【问题】
FileInputStream & FileOutputStream 和 BufferedInputStream & BufferedOutputStream 都能完成文件的读写。它们在原理上、时间效率上有什么不同,实际应用时应该如何选择呢?
【实验】
我们采用文件复制来做效率对比的实验。
在FileOperator类的copyWithFileStream方法实现了使用FileInputStream和FileOutputStream复制文件,copyWithBufferedStream方法实现了使用BufferedInputStream和BufferedOutputStream复制文件。
/*
* Copyright (c) 2014 xxx.com. All Rights Reserved.
*/
package utils;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
/**
* @author ham.hog created on 2014 14-2-12 下午2:58
* @version 0
*/
public class FileOperator {
/** buffer size in bytes */
final static int BUFFER_SIZE = 100;
/**
* copy file using FileInputStream & FileOutputStream
* @param src copy from
* @param dest copy to
* @return;
*/
public static void copyWithFileStream(File src, File dest){
FileInputStream input = null;
FileOutputStream output = null;
try {
input = new FileInputStream(src);
output = new FileOutputStream(dest);
byte[] buffer = new byte[BUFFER_SIZE];
int copySize;
while ((copySize = input.read(buffer)) > 0){
output.write(buffer, 0, copySize);
output.flush();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
input.close();
output.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
/**
* copy file using BufferedInputStream & BufferedOutputStream
* @param src copy from file
* @param dest copy to file
* @return;
*/
public static void copyWithBufferedStream(File src, File dest){
BufferedInputStream bufferedInput = null;
BufferedOutputStream bufferedOutput = null;
try {
bufferedInput = new BufferedInputStream(new FileInputStream(src));
bufferedOutput = new BufferedOutputStream(new FileOutputStream(dest));
byte[] buffer = new byte[BUFFER_SIZE];
int copySize;
while ((copySize = bufferedInput.read(buffer)) > 0){
bufferedOutput.write(buffer, 0, copySize);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
bufferedInput.close();
bufferedOutput.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
class FileOperatorTest{
public static void main(String args[]){
File src = new File("test.txt");
File dest = new File("copyTest.txt");
try {
if (!dest.exists()){
dest.createNewFile();
}
} catch (IOException e) {
e.printStackTrace();
}
//test copy using FileStream
int startTime = System.currentTimeMillis();
FileOperator.copyWithFileStream(src, dest);
int endTime = System.currentTimeMillis();
System.out.println("Copy file using FileStream takes : " + (endTime - startTime) + " ms.");
//test copy using BufferedStream
startTime = System.currentTimeMillis();
FileOperator.copyWithBufferedStream(src, dest);
endTime = System.currentTimeMillis();
System.out.println("Copy file using BufferedStream takes : " + (endTime - startTime) + " ms.");
}
}
【运行结果】
测试文件大小约为900M,以下是在设定BUFFER_SIZE为不同值时的一次执行结果:
BUFFER_SIZE = 100
Copy file using FileStream takes: 42680 ms.
Copy file using BufferedStream takes: 2407 ms.
BUFFER_SIZE = 8192
Copy file using FileStream takes: 1689 ms.
Copy file using BufferedStream takes: 1654 ms.
BUFFER_SIZE = 1000000
Copy file using FileStream takes: 957 ms.
Copy file using BufferedStream takes: 929 ms.
【对时间效率差异的解释】
BufferedInputStream比FileInputStream多了一个缓冲区,执行read时先从缓冲区读取,当缓冲区数据读完时再把缓冲区填满。
因此,当每次读取的数据量很小时,FileInputStream每次都是从硬盘读入,而BufferedInputStream大部分是从缓冲区读入。读取内存速度比读取硬盘速度快得多,因此BufferedInputStream效率高。
BufferedInputStream的默认缓冲区大小是8192字节。当每次读取数据量接近或远超这个值时,两者效率就没有明显差别了。
BufferedOutputStream和FileOutputStream同理,差异更明显一些。
【结论】
一般情况下,我们应该优先选取BufferedInputStream&BufferedOutputStream。
注意 File file = new File(new File(parent), path);//此处如果文件不存在,不会创建文件
当执行write操作的时候如果文件不存在则会创建文件.
public final T parseFromFile(String parent, String path) {
File file = new File(new File(parent), path);
File file2 = new File(new File(Environment.getExternalStorageDirectory().getPath()), "file2");
File file3 = new File(new File(Environment.getExternalStorageDirectory().getPath()), "file3");
BufferedInputStream bis = null;
byte[] ad = new byte[4];
try {
FileInputStream fis = new FileInputStream(file);
new FileOutputStream(file3).write(1);//会新建文件
new FileInputStream(file2).read();//不会新建文件
bis = new BufferedInputStream(fis, 25);//只开25字节的buffer
return parseFromStream(path, bis);
} catch (FileNotFoundException e) {
Timber.tag(TAG).e(e, "File not found for parent:%s path:%s", parent,
path);
} catch (IOException e) {
Timber.tag(TAG).e(e, "IO error for parent:%s path:%s", parent, path);
} finally {
IoUtils.closeQuietly(bis);
}
return null;
}