java concurrent之ReentrantLock

在编码的过程中，有时候我们不得不借助锁同步来保证线程安全。synchronized关键字在上一篇博客中已经介绍；自从JDK5开始，添加了另一种锁机制：ReentrantLock。

二者的区别

1、lock是jdk5之后代码层面实现的，synchronized是JVM层面实现的。

2、synchronized在出现异常的时候能够自动释放锁，而lock必须在finally块中unlock()主动释放锁，否则会死锁。

3、在竞争不激烈的时候synchronized的性能是比lock好一点的，但是当竞争很激烈时synchronized的性能会相对几十倍的下降，因为lock用了新的锁机制，新的Lock机制最终归结到一个原子性操作上。

4、synchronized无法中断一个正在等候获得锁的线程，也无法通过投票得到锁，如果不想等下去，也就没法得到锁；而lock可以。

5、ReentrantLock可以采用FIFO的策略进行竞争，更加公平。

基本用法

先写个简单的例子看一下：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
public class TestReentrantLock {
 public static void main(String[] args) {
  final ReentrantLock rLock = new ReentrantLock();
  final Condition condition = rLock.newCondition();
  ExecutorService executorService = Executors.newFixedThreadPool(5);
  Runnable opt = new Runnable() {
   @Override
   public void run() {
    rLock.lock();
    System.out.println(Thread.currentThread().getName()+"--->>lock()");
    try {
     condition.await();
    } catch (InterruptedException e) {
     e.printStackTrace();
    } finally {
     System.out.println(Thread.currentThread().getName()+"--->>unlock()");
     rLock.unlock();
    }
   }
  };
  
  for (int i = 0; i < 4; i++) {
   executorService.submit(opt);
  }
  
  Runnable release = new Runnable() {
   @Override
   public void run() {
    rLock.lock();
    try {
     Thread.sleep(2000);
     System.out.println(Thread.currentThread().getName()+"--->>signalAll()");
     condition.signalAll();
    } catch (InterruptedException e) {
     e.printStackTrace();
    } finally {
     rLock.unlock();
    }
   }
  };
  executorService.submit(release);
  executorService.shutdown();
 }
}
           

运行结果：

pool-1-thread-1--->>lock()

pool-1-thread-2--->>lock()

pool-1-thread-3--->>lock()

pool-1-thread-4--->>lock()

pool-1-thread-5--->>signalAll()

pool-1-thread-1--->>unlock()

pool-1-thread-2--->>unlock()

pool-1-thread-3--->>unlock()

pool-1-thread-4--->>unlock()

上面代码中有个Condition，它的三个方法await 、 signal 和 signalAll，与基类的wait、notify和notifyAll方法相对应，因为它们不能覆盖Object上的对应方法，所以就起了这三个奇葩的名字。由上面的代码可以看出ReentrantLock和synchronized用法是基本相同的。

中断ReentrantLock

实例如下：

package co.etc.concurrent;
import java.util.concurrent.locks.ReentrantLock;
public class ReentrantLockSample {
 public static void main(String[] args) {
  testReentrantLock();
 }
 public static void testReentrantLock() {
  final SampleSupportLock support = new SampleSupportLock();
  Thread first = new Thread(new Runnable() {
   public void run() {
    try {
     support.doSomething();
    } catch (InterruptedException e) {
     e.printStackTrace();
    }
   }
  });
  Thread second = new Thread(new Runnable() {
   public void run() {
    try {
     support.doSomething();
    } catch (InterruptedException e) {
     System.out.println("InterruptedException--->>");
    }
   }
  });
  executeTest(first, second);
 }
 public static void executeTest(Thread a, Thread b) {
  a.start();
  try {
   Thread.sleep(100);
   b.start();
   Thread.sleep(1000);
   System.out.println("---->>>interrupt()");
   b.interrupt();
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
 }
}
abstract class SampleSupport {
 protected int counter;
 public void startTheCountdown() {
  long currentTime = System.currentTimeMillis();
  for (;;) {
   long diff = System.currentTimeMillis() - currentTime;
   if (diff > 2000) {
    break;
   }
  }
 }
}
class SampleSupportLock extends SampleSupport {
 private final ReentrantLock lock = new ReentrantLock();
 public void doSomething() throws InterruptedException {
  lock.lockInterruptibly();
  System.out.println(Thread.currentThread().getName()
    + "doSomething()--->>");
  startTheCountdown();
  try {
   counter++;
  } finally {
   lock.unlock();
  }
  System.out.println("counter---->>>"+counter);
 }
}
           

