一.  解码方法
mina中有个内置类CumulativeProtocolDecoder是专门用来处理断包和粘包的。该类的api文档中有个实现的例子。
类org.apache.mina.filter.codec.CumulativeProtocolDecoder

public abstract class CumulativeProtocolDecoder extends ProtocolDecoderAdapter {private final AttributeKey BUFFER = new AttributeKey(getClass(), "buffer");public void decode(IoSession session, IoBuffer in,ProtocolDecoderOutput out) throws Exception {if (!session.getTransportMetadata().hasFragmentation()) {       //用来判断是否还有分帧(断包)while (in.hasRemaining()) {if (!doDecode(session, in, out)) {break;}}return;}处理断包，省略// ............................................}//需要实现的方法protected abstract boolean doDecode(IoSession session, IoBuffer in,ProtocolDecoderOutput out) throws Exception;
}

CumulativeProtocolDecoder是一个抽象类，必须继承并实现其doDecode方法，用户自定义协议的拆分就应该写在doDecode方法中，下面的类MessageDecoder是一个实现的例子。MessageDecoder解码网络数据到一种有两字节长度头的自定义消息协议格式。

/*** 断包和粘包处理，处理后的消息为一个或多个完整的数据消息*/
public class MessageDecoder extends CumulativeProtocolDecoder {/** (non-Javadoc)* @see org.apache.mina.filter.codec.CumulativeProtocolDecoder#doDecode(org.apache.mina.core.session.IoSession, org.apache.mina.core.buffer.IoBuffer,* org.apache.mina.filter.codec.ProtocolDecoderOutput)*/@Overrideprotected boolean doDecode(IoSession session, IoBuffer in,ProtocolDecoderOutput out) throws Exception {in.order(ServerConfig.ByteEndian);    //字节序, ServerConfig.ByteEndian = ByteOrder.LITTLE_ENDIAN//消息bufIoBuffer buf = IoBuffer.allocate(ServerConfig.MessageMaxByte);   //ServerConfig.MessageMaxByte 最大消息字节数buf.order(ServerConfig.ByteEndian);//考虑以下几种情况：//    1. 一个ip包中只包含一个完整消息//    2. 一个ip包中包含一个完整消息和另一个消息的一部分//    3. 一个ip包中包含一个消息的一部分//    4. 一个ip包中包含两个完整的数据消息或更多（循环处理在父类的decode中）if (in.remaining() > 1) {int length = in.getShort(in.position());
if (length < 4) {throw new ServerException("Error net message. (Message Length="+length+")");}if (length > ServerConfig.MessageMaxByte) {throw new ServerException("Error net message. Message Length("+length+") > MessageMaxByte("+ServerConfig.MessageMaxByte+")");}if (length > in.remaining()) return false;//复制一个完整消息byte[] bytes = new byte[length];in.get(bytes);buf.put(bytes);buf.flip();out.write(buf);return true;} else {return false;}}
}

二.  使用
将上面的解码器作为一个过滤器配置到mina中即可，在spring中的配置方法如下：

<!-- 协议过滤器，包括解码和译码 --><bean id="protocolCodecFilter" class="org.apache.mina.filter.codec.ProtocolCodecFilter"><constructor-arg><bean id="factory" class="server.ClientConnServer.MessageCodecFactory"></bean></constructor-arg></bean><!-- 将协议过滤器配置到mina的过滤链中 --><bean id="filterChainBuilder" class="org.apache.mina.core.filterchain.DefaultIoFilterChainBuilder"><property name="filters"><map><entry key="protocolCodecFilter" value-ref="protocolCodecFilter" /></map></property></bean><!-- 处理器 --><bean id="clientConnHandler" class="server.ClientConnServer.ClientConnHandler" /><!-- socket接收器，接收客户端连接 --><bean id="ioAcceptor" class="org.apache.mina.transport.socket.nio.NioSocketAcceptor" destroy-method="unbind"><!--        <property name="defaultLocalAddress" value=":161" />--><property name="handler" ref="clientConnHandler" /><property name="reuseAddress" value="true" /><property name="filterChainBuilder" ref="filterChainBuilder" /></bean>

要配置协议过滤器，必须使用一个ProtocolCodecFactory ，下面是简单实现:

public class MessageCodecFactory implements ProtocolCodecFactory {private final MessageEncoder encoder;private final MessageDecoder decoder;public MessageCodecFactory() {encoder = new MessageEncoder();decoder = new MessageDecoder();}/* (non-Javadoc)* @see org.apache.mina.filter.codec.ProtocolCodecFactory#getDecoder(org.apache.mina.core.session.IoSession)*/@Overridepublic ProtocolDecoder getDecoder(IoSession session) throws Exception {return decoder;}/* (non-Javadoc)* @see org.apache.mina.filter.codec.ProtocolCodecFactory#getEncoder(org.apache.mina.core.session.IoSession)*/@Overridepublic ProtocolEncoder getEncoder(IoSession session) throws Exception {return encoder;}
}

相关的编码器，此处不做任何事情

public class MessageEncoder extends ProtocolEncoderAdapter {/* (non-Javadoc)* @see org.apache.mina.filter.codec.ProtocolEncoder#encode(org.apache.mina.core.session.IoSession, java.lang.Object, org.apache.mina.filter.codec.ProtocolEncoderOutput)*/@Overridepublic void encode(IoSession session, Object message,ProtocolEncoderOutput out) throws Exception {//Do nothing}}