Mina 协议编解码过滤器二（协议解码器）
[url]http://donald-draper.iteye.com/blog/2376663[/url]
[size=medium][b]引言：[/b][/size]
上一篇文章，我们看了协议编解码过滤器，其中涉及到协议编解码工厂、协议编码器、和协议解码器，
由于篇幅原因，我们只看了协议编解码工厂、协议编码器，先来回顾一下：
协议编解码过滤器ProtocolCodecFilter关联一个协议编解码工厂ProtocolCodecFactory，协议编解码工厂提供协议编码器ProtocolEncoder和解码器ProtocolDecoder；编码器将消息对象编码成二进制或协议数据，解码器将二进制或协议数据解码成消息对象。编码器ProtocolEncoder主要有两个方法，encode和dispose；encode用于，编码上层的消息对象为二进制或协议数据。mina调用编码器的#encode方法将从会话写请求队列中pop的消息，然后调用ProtocolEncoderOutput#write方法将编码后的消息放在ByteBuffer；dispose方法释放编码器资源。ProtocolEncoderAdapter为编码器抽象实现，默认实现了dispose，不做任何事情，对于不需要释放资源的编码器继承ProtocolEncoderAdapter。ProtocolEncoderOutput主要的工作是将协议编码器编码后的
字节buffer，缓存起来，等待flush方法调用时，则将数据发送出去。SimpleProtocolEncoderOutput为ProtocolEncoderOutput的简单实现内部有一个buffer队列bufferQueue（Queue），用于存放write（ByteBuffer）方法，传入的字节buffer；
mergeAll方法为合并buffer队列的所有buffer数据到一个buffer；flush方法为发送buffer队列中的所有buffer，实际发送工作委托给doFlush方法待子类实现。ProtocolEncoderOutputImpl为协议编解码过滤器的内部类，ProtocolEncoderOutputImpl的doFlush，首先将会话包装成DefaultWriteFuture，将会话，写请求信息传递给NextFilter。
今天这篇文章我们来看一下协议解码器：
先贴出协议编解码过滤器及协议编解码工厂ProtocolCodecFactory的代码，以便理解
//ProtocolCodecFilter
/**
* An {@link IoFilter} which translates binary or protocol specific data into
* message object and vice versa using {@link ProtocolCodecFactory},
* {@link ProtocolEncoder}, or {@link ProtocolDecoder}.
*
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*/
public class ProtocolCodecFilter extends IoFilterAdapter
{
private static final Logger LOGGER = LoggerFactory.getLogger(org/apache/mina/filter/codec/ProtocolCodecFilter);
private static final Class EMPTY_PARAMS[] = new Class[0];
private static final IoBuffer EMPTY_BUFFER = IoBuffer.wrap(new byte[0]);
//编码器属性key
private static final AttributeKey ENCODER =
new AttributeKey(org/apache/mina/filter/codec/ProtocolCodecFilter, "encoder");
//编码器输出属性key
private static final AttributeKey ENCODER_OUT =
new AttributeKey(org/apache/mina/filter/codec/ProtocolCodecFilter, "encoderOut");
//解码器属性key
private static final AttributeKey DECODER =
new AttributeKey(org/apache/mina/filter/codec/ProtocolCodecFilter, "decoder");
//解码器输出属性key
private static final AttributeKey DECODER_OUT =
new AttributeKey(org/apache/mina/filter/codec/ProtocolCodecFilter, "decoderOut");
private final ProtocolCodecFactory factory;//协议编解码器工厂
}//ProtocolCodecFactory
/**
* Provides {@link ProtocolEncoder} and {@link ProtocolDecoder} which translates
* binary or protocol specific data into message object and vice versa.
* 协议编解码工厂ProtocolCodecFactory提供协议编码器和解码器，解码器二进制数据或
协议数据到消息对象；编码器反之。
* Please refer to
* [url=../../../../../xref-examples/org/apache/mina/examples/reverser/ReverseProtocolProvider.html]ReverserProtocolProvider[/url]
* example.
*
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*/
public interface ProtocolCodecFactory {
/**
* Returns a new (or reusable) instance of {@link ProtocolEncoder} which
* encodes message objects into binary or protocol-specific data.
返回一个编码器实例，用于将消息对象编码成二进制或协议数据
*/
ProtocolEncoder getEncoder() throws Exception;
/**
* Returns a new (or reusable) instance of {@link ProtocolDecoder} which
* decodes binary or protocol-specific data into message objects.
