Typesafe AttributeKeys
----------------------

                 Key: DIRMINA-874
                 URL: https://issues.apache.org/jira/browse/DIRMINA-874
             Project: MINA
          Issue Type: Improvement
          Components: Core
    Affects Versions: 3.0.0-M1
            Reporter: Christian Schwarz


In Mina 2 we can add attributes to a session and get them as well. Because 
IoSession#getAttribute(Object key)  returns Object we always have to remember 
what attribute type is associated with the key and cast the result to the 
expected type.

Mina 2 example:
final static Object KEY = new AttributeKey(SourceClass.class,"myKey");
...
session.set(KEY,"myAttribute");
String attributeValue= (String)session.get(KEY);


Instead of using plain Object-keys, the key type should contain information 
about its attributes. The aim is to get type-safe access to attributes.

Assume we have the following new AttributeKey-Class:
/**
* @parmeter T Type of the referenced Attribute
*/
class AttributeKey<T> {
  public TypesafeAttributeKey(Class<T> attributeType, String attributeKey){
   ...
  }
}

The IoSession should have Attribute related accessors like these:
 
 void setAttribute(AttributeKey<T> key, T value);
 T getAttribute(AttributeKey<T> key);


So in Mina 3 the example could look like this:

final static AttributeKey<String> KEY = new 
AttributeKey<String>(String.class,"myKey");
...
session.set(KEY,"myAttribute");
String attributeValue=session.get(KEY);

These 2 cases won't compile:

session.set(KEY,new Date());
Integer attributeValue=session.get(KEY);

This pattern would simplify the use of Attributes, because the programmer don't 
have to care about the types and can concentrate on more improtant things. In 
my humble opinion the Objekt keys should be removed at all, i don't see the 
purpose for such unspecific keys.



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