ByteBuffer使用实例

　　ByteBuffer作为JDK的字节流处理对象，这里举个小例子说明下用法，直接上代码：

package com.wlf.netty.nettyserver;

import org.junit.Assert;
import org.junit.Test;

import java.nio.ByteBuffer;

public class ByteBufferTest {

    @Test
    public void byteBufferTest() {

        // 写入消息体
        ByteBuffer byteBuffer = ByteBuffer.allocate(10);
        byteBuffer.putInt(0xabef0101);
        byteBuffer.putInt(1024); // 今天过节
        byteBuffer.put((byte) 1);
        byteBuffer.put((byte) 0);

        // 读取消息头，因为写完后position已经到10了，所以需要先反转为0，再从头读取
        byteBuffer.flip();
        printDelimiter(byteBuffer);

        // 读取length
        printLength(byteBuffer);

        // 继续读取剩下数据
        byteBuffer.get();
        byteBuffer.get();
        printByteBuffer(byteBuffer);

        // 我再反转一下，我还可以从头开始读
        byteBuffer.flip();
        printDelimiter(byteBuffer);

        // clear清空一下，再从头开始读
        byteBuffer.clear();
        printDelimiter(byteBuffer);

        // rewind重绕一下
        byteBuffer.rewind();
        printDelimiter(byteBuffer);

        // mark标记一下
        byteBuffer.mark();

        // 再读取length
        printLength(byteBuffer);

        // reset重置，回到读取delimiter的地方
        byteBuffer.reset();
        printByteBuffer(byteBuffer);
    }

    private void printDelimiter(ByteBuffer buf) {
        int newDelimiter = buf.getInt();
        System.out.printf("delimeter: %s\n", Integer.toHexString(newDelimiter));
        printByteBuffer(buf);
    }

    private void printLength(ByteBuffer buf) {
        int length = buf.getInt();
        System.out.printf("length: %d\n", length);
        printByteBuffer(buf);
    }

    private void printByteBuffer(ByteBuffer buf) {
        System.out.printf("position: %d, limit: %d, capacity: %d\n", buf.position(), buf.limit(), buf.capacity());
    }
}

　　输出结果：

delimeter: abef0101
position: 4, limit: 10, capacity: 10
length: 1024
position: 8, limit: 10, capacity: 10
position: 10, limit: 10, capacity: 10
delimeter: abef0101
position: 4, limit: 10, capacity: 10
delimeter: abef0101
position: 4, limit: 10, capacity: 10
delimeter: abef0101
position: 4, limit: 10, capacity: 10
length: 1024
position: 8, limit: 10, capacity: 10
position: 4, limit: 10, capacity: 10

Process finished with exit code 0

　　ByteBuffer的索引是唯一的。像上面的例子，初始索引是0，写完索引值为9，为了读取写入的值，我们再重新设置索引为0（调用flip方法）。ByteBuffer有4个索引值，分别是：

　　mask：就是你标记的索引，标记唯一的作用是调用reset重置回到过去

　　position：当前位置的索引，mask标记任何时候都不会大于position，因为你必须先读到当前位置之后，才能标记该位置；同时position也不能超过limit限制

　　limit：第一个不应该读取或写入的元素的索引，也就是读写禁地，默认是最大容量，如果你设置该值，那么理所让然它不能超过最大容量capacity

　　capacity：这个就不解释了

　　它们的大小关系始终是：

　　mask <= position <= limit <= capacity

　　我们上面的例子中就是capacity=limit。

　　初始索引：

　　+-----------------------------------------------------+

/                            bytes                              /

+-----------------------------------------------------+

/                            10                                  /

　　0=position                                               10=limit=capacity

　　我们写入delimiter之后：

　　+----------------+------------------------------------+

/      delimiter /     other  bytes                       /

+----------------+------------------------------------+

/             4     /          6                                    /

　  0　       4=position                                     10=limit=capacity

　　

　　至于反转flip如何切换读写模式、reset如何重置标记、clear清除如何重新设置索引值为0、rewind重绕如何让你重新读取，都不会动内容,所以你会看到上面不管怎么折腾我们都可以重复取出delimiter、length的值。看下源码就一清二楚了，无非就是对上面4个索引值进行赋值而已：

    /**
     * Resets this buffer's position to the previously-marked position.
     *
     * <p> Invoking this method neither changes nor discards the mark's
     * value. </p>
     *
     * @return  This buffer
     *
     * @throws  InvalidMarkException
     *          If the mark has not been set
     */
    public final Buffer reset() {
        int m = mark;
        if (m < 0)
            throw new InvalidMarkException();
        position = m;
        return this;
    }

    /**
     * Clears this buffer.  The position is set to zero, the limit is set to
     * the capacity, and the mark is discarded.
     *
     * <p> Invoke this method before using a sequence of channel-read or
     * <i>put</i> operations to fill this buffer.  For example:
     *
     * <blockquote><pre>
     * buf.clear();     // Prepare buffer for reading
     * in.read(buf);    // Read data</pre></blockquote>
     *
     * <p> This method does not actually erase the data in the buffer, but it
     * is named as if it did because it will most often be used in situations
     * in which that might as well be the case. </p>
     *
     * @return  This buffer
     */
    public final Buffer clear() {
        position = 0;
        limit = capacity;
        mark = -1;
        return this;
    }

    /**
     * Flips this buffer.  The limit is set to the current position and then
     * the position is set to zero.  If the mark is defined then it is
     * discarded.
     *
     * <p> After a sequence of channel-read or <i>put</i> operations, invoke
     * this method to prepare for a sequence of channel-write or relative
     * <i>get</i> operations.  For example:
     *
     * <blockquote><pre>
     * buf.put(magic);    // Prepend header
     * in.read(buf);      // Read data into rest of buffer
     * buf.flip();        // Flip buffer
     * out.write(buf);    // Write header + data to channel</pre></blockquote>
     *
     * <p> This method is often used in conjunction with the {@link
     * java.nio.ByteBuffer#compact compact} method when transferring data from
     * one place to another.  </p>
     *
     * @return  This buffer
     */
    public final Buffer flip() {
        limit = position;
        position = 0;
        mark = -1;
        return this;
    }

    /**
     * Rewinds this buffer.  The position is set to zero and the mark is
     * discarded.
     *
     * <p> Invoke this method before a sequence of channel-write or <i>get</i>
     * operations, assuming that the limit has already been set
     * appropriately.  For example:
     *
     * <blockquote><pre>
     * out.write(buf);    // Write remaining data
     * buf.rewind();      // Rewind buffer
     * buf.get(array);    // Copy data into array</pre></blockquote>
     *
     * @return  This buffer
     */
    public final Buffer rewind() {
        position = 0;
        mark = -1;
        return this;
    }