttemptLocking.java
// Locks in the concurrent library allow you
// to give up on trying to acquire a lock.
package concurrency;

import java.util.concurrent.*;
import java.util.concurrent.locks.*;

public class AttemptLocking {
  private ReentrantLock lock = new ReentrantLock();
  public void untimed() {
    boolean captured = lock.tryLock();
    try {
//        for(int i = 0; i < 10; i++) System.out.println("untime i: " + i);
      System.out.println("tryLock(): " + captured);
    } finally {
      if(captured)
        lock.unlock();
    }
  }
  public void timed() {
    boolean captured = false;
    try {
      captured = lock.tryLock(2, TimeUnit.SECONDS);
    } catch(InterruptedException e) {
      throw new RuntimeException(e);
    }
    try {
//        for(int i = 0; i < 10; i++) System.out.println("time i: " + i);
      System.out.println("tryLock(2, TimeUnit.SECONDS): " +
        captured);
    } finally {
      if(captured)
        lock.unlock();
    }
  }
  public static void main(String[] args) throws InterruptedException {
    final AttemptLocking al = new AttemptLocking();
    al.untimed(); // True -- lock is available
    al.timed();   // True -- lock is available
 // Now create a separate task to grab the lock:
    final CountDownLatch latch = new CountDownLatch(1);//1.增加一个"屏障"
    new Thread() {
        {
            setDaemon(false);
        }

        public void run() {
            al.lock.lock();
            System.out.println("acquired");
            latch.countDown();//2.屏障解除
        }
    }.start();
    Thread.yield(); // Give the 2nd task a chance
    latch.await();//3.阻塞在屏障处直到屏障解除
    al.untimed(); // False -- lock grabbed by task
    al.timed();   // False -- lock grabbed by task
  }
} /* Output:
tryLock(): true
tryLock(2, TimeUnit.SECONDS): true
acquired
tryLock(): false
tryLock(2, TimeUnit.SECONDS): false
*///:~