• 可中断
• 可以设置超时时间
• 可以设置为公平锁
• 支持多个条件变量
• 与 synchronized 一样都支持可重入

基本用法

• 获取和释放：

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

• 可重入：

  // 可重入演示：
  static ReentrantLock lock = new ReentrantLock();
  public static void main(String[] args) {
  	 method1();
  }
  public static void method1() {
  		 lock.lock();
  		 try {
  				 log.debug("execute method1");
  				 method2();
  		 } finally {
  				 lock.unlock();
  		 }
  }
  public static void method2() {
  		 lock.lock();
  		 try {
  				 log.debug("execute method2");
  				 method3();
  		 } finally {
  				 lock.unlock();
  		 }
  }
  public static void method3() {
  		 lock.lock();
  		 try {
  				 log.debug("execute method3");
  		 } finally {
  				 lock.unlock();
  		 }
  }
  

• 可打断演示：

  private static void test1() {
      ReentrantLock lock = new ReentrantLock();
  
      Thread t1 = new Thread(() -> {
          log.debug("启动...");
          try {
              lock.lockInterruptibly();
          } catch (InterruptedException e) {
              e.printStackTrace();
              log.debug("等锁的过程中被打断");
              return;
          }
          try {
              log.debug("获得了锁");
          } finally {
              lock.unlock();
          }
      }, "t1");
  
      lock.lock();
      log.debug("获得了锁");
      t1.start();
      try {
          sleep(1);
          t1.interrupt();
          log.debug("执行打断");
      } finally {
          lock.unlock();
      }
  }
  

• 锁超时（立刻失败）

  ReentrantLock lock = new ReentrantLock();
  Thread t1 = new Thread(() -> {
   log.debug("启动...");
   if (!lock.tryLock()) {
   log.debug("获取立刻失败，返回");
   return;
   }
   try {
   log.debug("获得了锁");
   } finally {
   lock.unlock();
   }
  }, "t1");
  lock.lock();
  log.debug("获得了锁");
  t1.start();
  try {
   sleep(2);
  } finally {
   lock.unlock();
  }
  

• 锁超时（超时失败）

  ReentrantLock lock = new ReentrantLock();
  Thread t1 = new Thread(() -> {
   log.debug("启动...");
   try {
   if (!lock.tryLock(1, TimeUnit.SECONDS)) {
   log.debug("获取等待 1s 后失败，返回");
   return;
   }
   } catch (InterruptedException e) {
   e.printStackTrace();
   }
   try {
   log.debug("获得了锁");
   } finally {
   lock.unlock();
   }
  }, "t1");
  lock.lock();
  log.debug("获得了锁");
  t1.start();
  try {
   sleep(2);
  } finally {
   lock.unlock();
  }
  

使用 ReentrantLock 解决哲学家就餐问题

public class TestDeadLock {
    public static void main(String[] args) {
        Chopstick c1 = new Chopstick("1");
        Chopstick c2 = new Chopstick("2");
        Chopstick c3 = new Chopstick("3");
        Chopstick c4 = new Chopstick("4");
        Chopstick c5 = new Chopstick("5");
        new Philosopher("苏格拉底", c1, c2).start();
        new Philosopher("柏拉图", c2, c3).start();
        new Philosopher("亚里士多德", c3, c4).start();
        new Philosopher("赫拉克利特", c4, c5).start();
        new Philosopher("阿基米德", c5, c1).start();
    }
}

@Slf4j(topic = "c.Philosopher")
class Philosopher extends Thread {
    Chopstick left;
    Chopstick right;

    public Philosopher(String name, Chopstick left, Chopstick right) {
        super(name);
        this.left = left;
        this.right = right;
    }

    @Override
    public void run() {
        while (true) {
            //　尝试获得左手筷子
            if (left.tryLock()) {
                try {
                    // 尝试获得右手筷子
                    if (right.tryLock()) {
                        try {
                            eat();
                        } finally {
                            right.unlock();
                        }
                    }
                } finally {
                    left.unlock();
                }
            }
        }
    }

    private void eat() {
        log.debug("eating...");
        Sleeper.sleep(1);
    }
}

class Chopstick extends ReentrantLock {
    String name;

    public Chopstick(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "筷子{" + name + '}';
    }
}

公平性

• ReentrantLock 默认是不公平的，可以通过构造方法来设置公平性；
• 公平锁一般没有必要，会降低并发度。

条件变量 Condition

@Slf4j(topic = "c.Test24")
public class Test24 {
    static final Object room = new Object();
    static boolean hasCigarette = false;
    static boolean hasTakeout = false;
    static ReentrantLock ROOM = new ReentrantLock();
    // 等待烟的休息室
    static Condition waitCigaretteSet = ROOM.newCondition();
    // 等外卖的休息室
    static Condition waitTakeoutSet = ROOM.newCondition();

    public static void main(String[] args) {

        new Thread(() -> {
            ROOM.lock();
            try {
                log.debug("有烟没？[{}]", hasCigarette);
                while (!hasCigarette) {
                    log.debug("没烟，先歇会！");
                    try {
                        waitCigaretteSet.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("可以开始干活了");
            } finally {
                ROOM.unlock();
            }
        }, "小南").start();

        new Thread(() -> {
            ROOM.lock();
            try {
                log.debug("外卖送到没？[{}]", hasTakeout);
                while (!hasTakeout) {
                    log.debug("没外卖，先歇会！");
                    try {
                        waitTakeoutSet.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                log.debug("可以开始干活了");
            } finally {
                ROOM.unlock();
            }
        }, "小女").start();

        sleep(1);
        new Thread(() -> {
            ROOM.lock();
            try {
                hasTakeout = true;
                waitTakeoutSet.signal();
            } finally {
                ROOM.unlock();
            }
        }, "送外卖的").start();

        sleep(1);

        new Thread(() -> {
            ROOM.lock();
            try {
                hasCigarette = true;
                waitCigaretteSet.signal();
            } finally {
                ROOM.unlock();
            }
        }, "送烟的").start();
    }

}