Java多线程设计模式(3)读写锁模式

1 Read-Write Lock Pattern

Read-Write Lock Pattern是一种将对于共享资源的访问与修改操作分离，称为读写分离。即访问是reader，修改是write，用单独的线程来处理。可以允许多个reader，但是不允许同时多个写入或者在读的过程中有写入。

由于对于实例状态的读取，并不会破坏状态的完整性且状态也不会修改，可以允许多个线程同时访问操作。但是若在写入的过程中，会更改实例的状态，此时就需要对于写入做保护，防止其他线程来进行读操作和写操作。

在多个线程共享一个实例的时候，会有参考实例状态的线程即仅仅读取实例状态的Reader参与者，并且也会有改变实例的状态的线程即Writer参与者，此时就需要运用这种模式。

  这种模式在读取不会冲突下，允许多个Reader参与者同时reader，提高了性能。不用单纯的在每次读的时候利用Synchronized来使得只能允许一个线程读操作，而是利用了外部定义的逻辑锁来设置允许多个Reader同时操作。在读取的操作比写入的操作多的时候，也可以使用这个模式。

  在这个模式中，主要有四个参与者，分别为：

1 Reader参与者。Reader参与者会对SharedResource参与者进行read。

2 Writer参与者。Writer参与者会对SharedResource参与者进行write。

3 SharedResource参与者。这个是Reader参与者与Writer参与者共享的资源。里面包含了提供不会改变内部状态的操作read，与会改变内部状态的操作write。注意对于这个参与者，不需要在进行read与write的方法加上synchronized。

在对于SharedResource参与者进行read和write操作时候，利用Before/After Pattern实现。

    前置处理(获取锁定)

      try{

      实际操作

       }finally{

        后续处理(释放锁)}

利用finally可以保证无论发生什么情况，都会释放锁，也不能把前置处理放置在try中，如果放置在里面，则finally就一定会执行，当前置处理发生异常，就不应该进而执行finally。

  4 ReadWriteLock参与者。提供了对于SharedResource参与者进行read和write时需要的锁，就是用户自定义的锁机制。包括readLock和readUnLock锁，writeLock和writeUnLock，注意必须要对这些操作进行同步处理，放置多个线程同时调用这些方法，在每个方法中必须加上synchronized。

实例：

有多个读者和多个写者共同操作一本书，只允许同时多个读者读取书，只允许同时一个写者写这本书，不允许同时读者与写者同时操作这本书籍。

SharedResource参与者。

在每次读写操作之间和之后都要获取锁与释放锁，不需要synchronized

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package  whut.readwritelock; public  class  Data {      private  final  char [] buffer;      private  final  ReadWriteLock lock= new  ReadWriteLock();                                                                                                                                                                                                                                                                                                           public  Data( int  size)      {          this .buffer= new  char [size];          for ( int  i= 0 ;i<buffer.length;i++)              buffer[i]= '*' ;      }                                                                                                                                                                                                                                                                                                           public  char [] read() throws  InterruptedException      {          lock.readLock();          try {              return  doRead();          } finally {              lock.readUnlock();          }      }                                                                                                                                                                                                                                                                                                           private  char [] doRead()      {          char [] newbuf= new  char [buffer.length];          for ( int  i= 0 ;i<buffer.length;i++)              newbuf[i]=buffer[i];          slowly();          return  newbuf;      }                                                                                                                                                                                                                                                                                                           public  void  write( char  c) throws  InterruptedException      {          lock.writeLock();          try {               doWrite(c);          } finally {              lock.writeUnlock();          }      }                                                                                                                                                                                                                                                                                                           private  void  doWrite( char  c)      {          for ( int  i= 0 ;i<buffer.length;i++)          {              buffer[i]=c;              slowly();              //这里的sleep并不会切换到别的线程              //这里就是体现了使用while的好处              //当该线程sleep时候，其余等待读取的还在wait中，而要写入的线程会判断它的状态，还没有释放锁          }      }                                                                                                                                                                                                                                                                                                           private  void  slowly()      {          try {              Thread.sleep( 50 );          } catch (InterruptedException e)          {                                                                                                                                                                                                                                                                                                                       }      }                                                                                                                                                                                                                                                                                                      }

读者线程Reader

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package  whut.readwritelock; public  class  ReaderThread  extends  Thread{      private  final  Data data;      public  ReaderThread(Data data)      {          this .data=data;      }                                                                                                                                                                                                                                                                                                  public  void  run()      {          try {              while ( true )              {                  char [] readbuf=data.read();                  System.out.println(Thread.currentThread().getName()                          + " reads " +String.valueOf(readbuf));              }          } catch (InterruptedException e)          {          }      } }

