Java NIO相关操作

1,buffer相关操作：

private static void bufferTest() {
    IntBuffer buffer = IntBuffer.allocate(100);

    System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
            + ",buffer capacity:" + buffer.capacity());

    buffer.put(10);
    int[] data = { 10, 2, 3, 4, 5, 6, 5, 6, 7, 8 };
    buffer.put(data);

    System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
            + ",buffer capacity:" + buffer.capacity());

    buffer.flip();

    System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
            + ",buffer capacity:" + buffer.capacity());

    while (buffer.hasRemaining()) {
        System.out.println(buffer.get());
    }
}

2，channel写数据相关操作：
private static void channelWriteTest() throws Exception {
String[] data = { “hadoop”, “spark”, “scala”, “bigdata”, “大数据” };
File file = new File(“d://nio_channel.txt”);
FileOutputStream out = new FileOutputStream(file);
FileChannel channel = out.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(1024);
for (String str : data) {
buffer.put((str + “\t”).getBytes(“UTF-8”));
}
buffer.flip();
channel.write(buffer);
channel.close();
out.close();
}

3，channel 读数据相关操作：

private static void chanelReadTest() throws Exception {
    File file = new File("D://settings.xml");
    FileInputStream inputStream = new FileInputStream(file);
    FileChannel channel = inputStream.getChannel();
    ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024);
    int tmp = 0;
    while ((tmp = channel.read(buffer)) != -1) {
        buffer.flip();
        channel.read(buffer);
        Charset charset = null;
        CharsetDecoder decoder = null;
        CharBuffer charBuffer = null;
        charset = Charset.forName("UTF-8");
        decoder = charset.newDecoder();
        // charBuffer = decoder.decode(buffer);//用这个的话，只能输出来一次结果，第二次显示为空
        charBuffer = decoder.decode(buffer.asReadOnlyBuffer());
        System.out.println(charBuffer.toString());
        buffer.clear();
    }
    inputStream.close();
}

4，内存映射的方式读取文件：

private static void chanelMapTest() throws Exception {
    File file = new File("D://settings.xml");
    FileInputStream inputStream = new FileInputStream(file);
    FileChannel fileChannel  = inputStream.getChannel();

    MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, file.length());//将文件映射到内存

    byte[] data = new byte[(int)file.length()];
    int index = 0;
    while(buffer.hasRemaining()){
        data[index++] = buffer.get();
    }
    System.out.println(new String(data,"UTF-8"));
    inputStream.close();
}

总结：
java nio 读写数据基于buffer,channel,来实现，channel是双向的，既可以把数据读入缓存区，也可以把缓存区的数据写入channel。
java读取文件的几种方式比较：
1，RundomAccessFile,读取文件较慢，能随机访问文件内容
2，传统java io操作，inputstream 读取文件较慢，效率不高
3，BufferderReader,包装读取文件较快
4，MappedByteBuffer 通过内存映射的方式读取文件最快，最为方便，如上第4中方式。

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