java 多线程饥饿现象的问题解决方法

java 多线程饥饿现象的问题解决方法

当有线程正在读的时候，不允许写 线程写，但是允许其他的读线程进行读。有写线程正在写的时候，其他的线程不应该读写。为了防止写线程出现饥饿现象，当线程正在读，如果写线程请求写，那么应该禁止再来的读线程进行读。

实现代码如下：

File.Java

package readerWriter; 

public class File {
private String name;
public File(String name)
{
  this.name=name; 

}
}

Pool.java

package readerWriter; 

public class Pool {
private int readerNumber=0;
private int writerNumber=0;
private boolean waittingWriten; 

public boolean isWaittingWriten() {
  return waittingWriten;
}
public void setWaittingWriten(boolean waittingWriten) {
  this.waittingWriten = waittingWriten;
} 

public File getFile() {
  return file;
}
public void setFile(File file) {
  this.file = file;
}
File file;
public Pool(File file)
{
  this.file=file; 

}
public int getReaderNumber() {
  return readerNumber;
}
public void setReaderNumber(int readerNumber) {
  this.readerNumber = readerNumber;
}
public int getWriterNumber() {
  return writerNumber;
}
public void setWriterNumber(int writerNumber) {
  this.writerNumber = writerNumber;
} 

}

Reader.java

package readerWriter; 

public class Reader implements Runnable{ 

  private String id;
  private Pool pool; 

  public Reader(String id,Pool pool)
  {
    this.id=id;
    this.pool=pool;
  } 

  @Override
  public void run()
  {
    // TODO Auto-generated method stub
    while(!Thread.currentThread().interrupted()){ 

    synchronized(pool){ 

        while(pool.getWriterNumber()>0 || pool.isWaittingWriten()==true)//当线程正在写或者
                                        //有线程正在等待写，则禁止读线程继续读
        { 

              try {
                pool.wait();
              } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
              } 

        } 

      { 

        pool.setReaderNumber(pool.getReaderNumber()+1);  

      }
    }
     System.out.println(id+" "+"is reading...."); 

     try {
      Thread.sleep(1000);
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } 

    synchronized(pool)
    {
      pool.setReaderNumber(pool.getReaderNumber()-1);
      System.out.println(id+"  "+"is existing the reader....");
      if(pool.getReaderNumber()==0)
          pool.notifyAll();
    } try {
      Thread.sleep(1000);
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }
    // pool.notifyAll(); 

  } 

  } 

}

Writer.java

package readerWriter; 

public class Writer implements Runnable{
  private Pool pool;
  String id;
  public Writer(String id,Pool pool)
  {
    this.id=id;
    this.pool=pool; 

  }
  @Override
  public void run() {
    // TODO Auto-generated method stub
    while(!Thread.currentThread().interrupted()){ 

    synchronized(pool){
      if(pool.getReaderNumber()>0)
        pool.setWaittingWriten(true);
      else
        pool.setWaittingWriten(false); 

      //当线程正在被读或者被写或者有线程等待读 

        while(pool.getWriterNumber()>0 ||  pool.getReaderNumber()>0)
        {
          try {
            pool.wait();
          } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
          } 

        }
      pool.setWaittingWriten(false);  //这个策略还算公平
      { 

        pool.setWriterNumber(pool.getWriterNumber()+1); 

      }
    }
     System.out.println(id+" "+"is writing...."); 

     try {
      Thread.sleep(1000);
    } catch (InterruptedException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } 

     //
    synchronized(pool)
    { 

      pool.setWriterNumber(pool.getWriterNumber()-1);
      System.out.println(id+"  "+"is existing the writer....");
      pool.notifyAll();
    }
     /* try {
        Thread.sleep(1000);
        //System.out.println("writer sleeping over");
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      } */ 

  } 

}
}

Main.java

package readerWriter; 

public class Main { 

  public static void main(String[] args) {
    // TODO Auto-generated method stub
Pool pool=new Pool(new File("dd file"));
for(int i=0;i<2;i++)
{
 Thread writer=new Thread(new Writer("writer "+i,pool));
 writer.start();
}
for(int i=0;i<5;i++)
{ 

  Thread reader=new Thread(new Reader("reader "+i,pool));
  reader.start(); 

} 

  } 

}

程序部分运行结果如下：

writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
reader 0 is reading....
reader 0  is existing the reader....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
reader 3 is reading....
reader 2 is reading....
reader 4 is reading....
reader 1 is reading....
reader 0 is reading....
reader 3  is existing the reader....
reader 1  is existing the reader....
reader 0  is existing the reader....
reader 4  is existing the reader....
reader 2  is existing the reader....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 1 is writing....
writer 1  is existing the writer....
reader 2 is reading....
reader 2  is existing the reader....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....
writer 0  is existing the writer....
writer 0 is writing....

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