在处理大文件时，如果利用普通的FileInputStream 或者FileOutputStream 抑或RandomAccessFile 来进行频繁的读写操作，都将导致进程因频繁读写外存而降低速度.如下为一个对比实验。
- 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) {
-
- e.printStackTrace();
- }
- long t=System.currentTimeMillis()-t1;
- System.out.println("sum:"+sum+" time:"+t);
- } catch (FileNotFoundException e) {
-
- 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) {
-
- e.printStackTrace();
- }
- long t=System.currentTimeMillis()-t1;
- System.out.println("sum:"+sum+" time:"+t);
- } catch (FileNotFoundException e) {
-
- 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) {
-
- e.printStackTrace();
- } catch (IOException e) {
-
- e.printStackTrace();
- }
- }
- }
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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
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