返回一个解码器实例，用于将二进制或协议数据解码成消息对象
*/
ProtocolDecoder getDecoder() throws Exception;
}
从上可以看出，协议编解码工厂ProtocolCodecFactory提供协议编码器ProtocolEncoder和解码器ProtocolDecoder；
编码器将消息对象编码成二进制或协议数据，解码器将二进制或协议数据解码成消息对象。
上一篇文章我们看了协议编码器，今天来看一下协议解码器:
/**
* Decodes binary or protocol-specific data into higher-level message objects.
* MINA invokes {@link #decode(IoSession, ByteBuffer, ProtocolDecoderOutput)}
* method with read data, and then the decoder implementation puts decoded
* messages into {@link ProtocolDecoderOutput} by calling
* {@link ProtocolDecoderOutput#write(Object)}.
*
* Please refer to
* [url=../../../../../xref-examples/org/apache/mina/examples/reverser/TextLineDecoder.html]TextLineDecoder[/url]
* example.
*
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*/
public interface ProtocolDecoder {
/**
* Decodes binary or protocol-specific content into higher-level message objects.
* MINA invokes {@link #decode(IoSession, ByteBuffer, ProtocolDecoderOutput)}
* method with read data, and then the decoder implementation puts decoded
* messages into {@link ProtocolDecoderOutput}.
* 将二级制或协议内存解码成上层消息对象。mina在读取数据时，调用解码器的#decode，
将解码后的消息放到ProtocolDecoderOutput
* @throws Exception if the read data violated protocol specification
*/
void decode(IoSession session, ByteBuffer in, ProtocolDecoderOutput out)
throws Exception;
/**
* Invoked when the specified session is closed. This method is useful
* when you deal with the protocol which doesn't specify the length of a message
* such as HTTP response without content-length header. Implement this
* method to process the remaining data that {@link #decode(IoSession, ByteBuffer, ProtocolDecoderOutput)}
* method didn't process completely.
* 到session关闭时，调用此方法。当处理没有精确长度协议时，特别有用，不如Http没有内容长度的回应。
实现此方法，主要是对于
* @throws Exception if the read data violated protocol specification
*/
void finishDecode(IoSession session, ProtocolDecoderOutput out)
throws Exception;
/**
* Releases all resources related with this decoder.
* 释放所有与解码器有关的资源。
* @throws Exception if failed to dispose all resources
*/
void dispose(IoSession session) throws Exception;
}
从上面来看解码器主要是将二级制或协议内存解码成上层消息对象。mina在读取数据时，调用解码器的#decode，将解码后的消息放到ProtocolDecoderOutput；当会话关闭时，调用finishDecode解码那些在#decode方法中没有处理完的数据。dispose主要是 释放所有与解码器有关的资源。
再来看协议解码器的抽象实现ProtocolDecoderAdapter
/**
* An abstract {@link ProtocolDecoder} implementation for those who don't need
* {@link ProtocolDecoder#finishDecode(IoSession, ProtocolDecoderOutput)} nor
* {@link ProtocolDecoder#dispose(IoSession)} method.
* 协议解码适配器主要是针对那些不需要#finishDecode和#dispose的解码器
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*/
public abstract class ProtocolDecoderAdapter implements ProtocolDecoder {
/**
* Override this method to deal with the closed connection.
* The default implementation does nothing.会话关闭
*/
public void finishDecode(IoSession session, ProtocolDecoderOutput out)
throws Exception {
}
/**
* Override this method to dispose all resources related with this decoder.
* The default implementation does nothing.释放资源
*/
public void dispose(IoSession session) throws Exception {
}
}ProtocolDecoderAdapter协议解码适配器主要是针对那些不需要#finishDecode和#dispose的解码器；
mina对于大部分的组件，都实现的相应的适配，使我们可以根据需要，去实现需要关注的方法或事件，
忽略不关心的方法或事件。比如我们前面看的IoFilter->IoFilterAdapter。
再来看编码输出ProtocolDecoderOutput，
/**
* Callback for {@link ProtocolDecoder} to generate decoded messages.
* {@link ProtocolDecoder} must call {@link #write(Object)} for each decoded
* messages.
*
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*/
public interface ProtocolDecoderOutput {
/**
* Callback for {@link ProtocolDecoder} to generate decoded messages.
* {@link ProtocolDecoder} must call {@link #write(Object)} for each
* decoded messages.
* 用于解码器，解码消息后，回调write方法。
* @param message the decoded message
*/
void write(Object message);
/**
* Flushes all messages you wrote via {@link #write(Object)} to
* the next filter.刷新所有write方法写的消息
*/
void flush();
}ProtocolDecoderOutput主要有两个方法，一个write方法，用于解码器，解完消息后回调；
一个flush方法，用于刷新所有解码器写到协议解码输出的消息对象。
再来看ProtocolDecoderOutput的简单实现SimpleProtocolDecoderOutput
/**
* A {@link ProtocolDecoderOutput} based on queue.
