RandomAcessFile、MappedByteBuffer和缓冲读/写文件

项目需要进行大文件的读写，调查测试的结果使我决定使用MappedByteBuffer及相关类进行文件的操作，效果不是一般的高。

网上参考资源很多，如下两篇非常不错：

1、花1K内存实现高效I/O的RandomAccessFile类

2、Java中Stream和Memory-mapped File的I/O性能对比

小结：

1、RandomAccessFile本身不带缓冲读写，和FileInputStream、FileOutputStream等一样，直接按字节读写时，性能不可接受；

2、使用MappedByteBuffer读写，固然性能会得到极大提升；其实只要自己处理缓冲，性能都会有非常大的提升，比如以下两种方式（见下记代码）中第一种使用了MappedByteBuffer，第二种自己进行缓冲，对于12M的文件，后者的效率甚至高于前者。

3、BufferedXXXX之类的缓冲流，如果仅使用默认的buffer size，性能不一定最优，要权衡不同情况各种因素设置大小。

package helloword.helloword;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
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) {
        // TODO Auto-generated method stub

    }

    public void test() throws IOException {
        /*
         * 测试结果与Buffer size有关
         */
        // 1、使用MappedByteBuffer: 0.7s
        String srcFile = "D://Noname1.txt";
        String destFile = "D://copy.txt";
        RandomAccessFile rafi = new RandomAccessFile(srcFile, "r");
        RandomAccessFile rafo = new RandomAccessFile(destFile, "rw");
        FileChannel fci = rafi.getChannel();
        FileChannel fco = rafo.getChannel();
        long size = fci.size();
        MappedByteBuffer mbbi = fci.map(FileChannel.MapMode.READ_ONLY, 0, size);
        MappedByteBuffer mbbo = fco.map(FileChannel.MapMode.READ_WRITE, 0, size);
        long start = System.currentTimeMillis();
        for (int i = 0; i < size; i++) {
            byte b = mbbi.get(i);
            mbbo.put(i, b);
        }
        fci.close();
        fco.close();
        rafi.close();
        rafo.close();
        System.out.println("Spend: " + (double) (System.currentTimeMillis() - start) / 1000 + "s");

    }

    public void test2() throws IOException {
        // 2、自己处理Buffer(RandomAccessFile): 0.13s
        String srcFile = "D://Noname1.txt";
        String destFile = "D://copy.txt";
        RandomAccessFile rafi = new RandomAccessFile(srcFile, "r");
        RandomAccessFile rafo = new RandomAccessFile(destFile, "rw");

        byte[] buf = new byte[1024 * 8];

        long start = System.currentTimeMillis();

        int c = rafi.read(buf);

        while (c > 0) {
            if (c == buf.length) {
                rafo.write(buf);
            } else {
                rafo.write(buf, 0, c);
            }

            c = rafi.read(buf);
        }
        rafi.close();
        rafo.close();
        System.out.println("Spend: " + (double) (System.currentTimeMillis() - start) / 1000 + "s");

    }

    public void test3() throws IOException {
        // 3、BufferedInputStream&BufferedOutputStream: 3.02s
        String srcFile = "D://Noname1.txt";
        String destFile = "D://copy.txt";
        FileInputStream rafi = new FileInputStream(srcFile);
        FileOutputStream rafo = new FileOutputStream(destFile);

        BufferedInputStream bis = new BufferedInputStream(rafi, 8192);
        BufferedOutputStream bos = new BufferedOutputStream(rafo, 8192);
        long size = rafi.available();

        long start = System.currentTimeMillis();

        for (int i = 0; i < size; i++) {
            byte b = (byte) bis.read();
            bos.write(b);
        }
        rafi.close();
        rafo.close();
        System.out.println("Spend: " + (double) (System.currentTimeMillis() - start) / 1000 + "s");
    }
}