java nio 读取大文件

Posted 2020-07-14 有梦就能实现

package com.yao.bigfile;

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

public class ReadBig {

    public static final String fileName = "D://log//192.168.56.1_debug_.log_2014-12-04";

    public static void main(String[] args) throws IOException {

        final int BUFFER_SIZE = 0x300000;// 缓冲区大小为3M

        File file = new File(fileName);

        /**
         * 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())
         */
        RandomAccessFile raf = new RandomAccessFile(file, "r");
        MappedByteBuffer inputBuffer = raf.getChannel().map(FileChannel.MapMode.READ_ONLY, file.length() / 2, file.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

        }

        raf.close();
        long end = System.currentTimeMillis();

        System.out.println("读取文件文件一半内容花费：" + (end - start) + "毫秒");

    }

}