*
* @author The Apache Directory Project ([email protected])
* @version $Rev$, $Date$
*
*/
public class SimpleProtocolDecoderOutput implements ProtocolDecoderOutput {
private final NextFilter nextFilter;//后继过滤器
private final IoSession session;//关联会话
private final Queue messageQueue = new Queue();//消息队列
//根据会话和后继过滤器构建简单协议解码输出
public SimpleProtocolDecoderOutput(IoSession session, NextFilter nextFilter) {
this.nextFilter = nextFilter;
this.session = session;
}
//写消息，直接将消息push到消息队列
public void write(Object message) {
messageQueue.push(message);
if (session instanceof BaseIoSession) {
((BaseIoSession) session).increaseReadMessages();
}
}
//将消息队列中的消息传给后继过滤器的messageReceived方法。
public void flush() {
while (!messageQueue.isEmpty()) {
nextFilter.messageReceived(session, messageQueue.pop());
}
}
}从上面来看，简单协议解码输出SimpleProtocolDecoderOutput，关联一个会话session，一个后继过滤器nextFilter，及一个消息队列messageQueue；所有解码器解码后的消息，回调协议解码输出的write方法，将消息暂时放在消息队列messageQueue中；flush方法主要是将消息队列中的消息传给后继过滤器的messageReceived方法。
再来看一些编解码过滤器获取协议解码输出的方法：
//ProtocolCodecFilter
//获取协议解码输出
private ProtocolDecoderOutput getDecoderOut(IoSession session,
NextFilter nextFilter) {
//从当前会话获取解码输出属性DECODER_OUT对一个的ProtocolDecoderOutput
ProtocolDecoderOutput out = (ProtocolDecoderOutput) session.getAttribute(DECODER_OUT);
if (out == null) {
//为null，则创建一个SimpleProtocolDecoderOutput，添加到会话中
out = new SimpleProtocolDecoderOutput(session, nextFilter);
session.setAttribute(DECODER_OUT, out);
}
return out;
}//获取协议编码输出
private ProtocolEncoderOutputImpl getEncoderOut(IoSession session,
NextFilter nextFilter, WriteRequest writeRequest) {
//这个就是我们上一篇文章讲的
return new ProtocolEncoderOutputImpl(session, nextFilter, writeRequest);
}从上面来看协议编解码过滤器ProtocolCodecFilter默认的协议编码输出为ProtocolEncoderOutputImpl，协议解码输出为SimpleProtocolDecoderOutput。
至此我们将协议编解码过滤器的所涉及到的相关概念看完，来整理一下：
协议编解码过滤器关联一个协议编解码工厂，协议编解码工厂用于创建协议编码和解码器；协议编码器将上层消息，编码成二级制或特定协议格式的数据，写到协议编码器输出的字节队列中，flush字节队列中的数据（filterWrite）给下一个过滤器。协议解码器将接收到的二级制或特定协议格式的数据，解码成上层消息，存放到协议解码器输出的消息队列，flush将消息队列中的消息传给后继过滤器的messageReceived方法。协议编解码过滤器ProtocolCodecFilter默认的协议编码输出为ProtocolEncoderOutputImpl，
协议解码输出为SimpleProtocolDecoderOutput。
结构如下：
ProtocolCodecFilter extends IoFilterAdapter
--ProtocolCodecFactory
--ProtocolEncoder
--ProtocolEncoderOutput（ProtocolEncoderOutputImpl）
--ProtocolDecoder
--ProtocolDecoderOutput（SimpleProtocolDecoderOutput）[size=medium][b]总结：[/b][/size]
[color=blue]解码器ProtocolDecoder将二级制或协议内存解码成上层消息对象。mina在读取数据时，调用解码器的#decode，将解码后的消息放到ProtocolDecoderOutput；当会话关闭时，调用finishDecode解码那些在#decode方法中没有处理完的数据。dispose主要是 释放所有与解码器有关的资源。针对不需要#finishDecode和#dispose的解码器，我们可以继承协议解码适配器ProtocolDecoderAdapter。ProtocolDecoderOutput有两个方法，一个write方法，用于解码器，解完消息后回调；一个flush方法，用于刷新所有解码器写到协议解码输出的消息对象。简单协议解码输出SimpleProtocolDecoderOutput，关联一个会话session，一个后继过滤器nextFilter，及一个消息队列messageQueue；所有解码器解码后的消息，回调协议解码输出的write方法，将消息暂时放在消息队列messageQueue中；flush方法主要是将消息队列中的消息传给后继过滤器的messageReceived方法。[/color]