Java NIO(一)-Channel和Buffer

1. Channel
   Channel类似于流，但和流不同，流是单向的只能读或写，Channel是既可以读又可以写。Channel常见的实现类有FileChannel、DatagramChannel、SocketChannel、ServerSocketChannel。BIO中的FileInputStream、FileOutputStream和RandomAccessFile能通过getChannel方法得到FileChannel对象
2. Buffer
   Buffer是唯一能与Channel交互的对象，它的常见实现有ByteBuffer、ShortBuffer、IntBuffer、LongBuffer、DoubleBuffer、FloatBuffer、CharBuffer。Buffer有四个属性position、limit、capacity和mark，Buffer初始化的时候需要指定容量capacity，Buffer它的底层实现是一个数组,如下图，以ByteBuffer为例，position表示数组中下一个要读或写的位置，当向Buffer写完后(不一定写满)，调用flip()方法，将Buffer的读写模式翻转，limit赋值为position，position赋值为0。mark是标记位置，可以通过reset()方法，将position值变为mark值。

public abstract class ByteBuffer
    extends Buffer
    implements Comparable<ByteBuffer>
{
    final byte[] hb;                  // Non-null only for heap buffers
    final int offset;
    boolean isReadOnly;                 // Valid only for heap buffers
}

直接看看ByteBuffer的简单使用：

public class ByteBufferTest {
    public static void main(String[] args) {
        try {
            FileInputStream is = new FileInputStream("/home/reda/Documents/test/1.txt");
            FileOutputStream fos = new FileOutputStream("/home/reda/Documents/test/1.txt");
            FileOutputStream os = new FileOutputStream("/home/reda/Documents/test/2.txt");
            FileChannel inChannel = is.getChannel();
            FileChannel fosChannel = fos.getChannel();
            FileChannel outChannel = os.getChannel();

            //特殊情况，直接在channel传输
//            inChannel.transferTo(0,inChannel.size(),outChannel);
//            outChannel.transferFrom(inChannel,0,inChannel.size());

            ByteBuffer buffer = ByteBuffer.allocate(16);
            //写入内容
            fosChannel.write(ByteBuffer.wrap("1234QAQ".getBytes("UTF-8")));
//            while(inChannel.read(buffer) != -1) {
//                buffer.flip();
//                outChannel.write(buffer);
//                //必须clear不然除了第一次，后面将读不进内容到Buffer
//                buffer.clear();
//            }

            buffer.clear();
            fosChannel.close();
            inChannel.read(buffer);
            buffer.flip();
            //将positon的位置标记在起始位置，将ByteBuffer中每个字节转为字符
            buffer.mark();
            while (buffer.hasRemaining()) {
                System.out.print((char)buffer.get());
            }
            System.out.println();
            //将positon的位置重置
            buffer.reset();
            CharBuffer charBuffer = buffer.asCharBuffer();
            //打印乱码,ByteBuffer直接转成CharBuffer不行
            System.out.println(charBuffer.toString());
            //将ByteBuffer解码成utf8后，则能重新正确打印
            CharBuffer charBuffer2 = Charset.forName("UTF-8").decode(buffer);
            System.out.println(charBuffer2.toString());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

ByteBuffer类有方法能转换成其他类型的Buffer

public class BufferTest {
    public static void main(String[] args) {
        ByteBuffer buffer = ByteBuffer.wrap(new byte[]{0,0,0,0,0,0,0,'a'});
        //byteBUffer result:0 0 0 0 0 0 0 97
        while (buffer.hasRemaining())
            System.out.print(buffer.get() + " ");
        //CharBuffer result:      a
        CharBuffer charBuffer = ((ByteBuffer)buffer.rewind()).asCharBuffer();
        while (charBuffer.hasRemaining())
            System.out.print(charBuffer.get() + " ");
        //FloatBuffer result: 0.0 1.36E-43
        FloatBuffer floatBuffer = ((ByteBuffer)buffer.rewind()).asFloatBuffer();
        while (floatBuffer.hasRemaining())
            System.out.print(floatBuffer.get() + " ");
        //IntBuffer result:0 97
        IntBuffer intBuffer = ((ByteBuffer)buffer.rewind()).asIntBuffer();
        while (intBuffer.hasRemaining())
            System.out.print(intBuffer.get() + " ");
        //ShortBuffer result:0 0 0 97
        ShortBuffer shortBuffer = ((ByteBuffer)buffer.rewind()).asShortBuffer();
        while (shortBuffer.hasRemaining())
            System.out.print(shortBuffer.get() + " ");
        //LongBuffer result:97
        LongBuffer longBuffer = ((ByteBuffer)buffer.rewind()).asLongBuffer();
        while (longBuffer.hasRemaining())
            System.out.print(longBuffer.get() + " ");
        //DoubleBuffer result：4.8E-322
        DoubleBuffer doubleBuffer = ((ByteBuffer)buffer.rewind()).asDoubleBuffer();
        while (doubleBuffer.hasRemaining())
            System.out.print(doubleBuffer.get() + " ");
    }
}

• 一字节：byte boolean
• 二字节： char short
• 四字节：int float
• 八字节：long double
  ByteBuffer包装了一个8字节的数组,byteBuffer每个值有一个字节所以有8个值，同理char、short4个值(char的前三个值为空，没显示)，int、float2个值，long和double一个值。