【JavaNIO的深入研究4】内存映射文件I/O，大文件读写操作，Java nio之MappedByteBuffer，高效文件/内存映射

public class UseMappedFile
{
  static private final int start = 0;
  static private final int size = 1024;

  static public void main( String args[] ) throws Exception {
    RandomAccessFile raf = new RandomAccessFile( "usemappedfile.txt", "rw" );
    FileChannel fc = raf.getChannel();

    MappedByteBuffer mbb = fc.map( FileChannel.MapMode.READ_WRITE,
      start, size );

    mbb.put( 0, (byte)97 );
    mbb.put( 1023, (byte)122 );
    for (int i = start; i < size; i++) {  
         System.out.print((char) mbb.get(i));  
    }  
    raf.close();
  }
}

public class LargeMappedFiles {  
    static int length = 0x8000000; // 128 Mb  
  
    public static void main(String[] args) throws Exception {  
        // 为了以可读可写的方式打开文件，这里使用RandomAccessFile来创建文件。  
        FileChannel fc = new RandomAccessFile("test.dat", "rw").getChannel();  
        //注意，文件通道的可读可写要建立在文件流本身可读写的基础之上  
        MappedByteBuffer out = fc.map(FileChannel.MapMode.READ_WRITE, 0, length);  
        //写128M的内容  
        for (int i = 0; i < length; i++) {  
            out.put((byte) ‘x‘);  
        }  
        System.out.println("Finished writing");  
        //读取文件中间6个字节内容  
        for (int i = length / 2; i < length / 2 + 6; i++) {  
            System.out.print((char) out.get(i));  
        }  
        fc.close();  
    }  
}  

package study;  
import java.io.FileInputStream;  
import java.io.FileOutputStream;  
import java.nio.ByteBuffer;  
import java.nio.MappedByteBuffer;  
import java.nio.channels.FileChannel;  
  
public class MapMemeryBuffer {  
  
    public static void main(String[] args) throws Exception {  
        ByteBuffer byteBuf = ByteBuffer.allocate(1024 * 14 * 1024);  
        byte[] bbb = new byte[14 * 1024 * 1024];  
        FileInputStream fis = new FileInputStream("e://data/other/UltraEdit_17.00.0.1035_SC.exe");  
        FileOutputStream fos = new FileOutputStream("e://data/other/outFile.txt");  
        FileChannel fc = fis.getChannel();  
        long timeStar = System.currentTimeMillis();// 得到当前的时间  
        fc.read(byteBuf);// 1 读取  
        //MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());  
        System.out.println(fc.size()/1024);  
        long timeEnd = System.currentTimeMillis();// 得到当前的时间  
        System.out.println("Read time :" + (timeEnd - timeStar) + "ms");  
        timeStar = System.currentTimeMillis();  
        fos.write(bbb);//2.写入  
        //mbb.flip();  
        timeEnd = System.currentTimeMillis();  
        System.out.println("Write time :" + (timeEnd - timeStar) + "ms");  
        fos.flush();  
        fc.close();  
        fis.close();  
    }  
  
}  
运行结果：  
14235  
Read time :24ms  
Write time :21ms  
我们把标注1和2语句注释掉，换成它们下面的被注释的那条语句，再来看运行效果。14235  
Read time :2ms  
Write time :0ms