Mina中关于多个同类型Filter（如ProtocolCodecFilter）实例共存问题的解决

    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;
     }
     ...
 }

    假设有两个ProtocolCodecFilter实例，那么同一时刻它们在各自的实例中AttributeKey其实是一样的，即如果通过

 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://mina.apache.org">Apache MINA Project</a>
  */
 public final class AttributeKey implements Serializable {
     /** The serial version UID */
     private static final long serialVersionUID = -583377473376683096L;

     /** The attribute's name */
     private final String name;

     public AttributeKey(Class<?> source, String name) {
         this.name = source.getName() + '.' + name + '@' + Integer.toHexString(this.hashCode());
     }

     @Override
     public String toString() {
         return name;
     }

     @Override
     public int hashCode() {
         int h = 17 * 37 + ((name == null) ? 0 : name.hashCode());
         return h;
     }

     @Override
     public boolean equals(Object obj) {
         if (this == obj) {
             return true;
         }

         if (!(obj instanceof AttributeKey)) {
             return false;
         }

         AttributeKey other = (AttributeKey) obj;

         return name.equals(other.name);
     }
 }

    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