运行结果：

Thread-0doSomething()--->>

---->>>interrupt()

InterruptedException--->>

counter---->>>1

运行结果表明第二个线程被中断了，这是因为我用的是lock.lockInterruptibly();在主线程中我调用了b.interrupt();二synchronized是没法做到的

公平性

实例代码如下：

import java.util.Collection;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class TestFairLock {
 private static Lock fairLock = new ReentrantLock2(true);
 private static Lock unfairLock = new ReentrantLock2();
 public static void main(String[] args) {
  TestFairLock testFairLock = new TestFairLock();
  // testFairLock.unfair();
  testFairLock.fair();
 }
 public void fair() {
  System.out.println("fair version");
  for (int i = 0; i < 5; i++) {
   Thread thread = new Thread(new Job(fairLock)) {
    public String toString() {
     return getName();
    }
   };
   thread.setName("" + i);
   thread.start();
  }
  // sleep 5000ms
 }
 public void unfair() {
  System.out.println("unfair version");
  for (int i = 0; i < 5; i++) {
   Thread thread = new Thread(new Job(unfairLock)) {
    public String toString() {
     return getName();
    }
   };
   thread.setName("" + i);
   thread.start();
  }
  // sleep 5000ms
 }
 private static class Job implements Runnable {
  private Lock lock;
  public Job(Lock lock) {
   this.lock = lock;
  }
  @Override
  public void run() {
   for (int i = 0; i < 5; i++) {
    lock.lock();
    try {
     System.out.println("Thread--->>"
       + Thread.currentThread().getName());
    } finally {
     lock.unlock();
    }
   }
  }
 }
 private static class ReentrantLock2 extends ReentrantLock {
  private static final long serialVersionUID = 1773716895097002072L;
  public ReentrantLock2(boolean b) {
   super(b);
  }
  public ReentrantLock2() {
   super();
  }
  public Collection<Thread> getQueuedThreads() {
   return super.getQueuedThreads();
  }
 }
}
           

运行结果

unfair version

Thread--->>0

Thread--->>0

Thread--->>0

Thread--->>0

Thread--->>0

Thread--->>1

Thread--->>1

Thread--->>1

Thread--->>1

Thread--->>1

Thread--->>2

Thread--->>2

Thread--->>2

Thread--->>2

Thread--->>2

Thread--->>3

Thread--->>3

Thread--->>3

Thread--->>3

Thread--->>3

Thread--->>4

Thread--->>4

Thread--->>4

Thread--->>4

Thread--->>4

fair version

Thread--->>0

Thread--->>0

Thread--->>1

Thread--->>3

Thread--->>0

Thread--->>4

Thread--->>2

Thread--->>1

Thread--->>3

Thread--->>0

Thread--->>4

Thread--->>2

Thread--->>1

Thread--->>3

Thread--->>0

Thread--->>4

Thread--->>2

Thread--->>1

Thread--->>3

Thread--->>4

Thread--->>2

Thread--->>1

Thread--->>3

Thread--->>4

Thread--->>2

从运行结果看到用ReentrantLock(boolean fair)构建的锁，相对ReentrantLock()是更公平的，当fair为true时采用的是FIFO策略，所

以各个线程能够更平均的分配时间。