读写锁小总结

今天小组组织了一次java性能培训，提到了读写锁，以前没有关注过这个东西，于是培训结束后写个程序探索一下读写锁的使用方式。

 

final ReadWriteLock lock = new ReentrantReadWriteLock();

ReadWriteLock具有两把锁，lock.readLock()和lock.writeLock()，特性总结如下：

1、readLock与readLock之间不是互斥的，可以并发执行。实验代码如下：

import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; public class Main {    public static void main(String[] args)    {       final ReadWriteLock lock = new ReentrantReadWriteLock();       new Thread(new Runnable()       {           @Override           public void run()           {               lock.readLock().lock();               System.out.println(Thread.currentThread().getName() + ":Sleep begin.");               // 休眠5秒               sleepInSeconds(5);               System.out.println(Thread.currentThread().getName()+ ":Sleep complete.");               lock.readLock().unlock();            }        }, "Thread-1").start();         //此处休眠1秒，确保上面的代码得以先执行       sleepInSeconds(1);         new Thread(new Runnable()       {           @Override           public void run()          {              lock.readLock().lock();              System.out.println(Thread.currentThread().getName()+ ":Sleep begin.");              // 休眠2秒              sleepInSeconds(2);              System.out.println(Thread.currentThread().getName() + ":Sleep complete.");              lock.readLock().unlock();         }      }, "Thread-2").start();  }    /**     * 休眠一定时间，单位5秒     * @param seconds 要休眠的秒数     */     private static void sleepInSeconds(int seconds)     {         try         {             Thread.sleep(seconds * 1000);         }         catch (InterruptedException e)         {             e.printStackTrace();         }      } }

Thread-1:Sleep begin. Thread-2:Sleep begin. Thread-2:Sleep complete. Thread-1:Sleep complete.

 

根据输出可以看出，虽然Thread-1已经调用了，lock.readLock().lock()，但是Thread-2还是在Thread-1调用lock.readLock().unlock()之前就执行打印了，说明两个readLock之间是互不影响的。

 

2、readLock和writeLock是互斥的，实验代码如下：

import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; public class Main {    public static void main(String[] args)    {       final ReadWriteLock lock = new ReentrantReadWriteLock();       new Thread(new Runnable()       {           @Override           public void run()           {               lock.readLock().lock();               System.out.println(Thread.currentThread().getName() + ":Sleep begin.");               // 休眠5秒               sleepInSeconds(5);               System.out.println(Thread.currentThread().getName()+ ":Sleep complete.");               lock.readLock().unlock();            }        }, "Thread-1").start();         //此处休眠1秒，确保上面的代码得以先执行       sleepInSeconds(1);         new Thread(new Runnable()       {           @Override           public void run()          {              lock.writeLock().lock();              System.out.println(Thread.currentThread().getName()+ ":Sleep begin.");              // 休眠2秒              sleepInSeconds(2);              System.out.println(Thread.currentThread().getName() + ":Sleep complete.");              lock.writeLock().unlock();         }      }, "Thread-2").start();  }    /**     * 休眠一定时间，单位5秒     * @param seconds 要休眠的秒数     */     private static void sleepInSeconds(int seconds)     {         try         {             Thread.sleep(seconds * 1000);         }         catch (InterruptedException e)         {             e.printStackTrace();         }      } }

Thread-1:Sleep begin. Thread-1:Sleep complete. Thread-2:Sleep begin. Thread-2:Sleep complete.

 

3、writeLock与writeLock之间是互斥的，实验代码如下

import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; public class Main {    public static void main(String[] args)    {       final ReadWriteLock lock = new ReentrantReadWriteLock();       new Thread(new Runnable()       {           @Override           public void run()           {               lock.writeLock().lock();               System.out.println(Thread.currentThread().getName() + ":Sleep begin.");               // 休眠5秒               sleepInSeconds(5);               System.out.println(Thread.currentThread().getName()+ ":Sleep complete.");               lock.writeLock().unlock();            }        }, "Thread-1").start();         //此处休眠1秒，确保上面的代码得以先执行       sleepInSeconds(1);         new Thread(new Runnable()       {           @Override           public void run()          {              lock.writeLock().lock();              System.out.println(Thread.currentThread().getName()+ ":Sleep begin.");              // 休眠2秒              sleepInSeconds(2);              System.out.println(Thread.currentThread().getName() + ":Sleep complete.");              lock.writeLock().unlock();         }      }, "Thread-2").start();  }    /**     * 休眠一定时间，单位5秒     * @param seconds 要休眠的秒数     */     private static void sleepInSeconds(int seconds)     {         try         {             Thread.sleep(seconds * 1000);         }         catch (InterruptedException e)         {             e.printStackTrace();         }      } }

Thread-1:Sleep begin. Thread-1:Sleep complete. Thread-2:Sleep begin. Thread-2:Sleep complete.

