Java NIO框架Netty教程(十)-Object对象的连续收发解析分析

如果您一直关注OneCoder，我们之前有两篇文章介绍关于Netty消息连续收发的问题。( 《Java NIO框架Netty教程（五）- 消息收发次数不匹配的问题 》、《 Java NIO框架Netty教程(七)-再谈收发信息次数问题 》)。如果您经常的”怀疑”和思考，我们刚介绍过了Object的传递，您是否好奇，在Object传递中是否会有这样的问题？如果Object流的字节截断错乱，那肯定是会出错的。Netty一定不会这么傻的，那么Netty是怎么做的呢？

我们先通过代码验证一下是否有这样的问题。（有问题的可能性几乎没有。）

/**
	 * 当绑定到服务端的时候触发，给服务端发消息。
	 * 
	 * @author lihzh
	 * @alia OneCoder
	 */
	@Override
	public void channelConnected(ChannelHandlerContext ctx, ChannelStateEvent e) {
		// 向服务端发送Object信息
		sendObject(e.getChannel());
	}

	/**
	 * 发送Object
	 * 
	 * @param channel
	 * @author lihzh
	 * @alia OneCoder
	 */
	private void sendObject(Channel channel) {
		Command command = new Command();
		command.setActionName("Hello action.");
		Command commandOne = new Command();
		commandOne.setActionName("Hello action. One");
		Command command2 = new Command();
		command2.setActionName("Hello action. Two");
		channel.write(command2);
		channel.write(command);
		channel.write(commandOne);
	}

打印结果：

Hello action. Two Hello action. Hello action. One

一切正常。那么Netty是怎么分割对象流的呢？看看ObjectDecoder怎么做的。 在ObjectDecoder的基类LengthFieldBasedFrameDecoder中注释中有详细的说明。我们这里主要介绍一下关键的代码逻辑：

@Override
    protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {

        if (discardingTooLongFrame) {
            long bytesToDiscard = this.bytesToDiscard;
            int localBytesToDiscard = (int) Math.min(bytesToDiscard, buffer.readableBytes());
            buffer.skipBytes(localBytesToDiscard);
            bytesToDiscard -= localBytesToDiscard;
            this.bytesToDiscard = bytesToDiscard;
            failIfNecessary(ctx, false);
            return null;
        }

        if (buffer.readableBytes() < lengthFieldEndOffset) {
            return null;
        }

        int actualLengthFieldOffset = buffer.readerIndex() + lengthFieldOffset;
        long frameLength;
        switch (lengthFieldLength) {
        case 1:
            frameLength = buffer.getUnsignedByte(actualLengthFieldOffset);
            break;
        case 2:
            frameLength = buffer.getUnsignedShort(actualLengthFieldOffset);
            break;
        case 3:
            frameLength = buffer.getUnsignedMedium(actualLengthFieldOffset);
            break;
        case 4:
            frameLength = buffer.getUnsignedInt(actualLengthFieldOffset);
            break;
……

我们这里进入的是4，还记得在编码时候的开头的4位占位字节吗？跟踪进去发现。

public int getInt(int index) {
        return  (array[index]     & 0xff) << 24 |
                (array[index + 1] & 0xff) << 16 |
                (array[index + 2] & 0xff) <<  8 |
                (array[index + 3] & 0xff) <<  0;
    }

原来，当初在编码时，在流开头增加的4字节的字符是做这个的。他记录了当前了这个对象流的长度，便于在解码时候准确的计算出该对象流的长度，正确解码。看来，我们如果我们自己写的对象编码解码的工具，要考虑的还有很多啊。

