线程读写文件

多进程读写文件：一个进程A写文件file，另一个进程B读文件file
Doug Lea 在他的书中提供一个示例代码
ReadWrite 为抽象类，允许并发的读操作，不允许并发的写操作，也不允许读写同时进行。
可以扩展ReadWrite为SingleFileReadWrite 实现 doRead和doWrite方法来读写文件
线程A和线程B使用同一个SingleFileReadWrite 实例

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SingleFileReadWrite rw;
threadB rw.read();
threadA rw.write();

Java代码?

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abstract class ReadWrite {    protected int activeReaders = 0;  // threads executing read    protected int activeWriters = 0;  // always zero or one      protected int waitingReaders = 0; // threads not yet in read    protected int waitingWriters = 0; // same for write      protected abstract void doRead(); // implement in subclasses    protected abstract void doWrite();      public void read() throws InterruptedException {     beforeRead();     try   { doRead(); }     finally { afterRead(); }    }      public void write() throws InterruptedException {     beforeWrite();     try     { doWrite(); }     finally { afterWrite(); }    }    protected boolean allowReader() {     return waitingWriters == 0 && activeWriters == 0;    }      protected boolean allowWriter() {     return activeReaders == 0 && activeWriters == 0;    }      protected synchronized void beforeRead()     throws InterruptedException {      ++waitingReaders;      while (!allowReader()) {       try { wait(); }       catch (InterruptedException ie) {         --waitingReaders; // roll back state         throw ie;       }     }     --waitingReaders;     ++activeReaders;    }      protected synchronized void afterRead()  {     --activeReaders;     notifyAll();    }      protected synchronized void beforeWrite()     throws InterruptedException {      ++waitingWriters;      while (!allowWriter()) {       try { wait(); }       catch (InterruptedException ie) {        --waitingWriters;        throw ie;       }      }      --waitingWriters;      ++activeWriters;    }      protected synchronized void afterWrite() {     --activeWriters;     notifyAll();     }   }  

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