Map<K,V>with JAXB. First, JAXB does not like interfaces. Second, it considers a
Map<K,V>to be a bean so it does not handle the internal implementation. The JAXB JIRA Issue is here JAXB-223
There are a number of articles published about how to adapt a
Map<K,V>to work in JAXB. However, all of them I have seen are very specific mappings like
Map<Integer,String>, etc. None of them address the more specific generic case
Map<K,V>. This example handles the more general case using generics. All the other examples I looked at online generally follow the example code from the
XmlAdapterjavadoc. The code snippet does not encapsulate the values, and generally does not adhere to good coding practices.
The process is to create an adapter is as follows:
- Create an mapping entry for
Map<K,V>. In our case, we will create one called:
MapEntryType<K, V>which will map our generic types.
- Next we create a value holder for our
Map<K,V>which contains a
List<MapEntryType<K, V>>of our entries. The value holder looks like
- Finally, we create an adapter which extends
XmlAdapter. In our case, it is very long since I am using Java 6. In Java 7, we can use the diamond <> operator to shorten it. The adapter is
XmlGenericMapAdapter<K, V> extends XmlAdapter<MapType<K, V>, Map<K, V>>.
MapEntryType<K, V>has the
@XmlAccessorType(XmlAccessType.PROPERTY)set to property and the getters are annotated as
The NetBeans 7.1 RC1 Apache Maven project can be downloaded here: generic-jaxb-map-adapter.zip
XmlGenericMapAdapter<K, V> extends XmlAdapter<MapType<K, V>, Map<K, V>>
Here is the output from our adapter.
We take advantage of JAXB to provide automatic XML Schema definitions for our keys and values. The only problem with this approach is that it is very "chatty". It provides a schema definition for each key and value.
If you know that you are going to be using simple types, it is more appropriate to use a specific type rather than using generics. We would replace all of the <K, V> with <String, String>. This will remove all of the schema definitions and produce an output which is much cleaner. The
Stringadapters are included in the source code for further review.