读取大文件（NIO）

package com.IO;

import java.io.File;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

/**
*
* map(FileChannel.MapMode mode,long position, long size)
*
* mode - 根据是按只读、读取/写入或专用（写入时拷贝）来映射文件，分别为 FileChannel.MapMode 类中所定义的
* READ_ONLY、READ_WRITE 或 PRIVATE 之一
*
* position - 文件中的位置，映射区域从此位置开始；必须为非负数
*
* size - 要映射的区域大小；必须为非负数且不大于 Integer.MAX_VALUE
*
* 所以若想读取文件后半部分内容，如例子所写；若想读取文本后1/8内容，需要这样写map(FileChannel.MapMode.
* READ_ONLY, f.length()*7/8,f.length()/8)
*
* 想读取文件所有内容，需要这样写map(FileChannel.MapMode.READ_ONLY, 0,f.length())
*/

public class ReadBigFile {

public static void main(String[] args) throws Exception {
final int BUFFER_SIZE = 0x300000;// 缓冲区大小为3M
File f = new File("D:\\test_data.txt");
MappedByteBuffer inputBuffer = new RandomAccessFile(f, "r").getChannel().map(FileChannel.MapMode.READ_ONLY,f.length() / 2, f.length() / 2);
byte[] dst = new byte[BUFFER_SIZE];// 每次读出3M的内容
long start = System.currentTimeMillis();
for (int offset = 0; offset < inputBuffer.capacity(); offset += BUFFER_SIZE) {
if (inputBuffer.capacity() - offset >= BUFFER_SIZE) {
for (int i = 0; i < BUFFER_SIZE; i++)
dst[i] = inputBuffer.get(offset + i);
} else {
for (int i = 0; i < inputBuffer.capacity() - offset; i++)
dst[i] = inputBuffer.get(offset + i);
}
int length = (inputBuffer.capacity() % BUFFER_SIZE == 0) ? BUFFER_SIZE
: inputBuffer.capacity() % BUFFER_SIZE;
// System.out.println(new String(dst,0,length));//new
// String(dst,0,length)这样可以取出缓存保存的字符串，可以对其进行操作
}
long end = System.currentTimeMillis();
System.out.println("读取文件文件一半内容花费：" + (end - start) + "毫秒");
}
}