写者线程Writer

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package  whut.readwritelock; import  java.util.Random; public  class  WriterThread  extends  Thread{                                                                                                                                                                                                                                                                                          private  static  final  Random random= new  Random();      private  final  Data data;      private  final  String filler;      private  int  index= 0 ;                                                                                                                                                                                                                                                                                          public  WriterThread(Data data,String filler)      {          this .data=data;          this .filler=filler;      }                                                                                                                                                                                                                                                                                          public  void  run()      {          try {              while ( true )              {                  char  c=nextChar();                  data.write(c);                  Thread.sleep(random.nextInt( 1000 ));              }          } catch (InterruptedException e)          {                                                                                                                                                                                                                                                                                                      }      }                                                                                                                                                                                                                                                                                          private  char  nextChar()      {          char  c=filler.charAt(index);          index++;          if (index>=filler.length())              index= 0 ;          return  c;      } }

WriteReadLock类

 主要就是利用自定义锁机制来控制读写操作。设置读写时的警戒条件，利用Guarded Suspension Pattern来处理。

  设置有四个字段：

readingReaders表示实例当前被读取的线程数目。是readLock之后与readUnlock之间的数。作为写入操作的警戒条件之一。

  writingReaders表示实例当被写操作的线程数目，只能是0或者1，它是作为写入操作和读取操作的警戒条件。

waitingReaders表示多少个处于等待写入的线程数目。当ReaderThread比WriterThread多的时候，由于在要进行写入操作过程中，如果有读取操作，则会一直等待，但是读取操作没有设置为互斥，则他们会一个个的执行读操作，致使写入操作无法进行。设置这个字段状态目的就是为了使得ReaderThread在警戒条件判断中当waitingReaders大于0的时候，该ReaderThread能wait，能让出控制权给WriterThread。

preferWriter表示true为写优先，表示false为读优先。虽然读取能让出控制权给写入操作，可是当写入操作拥有了控制权后，则可能使得读取操作无法执行。此时就需要这个preferWriter字段，这个就是目的使得ReaderThread和WriterThread能够轮流执行的。

   当读操作完后，设置为true以使得写入操作有可能获得执行，不过写入操作必须在满足preferWriter为true以及有等待写入的操作下才能真正的写入。当写入操作完毕后，设置该字段为false使得读操作能够执行。

总结：

  读取操作等待的警戒条件是：当前写入线程大于0，或者preferWriter为true且有等待写入的线程

  写入操作等待的警戒条件是：当前写入线程大于0，或者当前读取线程大于0

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package  whut.readwritelock; //关键部分 public  class  ReadWriteLock {      private  int  readingReaders= 0 ; //实际正在读取的线程数目      private  int  waitingWriters= 0 ; //正在等待写入的线程数目      private  int  writingWriters= 0 ; //实际正在写入的线程数目      private  boolean  preferWriter= true ; //写入优先的话，值为true                    //读取的时候获取锁      public  synchronized  void  readLock() throws  InterruptedException      {          //当有写入的时候，或者写入为优先级并且有等待的写入线程          while (writingWriters> 0 ||(preferWriter&&waitingWriters> 0 ))          {              wait();          }          readingReaders++;      }                    //读完毕后释放锁      public  synchronized  void  readUnlock() throws  InterruptedException      {          readingReaders--;          preferWriter= true ;          notifyAll();      }                    //写入的时候获取锁      public  synchronized  void  writeLock() throws  InterruptedException      {          waitingWriters++; //正在等待的写入的线程数目          try {              //有写入或者读入的时候              while (readingReaders> 0 ||writingWriters> 0 )              {                  wait();              }          } finally {              waitingWriters--; //被唤醒了，则就是进而真正写入          }          writingWriters++;      }                    //写入毕后释放锁      public  synchronized  void  writeUnlock() throws  InterruptedException      {          writingWriters--;          preferWriter= false ; //写入后马上更换优先级，让读者继续          notifyAll();      } }

测试类：

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package  whut.readwritelock; public  class  ReadWriteMain {      public  static  void  main(String[] args) {          // TODO Auto-generated method stub          Data data= new  Data( 10 );          //读取线程          new  ReaderThread(data).start();          new  ReaderThread(data).start();          new  ReaderThread(data).start();          new  ReaderThread(data).start();          new  ReaderThread(data).start();          new  ReaderThread(data).start();                      //写入线程          new  WriterThread(data, "ABCDEFGHIJKLMNOPQRSTUVWXYZ" ).start();          new  WriterThread(data, "abcdefghijklmnopqrstuvwxyz" ).start();      } }

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