JAXB 2.0, XMLAdaptor and HashMap for customized mapping

Question

I have a requirement to implement a custom mapping of HashMap in JAXB. Basically I want to be able to use a HashMap structure to represent key/value pairs instead of the default List. So I need to be able to store (marshal) a HashMap structure into XML, and then be able to read (unmarshal) the XML back into a HashMap.
I've discovered the XMLAdaptor class and I think this is the way to do it:
https://jaxb.dev.java.net/nonav/jaxb20-pr/api/javax/xml/bind/annotation/adapters/XmlAdapter.html
However, my knowledge of generics and annotations in 5.0, and XML schemas are somewhat elementary - so I'm a bit stuck.
If someone has a complete working example of the HashMap example they show in the API doc linked above, I would greatly appreciate it if you could post it here.
Otherwise, perhaps you could answer a couple of questions for me.
1) I tried using the XSD directly from the example in the API doc (Step 2).
     <xs:complexType name="myHashMapType">
       <xs:sequence>
         <xs:element name="entry" type="myHashMapEntryType"
                        maxOccurs="unbounded"/>
       </xs:sequence>
     </xs:complexType>
     <xs:complexType name="myHashMapEntryType">
       <xs:simpleContent>
         <xs:extension base="xs:string"/>
       </xs:simpleContent>
       <xs:attribute name="key" type="xs:int"/>
     </xs:complexType>xjc complains when I generate the classes and says that the 'xs:attribute name=key' shouldn't be there. If I move it up into the 'xs:extension' node it works. But then I get strange XML when I create some key/value pairs and marshal them. The first XML node looks okay, but the rest of the List items start with '<xsi:entry ...', e.g.
<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="myHashMapEntryType" key="key0">value0</entry>
<xsi:entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="myHashMapEntryType" key="key1">value1</xsi:entry>
...So my first question is, what is the proper XSD to represent a HashMap with XML like the following:
     <hashmap>
         <entry key="id123">this is a value</entry>
         <entry key="id312">this is another value</entry>
      </hashmap>2) Now for the HashMap part of it; you need to create a class that extends XmlAdaptor<HashMap, MyHashMapType> with marshal and unmarshal methods. This is where I get a little fuzzy on exactly what these methods need to do. The example in the API doc doesn't show this.
3) And finally; once I have all this in place, how do I actually marshal and unmarshal, presumably using the value types that I created instead of the value types that were auto generated by xjc? At least I think that's what I'm supposed to do.
Thanks.

Answer 1

Download the javaeetutorial5
Under this path is a working example.
\examples\jaxb\j2s-xmlAdapter-field
I've tried following the bad example you sited, and even with a great deal of work it still fails to work.
I think the example will help clarify how marshall and unmarshall methods are to be implemented. What would be really nice is for whoever wrote the example you sited to finish the job instead of expecting the reader of their documentation to figure it out.
I fail to understand why JaxB has failed to directly support a collection so prevalent in its use as a HashMap. I also don�t know why they can�t support a Map interface that by default maps to a HashMap.
Best of luck to you,
Robert