认识达内从这里开始

昆明IT培训的老师这一期给大家讲Java--Stream，NIO ByteBuffer，NIO MappedByteBuffer性能对比。

目前Java中最IO有多种文件读取的方法，本文章对比Stream，NIO ByteBuffer，NIO MappedByteBuffer的性能，让我们知道到底怎么能写出性能高的文件读取代码。

package com.seeyon.nio;

import org.junit.Test;

import java.io.*;

import java.nio.ByteBuffer;

import java.nio.MappedByteBuffer;

import java.nio.channels.FileChannel;

/**

* Created by yangyu on 16/12/28.

*/

/**

*比较Stream流，NIO ByteBuffer，NIO MappedByteBuffer性能对比

*其中Stream最慢，NIO MappedByteBuffer最快

* Stream：1000ms

* NIO ByteBuffer:220ms

* NIO MappedByteBuffer:112ms

*/

public class Compare {

/**

*使用stream作为IO流读取和写入文件

*速度：1000ms左右

*

* @throws IOException

*/

@Test

public void useStream() throws IOException {

long startTime = System.currentTimeMillis();

/**

* 4000000个整数长度的文件

*/

int num = 2000 * 2000;

/**

*带缓冲的输出流，写文件

*/

DataOutputStream dataOutputStream = new DataOutputStream(new BufferedOutputStream(new FileOutputStream("/Users/yangyu/Downloads/compare.tmp")));

for (int i = 0; i < num; i++) {

dataOutputStream.writeInt(i);

}

dataOutputStream.close();

int data = 0;

/**

*带缓冲的输入流，读文件

*/

DataInputStream in = new DataInputStream(new BufferedInputStream(new FileInputStream("/Users/yangyu/Downloads/compare.tmp")));

try {

while (true) {

data = in.readInt();

}

} catch (EOFException e) {

System.out.println("读取完成"+data);

}

in.close();

long endTime = System.currentTimeMillis();

System.out.println("ms:" + (endTime - startTime));

}

/**

*使用NIO ByteBuffer

*时间：220ms

* @throws IOException

*/

@Test

public void useNioByteBuffer() throws IOException {

long startTime = System.currentTimeMillis();

int num = 2000*2000;

/**

*文件输出流

*/

FileOutputStream fileOutputStream = new FileOutputStream("/Users/yangyu/Downloads/compare.tmp");

/**

* NIO Channel通道

*/

FileChannel fileChannel = fileOutputStream.getChannel();

/**

* ByteBuffer缓冲区

*/

ByteBuffer buffer = ByteBuffer.allocate(num*5);

for (int i = 0; i < num; i++) {

buffer.putInt(i);

}

/**

*为写做准备

*/

buffer.flip();

/**

*写操作

*/

fileChannel.write(buffer);

fileChannel.close();

/**

*缓冲区

*/

ByteBuffer buffer1 = ByteBuffer.allocate(num*5);

/**

*文件输入流

*/

FileInputStream in = new FileInputStream("/Users/yangyu/Downloads/compare.tmp");

/**

*输入通道

*/

FileChannel fin = in.getChannel();

/**

*为读取做准备

*/

buffer1.clear();

System.out.println(buffer1.limit());

/**

*读取

*/

fin.read(buffer1);

fin.close();

long endTime = System.currentTimeMillis();

System.out.println("ms:" + (endTime - startTime));

buffer1.flip();

System.out.println(buffer1.limit());

}

/**

*使用MappedByteBuffer，通过FileChannel将文件映射到内存

*时间：112ms

* @throws IOException

*/

@Test

public void useRandomAccess() throws IOException {

long startTime = System.currentTimeMillis();

int num = 2000*2000;

/**

*使用可随机访问位置的RandomAccessFile

*/

RandomAccessFile file = new RandomAccessFile("/Users/yangyu/Downloads/compare.tmp","rw");

/**

*获取通道Channel

*/

FileChannel fileChannel = file.getChannel();

/**

*将文件映射到缓冲区MappedByteBuffer

*/

MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE,0,num*4);

/**

*写文件

*/

for (int i = 0; i < num; i++) {

mappedByteBuffer.putInt(i);

}

fileChannel.close();

int data=0;

RandomAccessFile file1 = new RandomAccessFile("/Users/yangyu/Downloads/compare.tmp","rw");

FileChannel fc = file1.getChannel();

MappedByteBuffer mappedByteBuffer1 = fc.map(FileChannel.MapMode.READ_WRITE,0,file1.length());

/**

*读文件

*/

while (mappedByteBuffer1.hasRemaining()){

data = mappedByteBuffer1.getInt();

}

fc.close();

long endTime = System.currentTimeMillis();

System.out.println("ms:" + (endTime - startTime));

System.out.println(data);

}

}

结论非常明显啦，以后再使用IO读写文件的时候，多使用NIO MappedByteBuffer吧，毕竟NIO比老IO性能好太多啦。