Java中使用記憶體映射實作大檔案上傳實例

在處理大檔案時，如果利用普通的FileInputStream 或FileOutputStream 抑或RandomAccessFile 來進行頻繁的讀寫操作，都將導致進程因頻繁讀寫外存而降低速度.如下為一個對比實驗。
複製代碼代碼如下:
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();
}

}

}

測試檔案為一個大小為1253244位元組的檔案。測試結果：
複製代碼代碼如下:
sum:220152087 time:1464
sum:220152087 time:72
sum:220152087 time:25

說明讀數據無誤。刪除其中的資料處理部分。
複製代碼代碼如下:
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();
}

}

}

測試結果：
複製代碼代碼如下:
sum:0 time:1458
sum:0 time:67
sum:0 time:8

由此可見，將檔案部分或全部映射到記憶體後進行讀寫，速度將會提高很多。

這是因為記憶體映射檔案首先將外存上的檔案映射到記憶體中的一塊連續區域，被當成一個位元組數組進行處理，讀寫操作直接對記憶體進行操作，而後再將記憶體區域重新映射到外存文件，這節省了中間頻繁的對外存進行讀寫的時間，大大降低了讀寫時間。