最近在研读Mina框架的源码，刚好手头上有项目就直接用上了。直接进入正题……

    对Mina有点了解的人都知道，Mina中有一条FilterChain，我们可以向其中加入Filter实现链式处理。从源码看来，多数Filter（如ProtocolCodecFilter）在使用过程只允许有一个实例。若FilterChain中存在多个相同的实例，则有可能出现各种莫明其妙的问题，这些问题往往是超出你预料之外的，而且调试起来比较困难，除非你熟知Mina整个框架。究其原因，主要是在AttributeKey的问题，例：

public class ProtocolCodecFilter extends IoFilterAdapter {
    /** A logger for this class */
    private static final Logger LOGGER = LoggerFactory.getLogger(ProtocolCodecFilter.class);

    private static final Class<?>[] EMPTY_PARAMS = new Class[0];

    private static final IoBuffer EMPTY_BUFFER = IoBuffer.wrap(new byte[0]);

    private final AttributeKey ENCODER = new AttributeKey(ProtocolCodecFilter.class, "encoder");

    private final AttributeKey DECODER = new AttributeKey(ProtocolCodecFilter.class, "decoder");

    private final AttributeKey DECODER_OUT = new AttributeKey(ProtocolCodecFilter.class, "decoderOut");

    private final AttributeKey ENCODER_OUT = new AttributeKey(ProtocolCodecFilter.class, "encoderOut");
	
	...
	private ProtocolDecoderOutput getDecoderOut(IoSession session, NextFilter nextFilter) {
        ProtocolDecoderOutput out = (ProtocolDecoderOutput) session.getAttribute(DECODER_OUT);

        if (out == null) {
            // Create a new instance, and stores it into the session
            out = new ProtocolDecoderOutputImpl();
            session.setAttribute(DECODER_OUT, out);
        }

        return out;
    }
	...
}

ProtocolDecoderOutput out = (ProtocolDecoderOutput) session.getAttribute(DECODER_OUT);

它们获取的实例是一样的。因此，也就造成两个实例中的ProtocolCodecOutput产生相互干扰的情况。

    我们再来看看为什么两个实例中的AttributeKey为什么会是一样的。

package org.apache.mina.core.session;

import java.io.Serializable;

/**
 * Creates a Key from a class name and an attribute name. The resulting Key will
 * be stored in the session Map.<br>
 * For instance, we can create a 'processor' AttributeKey this way :
 * 
 * <pre>
 * private static final AttributeKey PROCESSOR = new AttributeKey(
 * 	SimpleIoProcessorPool.class, "processor");
 * </pre>
 * 
 * This will create the <b>SimpleIoProcessorPool.processor@7DE45C99</b> key
 * which will be stored in the session map.<br>
 * Such an attributeKey is mainly useful for debug purposes.
 * 
 * @author <a href=http://www.mamicode.com/"http://mina.apache.org">Apache MINA Project>    session.getAttrubute(...) 中存储<KEY, VALUE>对的其实是
private static class DefaultIoSessionAttributeMap implements IoSessionAttributeMap {
        private final ConcurrentHashMap<Object, Object> attributes = new ConcurrentHashMap<Object, Object>(4);
		...
}

是一个ConcurrentHashMap。因此，它是通过KEY的hashCode去寻找对象，上述两个实例是一样的，事实上间接说明了它们的hashCode()返回的是一样的值。
    我们看到AttributeKey的hashCode()函数，唯一不一样的地方就只能是name，name是String类型的，其hashCode()计算方法如下：
Open Declaration int java.lang.String.hashCode()


Returns a hash code for this string. The hash code for a String object is computed as 

 s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
 
using int arithmetic, where s[i] is the ith character of the string, n is the length of the string, and ^ indicates exponentiation. (The hash value of the empty string is zero.)

从上述String中hashCode()的算法我们可知，只要String的内容一样，它们的hashCode()就是一样的。所以，就有了两个ProtocolCodecFilter中通过session.getAttribute(DECODER_OUT)会拿到同一对象，也就可能产生相互干扰的情况。
    之于相应的解决方法，可以把相应的Filter类抓出来，如ProtocolCodecFitler：
public class MyProtocolCodecFilter extends IoFilterAdapter {
    /** A logger for this class */
    private static final Logger LOGGER = LoggerFactory.getLogger(MyProtocolCodecFilter.class);

    private static final Class<?>[] EMPTY_PARAMS = new Class[0];

    private static final IoBuffer EMPTY_BUFFER = IoBuffer.wrap(new byte[0]);

    private final AttributeKey ENCODER = new AttributeKey(MyProtocolCodecFilter.class, "encoder");

    private final AttributeKey DECODER = new AttributeKey(MyProtocolCodecFilter.class, "decoder");

    private final AttributeKey DECODER_OUT = new AttributeKey(MyProtocolCodecFilter.class, "decoderOut");

    private final AttributeKey ENCODER_OUT = new AttributeKey(MyProtocolCodecFilter.class, "encoderOut");
	
	...
	...
}

调用的时候，由原先的
connector.getFilterChain().addLast("my-codec", new ProtocolCodecFilter(new OwspMessageFactory()));

改成
connector.getFilterChain().addLast("my-codec", new MyProtocolCodecFilter(new OwspMessageFactory()));

注：转载请注明出处 http://blog.csdn.net/oathevil/article/details/37601391