... run() { readBuf(); } ...

  public void readBuf(){
    byte[] bytebuf=new byte[1024];
    int i=0;
    while(true){
      try{
        i=in.read(bytebuf);     //读到 byte[] bytebuf中
      }catch(IOException e){
        //...
      }
    }
  }

while(true){
  try{
    i=in.read(bytebuf);
    System.out.println("had read bytebuf and do smth ...");
  }catch(){
    //...
  }
}

ByteBuffer bytebuf=ByteBuffer.allocate(1024);   //allocateDirect(1024)也不行
bytebuf.put("new String to send\n".getBytes());
bytebuf.put("another string to send\n".getBytes());

byte[] buff=new buff[1024]; //如何取出到一个byte[]里面呢？
bytebuf.get(buff);          //我尝试了这两个步骤

//若取出的是byte[],则我可以输出到终端
System.out.println(new String(buff));

//结果：失败

          我查到原因了  是我程序里多了条语句 ...
          非常感谢 jiangns3000 大哥 的帮助 ....

  byte[] buff=new byte[len];
  bytebuf.rewind();
  bytebuf.get(buff);

public class BaseClient{
  private Socket client;
  ...
  private InputStream in;
  ...       ...               out;

  private Thread getDataThread;
  
  public BaseClient(String hostAddr, int hostPort){
    this.hostAddr=hostAddr;
    ...

    //创建一个读数据的线程
    getDataThread = new Thread(){
       public void run(){
         while((len=in.)!=-1){ getDataFunction(buff); }  //循环不停的读  [1]
       }
    }
    ...
  }

  //激活连接
  public setConnect(){
    try{
        client=new Socket(...);
        in=client.getInputStream();
        out=client.getOutputStream();
    }catch(...){}
  }

  //发送数据的方法 ...
  public void sendDataOut(byte[] buff){
    out.write(buff);
    out.flush();
  }

  //得到数据时 执行的函数 ...
  public void getDataFunction(byte[] buff){
    //供重载
  }

  //以及一些发生错误时 会被调用的方法 供重载
  ...(略)
}

我是想 能够随时停止、启动  socket

import java.net.*;
import java.io.*;

public class BaseClient{

  private static final int UNKNOWN_HOST_ERROR=0;
  private static final int CREATE_IO_ERROR=1;
  private static final int GET_DATA_ERROR=2;
  private static final int SEND_DATA_ERROR=3;
    
  private Socket client;

  private String hostAddr=null;
  private int    hostPort=0;

  private InputStream in;
  private OutputStream out;  

  private Thread getDataThread;
  
  private boolean connectionStatus=false;
  
  private boolean stopThread=false;
  
  /**
   *read in buffer
   */
  private byte[] buff;  
  private int len;
  
  public BaseClient(String hostAddr,int hostPort){
    this.hostAddr=hostAddr;
    this.hostPort=hostPort;
    createReadDataThread();
  }
  
  private void createReadDataThread(){
    getDataThread=new Thread(){
      public void run(){
        while(!stopThread){
          try{
            while((len=in.read(buff))!=-1){    //  出错在这里 ... 什么原因呢？
              getDataIn(buff);
            }
          }catch(IOException e){
            disconnect(GET_DATA_ERROR);
          }
        }
      }
    };  
  }
  
  private void disconnect(int connectionError){
    connectionStatus=false;
    try{
      in.close();
      out.close();
      client.close();
      stopThread=true;
    }catch(IOException e){}
    switch (connectionError){
      case UNKNOWN_HOST_ERROR:
        connectionError();
        break;
      case CREATE_IO_ERROR:
        connectionError();
        break;
      case GET_DATA_ERROR:
        getDataError();
        break;
      case SEND_DATA_ERROR:
        sendDataError();
    }
  }
  
  
    //////////////////////////////////////
    ///////// Connection Methods /////////
    //////////////////////////////////////
  
  
  public void setConnect(){
    try{
      client=new Socket(hostAddr,hostPort);
      out=client.getOutputStream();
      in=client.getInputStream();

      getDataThread.start();                        // 在这里才启动 线程
      
      connectionStatus=true;
      connectionReady();
      
    }catch(UnknownHostException unkhex){
      disconnect(UNKNOWN_HOST_ERROR);
    }catch(IOException ioex){
      disconnect(CREATE_IO_ERROR);
    }
  }
  
  public void setDisconnect(){
    connectionStatus=false;
    try{
      in.close();
      out.close();
      client.close();
      stopThread=true;
    }catch(IOException e){}
  }
  
  /**
   *send data out method
   */
  public void sendDataOut(byte[] buff){
    try{
      out.write(buff);
      out.flush();
    }catch(IOException e){
      disconnect(SEND_DATA_ERROR);
    }
  }
  
  public InputStream getInputStream(){
    return in;
  }
  
  public OutputStream getOutputStream(){
    return out;
  }
  

    //////////////////////////////////////
    ///////// Connection Events //////////
    //////////////////////////////////////
  
  public void connectionReady(){
    System.out.println("lxd.BaseClient.connectionReady()");
  }
    
  public void connectionError(){
    System.out.println("lxd.BaseClient.connectError()");
  }
  
  public void sendDataError(){
    System.out.println("lxd.BaseClient.sendDataError()");
  }
  
  public void getDataError(){
    System.out.println("lxd.BaseClient.geDataError()");
  }
  
  public void getDataIn(byte[] buff){
    System.out.println("lxd.BaseClient.getDataIn()");
  }
  //main()
  public static void main(String[] args){
    BaseClient baseclient=new BaseClient("192.168.0.22",1080);
    baseclient.setConnect();
  }  
}

lxd.BaseClient.connectionReady()
java.lang.NullPointerException
        at java.net.SocketInputStream.read(SocketInputStream.java:90)
        at BaseClient$1.run(BaseClient.java:42)
Press any key to continue...

byte[] buff;                           //这里错了  ... 一定要分配好长度 

该成

byte buff=new byte[1024];   //可以了