 

由于readLock与writeLock是互斥的，而readLock与readLock之间是非互斥的，产生了一种猜想，假设有这么一种场景：

Thread-1在第0秒的时候执行了readLock().lock()

Thread-1在第5秒的时候执行了readLock().unlock()

 

Thread-2在第2秒的时候执行了writeLock().lock()

Thread-2在第7秒的时候执行了writeLock().unlock()

 

Thread-3在第4秒的时候执行了writeLock().lock()

Thread-3在第9秒的时候执行了writeLock().unlock()

 

由于Thread-2的writeLock与Thread-1的readLock是互斥的，所以在第5秒之前Thread-2的writeLock之间的代码都不会执行，但Thread-1和Thread-3都是读锁，会不会出现Thread-3的readLock之间的代码先于Thread-2的writeLock之间的代码执行的情况，做个实验便知

 

import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; public class Main {  private static long beginTime;  public static void main(String[] args)  {   final ReadWriteLock lock = new ReentrantReadWriteLock();   beginTime = System.currentTimeMillis();   new Thread(new Runnable()   {    @Override    public void run()    {     System.out.println(Thread.currentThread().getName()+ " begin running."+getTime());     lock.writeLock().lock();     System.out.println(Thread.currentThread().getName()+ ":Sleep begin."+getTime());     // 休眠5秒     sleepInSeconds(5);     System.out.println(Thread.currentThread().getName()+ ":Sleep complete."+getTime());     lock.writeLock().unlock();    }   }, "Thread-1").start();      //此处休眠1秒，确保上面的代码得以先执行   sleepInSeconds(2);   new Thread(new Runnable()   {    @Override    public void run()    {     System.out.println(Thread.currentThread().getName()+ " begin running."+getTime());     lock.writeLock().lock();     System.out.println(Thread.currentThread().getName()+ ":Sleep begin."+getTime());     // 休眠2秒     sleepInSeconds(5);     System.out.println(Thread.currentThread().getName()+ ":Sleep complete."+getTime());     lock.writeLock().unlock();    }   }, "Thread-2").start();      //此处休眠1秒，确保上面的代码得以先执行   sleepInSeconds(2);   new Thread(new Runnable()   {    @Override    public void run()    {     System.out.println(Thread.currentThread().getName()+ " begin running."+getTime());     lock.readLock().lock();     System.out.println(Thread.currentThread().getName()+ ":Sleep begin."+getTime());     // 休眠2秒     sleepInSeconds(5);     System.out.println(Thread.currentThread().getName()+ ":Sleep complete."+getTime());     lock.readLock().unlock();    }   }, "Thread-3").start();  }  /**   * 休眠一定时间，单位5秒   * @param seconds 要休眠的秒数   */  private static void sleepInSeconds(int seconds)  {   try   {    Thread.sleep(seconds * 1000);   }   catch (InterruptedException e)   {    e.printStackTrace();   }  }    /**   * 获取时间字符串   * @return   */  private static String getTime()  {   return " Time:"+(System.currentTimeMillis() - beginTime)/1000;  } }

Thread-1 begin running. Time:0 Thread-1:Sleep begin. Time:0 Thread-2 begin running. Time:2 Thread-3 begin running. Time:4 Thread-1:Sleep complete. Time:5 Thread-2:Sleep begin. Time:5 Thread-2:Sleep complete. Time:10 Thread-3:Sleep begin. Time:10 Thread-3:Sleep complete. Time:15

 

从实验结果来看，Thread-1，Thread-2，Thread-3的代码已经按照原先设计在分别在0秒，2秒，4秒的时刻被调用，但是其lock部分的代码却是：Thread-1的lock代码先执行，Thread-1执行完了才执行Thread-2的lock代码，Thread-2执行完了才执行Thread-3的lock代码。

 

之前猜想的Thread-3的lock代码先于Thread-2的lock代码执行的情况没有出现，也就是说虽然当前正在执行的代码是readLock的代码，但如果存在一个writeLock排队，那么其它readLock是无法与当前正在执行的readLock并行执行的。

 

欢迎拍砖