工作中有一次发现,利用FileInputStream处理文件时很慢,于是想有没有读取大文件时比较快的方法,网上有说利用内存映射处理大文件,下面我们一起看一组对比试验:
package test.api; import java.io.*; import java.nio.MappedByteBuffer; import java.nio.channels.FileChannel; public class GCTest { public static void main(String[] args) { try { FileInputStream fis=new FileInputStream("D:\\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("D:\\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("D:\\res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 151552); int sum=0; int n; long t1=System.currentTimeMillis(); for(int i=0;i<151552;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151552KB,测试结果为:
sum:13430346 time:186
sum:13430346 time:13
sum:13430346 time:4
说明数据读取无误,下面删除数据处理部分
package test.api; import java.io.*; import java.nio.MappedByteBuffer; import java.nio.channels.FileChannel; public class GCTest { public static void main(String[] args) { try { FileInputStream fis=new FileInputStream("D:\\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("D:\\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("D:\\res.txt","rw").getChannel().map(FileChannel.MapMode.READ_WRITE, 0, 151552); int sum=0; int n; long t1=System.currentTimeMillis(); for(int i=0;i<151552;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(); } } }
处理结果:
sum:0 time:151
sum:0 time:7
sum:0 time:1
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。