FileReader f=new FileReader("f:\\lissdy.txt");
BufferedReader bw=new BufferedReader(f);

String num=bw.readLine();

bw.close();

package com.lissdy;  
  
import java.io.BufferedReader;  
import java.io.IOException;  
import java.io.PrintWriter;  
  
import javax.servlet.http.HttpServlet;  
import javax.servlet.http.HttpServletRequest;  
import javax.servlet.http.HttpServletResponse;  
import java.io.*;  
@SuppressWarnings("serial")  
public class ReadFile extends HttpServlet{  
  public void doGet(HttpServletRequest req,HttpServletResponse res)  
  {  
    try {  
    res.setContentType("text/html;charset=gbk");  
    PrintWriter pw=res.getWriter();  
    FileReader f=new FileReader("d:\\lissdy.txt");  
    BufferedReader bw=new BufferedReader(f);  
    String num=bw.readLine();  
    bw.close();  
    pw.println("从文件中读取的数据是："+num);  
  } catch (IOException e) {  
    // TODO Auto-generated catch block  
    e.printStackTrace();  
  }  
  }  
}  

package com.lissdy;  
  
import java.io.IOException;  
import java.io.PrintWriter;  
import javax.servlet.http.HttpServlet;  
import javax.servlet.http.HttpServletRequest;  
import javax.servlet.http.HttpServletResponse;  
import java.io.*;  
@SuppressWarnings("serial")  
public class WriteFile extends HttpServlet{  
  public void doGet(HttpServletRequest req,HttpServletResponse res)  
  {  
    try {  
    res.setContentType("text/html;charset=gbk");  
    PrintWriter pw=res.getWriter();  
    //从内存到磁盘流动为写(输出流)  
    //从磁盘文件到内存流动为读(输入流)  
    FileWriter f=new FileWriter("d:\\lissdy.txt");  
    BufferedWriter bw=new BufferedWriter(f);  
    bw.write("试一试文件怎么写");  
    pw.println("向文件写入数据");  
    bw.close();  
  } catch (IOException e) {  
    // TODO Auto-generated catch block  
    e.printStackTrace();  
  }  
  }  
}