Java 内存映射读取文件_Java内存映射文件

package com.what21.nio03;

import java.io.File;

import java.io.RandomAccessFile;

import java.nio.ByteBuffer;

import java.nio.MappedByteBuffer;

import java.nio.channels.FileChannel;

public class MapFiles {

/**

* @param argv

* @throws Exception

*/

public static void main(String[] argv) throws Exception {

// 创建一个临时文件链接和管道

File tempFile = File.createTempFile("mmaptest", null);

RandomAccessFile file = new RandomAccessFile(tempFile, "rw");

FileChannel channel = file.getChannel();

ByteBuffer temp = ByteBuffer.allocate(100);

// 从位置0存放一些内容到文件

temp.put("This is the file content".getBytes());

temp.flip();

channel.write(temp, 0);

temp.clear();

temp.put("This is more file content".getBytes());

temp.flip();

channel.write(temp, 8192);

// 针对同一个文件创建三种映射文件模式

MappedByteBuffer ro = channel.map(FileChannel.MapMode.READ_ONLY, 0,

channel.size());

MappedByteBuffer rw = channel.map(FileChannel.MapMode.READ_WRITE, 0,

channel.size());

MappedByteBuffer cow = channel.map(FileChannel.MapMode.PRIVATE, 0,

channel.size());

System.out.println("Begin");

showBuffers(ro, rw, cow);

// 修改READ模式拷贝位置

cow.position(8);

cow.put("COW".getBytes());

System.out.println("Change to COW buffer");

showBuffers(ro, rw, cow);

// 修改READ/WRITE模式拷贝位置

rw.position(9);

rw.put(" R/W ".getBytes());

rw.position(8194);

rw.put(" R/W ".getBytes());

rw.force();

System.out.println("Change to R/W buffer");

showBuffers(ro, rw, cow);

temp.clear();

temp.put("Channel write ".getBytes());

temp.flip();

channel.write(temp, 0);

temp.rewind();

channel.write(temp, 8202);

System.out.println("Write on channel");

showBuffers(ro, rw, cow);

// 再次修改

cow.position(8207);

cow.put(" COW2 ".getBytes());

System.out.println("Second change to COW buffer");

showBuffers(ro, rw, cow);

rw.position(0);

rw.put(" R/W2 ".getBytes());

rw.position(8210);

rw.put(" R/W2 ".getBytes());

rw.force();

System.out.println("Second change to R/W buffer");

showBuffers(ro, rw, cow);

channel.close();

file.close();

tempFile.delete();

}

/**

* 显示目前的缓冲区内容

*

* @param ro

* @param rw

* @param cow

* @throws Exception

*/

public static void showBuffers(ByteBuffer ro, ByteBuffer rw, ByteBuffer cow)

throws Exception {

dumpBuffer("R/O", ro);

dumpBuffer("R/W", rw);

dumpBuffer("COW", cow);

System.out.println("");

}

/**

* @param prefix

* @param buffer

* @throws Exception

*/

public static void dumpBuffer(String prefix, ByteBuffer buffer)

throws Exception {

System.out.print(prefix + ": '");

int nulls = 0;

int limit = buffer.limit();

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

char c = (char) buffer.get(i);

if (c == '\u0000') {

nulls++;

continue;

}

if (nulls != 0) {

System.out.print("|[" + nulls + " nulls]|");

nulls = 0;

}

System.out.print(c);

}

System.out.println("'");

}

}