java读写文件，读超大文件

一直在处理爬虫，经常能遇到读写文件的操作，很多时候都是读写超大文件，记录如下：

一、读文件

    import java.io.BufferedOutputStream;

    import  java.io.BufferedReader;

    import java.io.File;

    import java.io.FileOutputStream;

   import java.io.FileReader;

   import java.io.FileWriter;

   import java.io.IOException;

   import java.io.OutputStreamWriter;

     public void ReadFile(String filename) {

          File file=new File(filename);

         BufferedReader reader=null;

      try{

               reader=new BufferedReader(new FileReader(file));   //如果是读大文件  则  new BufferedReader(new FileReader(file),5*1024*1024);  即，设置缓存

               String tempString=null;

               while((tempString=reader.readLine())!=null)

               {

                    //进行操作.....

              }

            reader.close();

       }catch(IOException e){

             e.printStackTrace();

      }finally{

           if(reader!=null)

         {

               try{

                      reader.close();

               }catch(IOException e)

             {

             e.printStackTrace();

            }

          }

       }

}

二、写文件

    import java.io.BufferedOutputStream;

    import  java.io.BufferedReader;

   import  java.io.BufferedWriter;

    import java.io.File;

    import java.io.FileOutputStream;

   import java.io.FileReader;

   import java.io.FileWriter;

   import java.io.IOException;

   import java.io.OutputStreamWriter;

 

    public void method1(String file,String content){

     BufferedWriter out=null;

     try{

           out=new BufferedWriter(new OutputStreamWriter(new FileOutputStream(file,true)));  //追加的方法

          out.write(content);

          out.write("\r\n");

   }catch(IOException e){

             e.printStackTrace();

      }finally{

     try{

    out.close();

  }catch(IOException e){

             e.printStackTrace();

      }

}

}

public void main(String[] args)

{

   String filename="D:\a.txt";

  File f=new File(filename);

 if(f.exists()){

    f.delete();

}

f.createNewFile();

String ss="sssssss";

method1(filename,ss);

}

===========================华丽丽的分隔符=============================

在实践中发现上面的方法最多也就能处理10M以内的数据，从而编辑此文章。

想要真正处理上百M，甚至上G的文件，那就要用到java的nio包：

下面的代码转自【http://www.oschina.net/code/snippet_54100_7938】

import java.io.BufferedReader; 
import java.io.File; 
import java.io.FileInputStream; 
import java.io.IOException; 
import java.io.RandomAccessFile; 
import java.nio.ByteBuffer; 
import java.nio.channels.FileChannel; 


public class TestNio { 

public static void main(String args[]) throws Exception{ 
int bufSize = 100; 
File fin = new File("D:\\workspace\\test\\usagetracking.log"); 
File fout = new File("D:\\workspace\\test\\usagetracking2.log"); 


FileChannel fcin = new RandomAccessFile(fin, "r").getChannel(); 
ByteBuffer rBuffer = ByteBuffer.allocate(bufSize); 


FileChannel fcout = new RandomAccessFile(fout, "rws").getChannel(); 
ByteBuffer wBuffer = ByteBuffer.allocateDirect(bufSize); 




readFileByLine(bufSize, fcin, rBuffer, fcout, wBuffer); 


System.out.print("OK!!!"); 
} 

/*读文件同时写文件*/
public static void readFileByLine(int bufSize, FileChannel fcin, ByteBuffer rBuffer, FileChannel fcout, ByteBuffer wBuffer){ 
String enterStr = "\n"; 
try{ 
byte[] bs = new byte[bufSize]; 


int size = 0; 
StringBuffer strBuf = new StringBuffer(""); 
//while((size = fcin.read(buffer)) != -1){ 
while(fcin.read(rBuffer) != -1){ 
     int rSize = rBuffer.position(); 
     rBuffer.rewind(); 
     rBuffer.get(bs); 
     rBuffer.clear(); 
     String tempString = new String(bs, 0, rSize); 
     //System.out.print(tempString); 
     //System.out.print("<200>"); 


     int fromIndex = 0; 
     int endIndex = 0; 
     while((endIndex = tempString.indexOf(enterStr, fromIndex)) != -1){ 
      String line = tempString.substring(fromIndex, endIndex); 
      line = new String(strBuf.toString() + line); 
      //System.out.print(line); 
      //System.out.print("</over/>"); 
      //write to anthone file 
      writeFileByLine(fcout, wBuffer, line); 


      
      strBuf.delete(0, strBuf.length()); 
      fromIndex = endIndex + 1; 
     } 
     if(rSize > tempString.length()){ 
     strBuf.append(tempString.substring(fromIndex, tempString.length())); 
     }else{ 
     strBuf.append(tempString.substring(fromIndex, rSize)); 
     } 
} 
} catch (IOException e) { 
// TODO Auto-generated catch block 
e.printStackTrace(); 
} 
} 

/*写文件*/
public static void writeFileByLine(FileChannel fcout, ByteBuffer wBuffer, String line){ 
try { 
//write on file head 
//fcout.write(wBuffer.wrap(line.getBytes())); 
//wirte append file on foot 
fcout.write(wBuffer.wrap(line.getBytes()), fcout.size()); 


} catch (IOException e) { 
// TODO Auto-generated catch block 
e.printStackTrace(); 
} 
} 


}