Java中如何用内存映射处理大文件

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package test;   import java.io.BufferedInputStream;  import java.io.FileInputStream;  import java.io.FileNotFoundException;  import java.io.IOException;  import java.io.RandomAccessFile;  import java.nio.MappedByteBuffer;  import java.nio.channels.FileChannel;   public class Test {             public static void main(String[] args) {          try {              FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt");              int sum=0;              int n;              long t1=System.currentTimeMillis();              try {                  while((n=fis.read())>=0){                      sum+=n;                  }              } catch (IOException e) {                  // TODO Auto-generated catch block                  e.printStackTrace();              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }                    try {              FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt");              BufferedInputStream bis=new BufferedInputStream(fis);              int sum=0;              int n;              long t1=System.currentTimeMillis();              try {                  while((n=bis.read())>=0){                      sum+=n;                  }              } catch (IOException e) {                  // TODO Auto-generated catch block                  e.printStackTrace();              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }                    MappedByteBuffer buffer=null;          try {              buffer=new RandomAccessFile("/home/tobacco/test/res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 1253244);              int sum=0;              int n;              long t1=System.currentTimeMillis();              for(int i=0;i<1253244;i++){                  n=0x000000ff&buffer.get(i);                  sum+=n;              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          } catch (IOException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }       }   }

测试文件为一个大小为1253244字节的文件。测试结果：

sum:220152087 time:1464  sum:220152087 time:72  sum:220152087 time:25

说明读数据无误。删去其中的数据处理部分。

package test;   import java.io.BufferedInputStream;  import java.io.FileInputStream;  import java.io.FileNotFoundException;  import java.io.IOException;  import java.io.RandomAccessFile;  import java.nio.MappedByteBuffer;  import java.nio.channels.FileChannel;   public class Test {             public static void main(String[] args) {          try {              FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt");              int sum=0;              int n;              long t1=System.currentTimeMillis();              try {                  while((n=fis.read())>=0){                      //sum+=n;                  }              } catch (IOException e) {                  // TODO Auto-generated catch block                  e.printStackTrace();              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }                    try {              FileInputStream fis=new FileInputStream("/home/tobacco/test/res.txt");              BufferedInputStream bis=new BufferedInputStream(fis);              int sum=0;              int n;              long t1=System.currentTimeMillis();              try {                  while((n=bis.read())>=0){                      //sum+=n;                  }              } catch (IOException e) {                  // TODO Auto-generated catch block                  e.printStackTrace();              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }                    MappedByteBuffer buffer=null;          try {              buffer=new RandomAccessFile("/home/tobacco/test/res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 1253244);              int sum=0;              int n;              long t1=System.currentTimeMillis();              for(int i=0;i<1253244;i++){                  //n=0x000000ff&buffer.get(i);                  //sum+=n;              }              long t=System.currentTimeMillis()-t1;              System.out.println("sum:"+sum+"  time:"+t);          } catch (FileNotFoundException e) {              // TODO Auto-generated catch block              e.printStackTrace();          } catch (IOException e) {              // TODO Auto-generated catch block              e.printStackTrace();          }       }   }

测试结果：

sum:0 time:1458  sum:0 time:67  sum:0 time:8

由此可见，将文件部分或者全部映射到内存后进行读写，速度将提高很多。

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