附：LengthFieldBasedFrameDecoder的JavaDoc

/**
* A decoder that splits the received {@link ChannelBuffer}s dynamically by the
* value of the length field in the message.  It is particularly useful when you
* decode a binary message which has an integer header field that represents the
* length of the message body or the whole message.
* <p>
* {@link LengthFieldBasedFrameDecoder} has many configuration parameters so
* that it can decode any message with a length field, which is often seen in
* proprietary client-server protocols. Here are some example that will give
* you the basic idea on which option does what.
*
* <h3>2 bytes length field at offset 0, do not strip header</h3>
*
* The value of the length field in this example is <tt>12 (0x0C)</tt> which
* represents the length of "HELLO, WORLD".  By default, the decoder assumes
* that the length field represents the number of the bytes that follows the
* length field.  Therefore, it can be decoded with the simplistic parameter
* combination.
* <pre>
* <b>lengthFieldOffset</b>   = <b>0</b>
* <b>lengthFieldLength</b>   = <b>2</b>
* lengthAdjustment    = 0
* initialBytesToStrip = 0 (= do not strip header)
*
* BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
* +--------+----------------+      +--------+----------------+
* | Length | Actual Content |----->| Length | Actual Content |
* | 0x000C | "HELLO, WORLD" |      | 0x000C | "HELLO, WORLD" |
* +--------+----------------+      +--------+----------------+
* </pre>
*
* <h3>2 bytes length field at offset 0, strip header</h3>
*
* Because we can get the length of the content by calling
* {@link ChannelBuffer#readableBytes()}, you might want to strip the length
* field by specifying <tt>initialBytesToStrip</tt>.  In this example, we
* specified <tt>2</tt>, that is same with the length of the length field, to
* strip the first two bytes.
* <pre>
* lengthFieldOffset   = 0
* lengthFieldLength   = 2
* lengthAdjustment    = 0
* <b>initialBytesToStrip</b> = <b>2</b> (= the length of the Length field)
*
* BEFORE DECODE (14 bytes)         AFTER DECODE (12 bytes)
* +--------+----------------+      +----------------+
* | Length | Actual Content |----->| Actual Content |
* | 0x000C | "HELLO, WORLD" |      | "HELLO, WORLD" |
* +--------+----------------+      +----------------+
* </pre>
*
* <h3>2 bytes length field at offset 0, do not strip header, the length field
*     represents the length of the whole message</h3>
*
* In most cases, the length field represents the length of the message body
* only, as shown in the previous examples.  However, in some protocols, the
* length field represents the length of the whole message, including the
* message header.  In such a case, we specify a non-zero
* <tt>lengthAdjustment</tt>.  Because the length value in this example message
* is always greater than the body length by <tt>2</tt>, we specify <tt>-2</tt>
* as <tt>lengthAdjustment</tt> for compensation.
* <pre>
* lengthFieldOffset   =  0
* lengthFieldLength   =  2
* <b>lengthAdjustment</b>    = <b>-2</b> (= the length of the Length field)
* initialBytesToStrip =  0
*
* BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
* +--------+----------------+      +--------+----------------+
* | Length | Actual Content |----->| Length | Actual Content |
* | 0x000E | "HELLO, WORLD" |      | 0x000E | "HELLO, WORLD" |
* +--------+----------------+      +--------+----------------+
* </pre>
*
* <h3>3 bytes length field at the end of 5 bytes header, do not strip header</h3>
*
* The following message is a simple variation of the first example.  An extra
* header value is prepended to the message.  <tt>lengthAdjustment</tt> is zero
* again because the decoder always takes the length of the prepended data into
* account during frame length calculation.
* <pre>
* <b>lengthFieldOffset</b>   = <b>2</b> (= the length of Header 1)
* <b>lengthFieldLength</b>   = <b>3</b>
* lengthAdjustment    = 0
* initialBytesToStrip = 0
*
* BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
* +----------+----------+----------------+      +----------+----------+----------------+
* | Header 1 |  Length  | Actual Content |----->| Header 1 |  Length  | Actual Content |
* |  0xCAFE  | 0x00000C | "HELLO, WORLD" |      |  0xCAFE  | 0x00000C | "HELLO, WORLD" |
* +----------+----------+----------------+      +----------+----------+----------------+
* </pre>
*
* <h3>3 bytes length field at the beginning of 5 bytes header, do not strip header</h3>
*
* This is an advanced example that shows the case where there is an extra
* header between the length field and the message body.  You have to specify a
* positive <tt>lengthAdjustment</tt> so that the decoder counts the extra
* header into the frame length calculation.
* <pre>
* lengthFieldOffset   = 0
* lengthFieldLength   = 3
* <b>lengthAdjustment</b>    = <b>2</b> (= the length of Header 1)
* initialBytesToStrip = 0
*
* BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
* +----------+----------+----------------+      +----------+----------+----------------+
* |  Length  | Header 1 | Actual Content |----->|  Length  | Header 1 | Actual Content |
* | 0x00000C |  0xCAFE  | "HELLO, WORLD" |      | 0x00000C |  0xCAFE  | "HELLO, WORLD" |
* +----------+----------+----------------+      +----------+----------+----------------+
* </pre>
*
* <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
*     strip the first header field and the length field</h3>
*
* This is a combination of all the examples above.  There are the prepended
* header before the length field and the extra header after the length field.
* The prepended header affects the <tt>lengthFieldOffset</tt> and the extra
* header affects the <tt>lengthAdjustment</tt>.  We also specified a non-zero
* <tt>initialBytesToStrip</tt> to strip the length field and the prepended
* header from the frame.  If you don't want to strip the prepended header, you
* could specify <tt>0</tt> for <tt>initialBytesToSkip</tt>.
* <pre>
* lengthFieldOffset   = 1 (= the length of HDR1)
* lengthFieldLength   = 2
* <b>lengthAdjustment</b>    = <b>1</b> (= the length of HDR2)
* <b>initialBytesToStrip</b> = <b>3</b> (= the length of HDR1 + LEN)
*
* BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
* +------+--------+------+----------------+      +------+----------------+
* | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
* | 0xCA | 0x000C | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
* +------+--------+------+----------------+      +------+----------------+
* </pre>
*
* <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
*     strip the first header field and the length field, the length field
*     represents the length of the whole message</h3>
*
* Let's give another twist to the previous example.  The only difference from
* the previous example is that the length field represents the length of the
* whole message instead of the message body, just like the third example.
* We have to count the length of HDR1 and Length into <tt>lengthAdjustment</tt>.
* Please note that we don't need to take the length of HDR2 into account
* because the length field already includes the whole header length.
* <pre>
* lengthFieldOffset   =  1
* lengthFieldLength   =  2
* <b>lengthAdjustment</b>    = <b>-3</b> (= the length of HDR1 + LEN, negative)
* <b>initialBytesToStrip</b> = <b> 3</b>
*
* BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
* +------+--------+------+----------------+      +------+----------------+
* | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
* | 0xCA | 0x0010 | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
* +------+--------+------+----------------+      +------+----------------+
* </pre>
*
* @see LengthFieldPrepender
*/