[FIXED] Spring/json: Convert a typed collection like List<MyPojo>

Issue

I'm trying to marshal a list: List<Pojo> objects via the Spring Rest Template.

I can pass along simple Pojo objects, but I can't find any documentation that describes how to send a List<Pojo> objects.

Spring is using Jackson JSON to implement the HttpMessageConverter. The jackson documentation covers this:

In addition to binding to POJOs and "simple" types, there is one additional variant: that of binding to generic (typed) containers. This case requires special handling due to so-called Type Erasure (used by Java to implement generics in somewhat backwards compatible way), which prevents you from using something like Collection<String>.class (which does not compile).

So if you want to bind data into a Map<String,User> you will need to use:

Map<String,User> result = mapper.readValue(src, new TypeReference<Map<String,User>>() {});

where TypeReference is only needed to pass generic type definition (via anynomous inner class in this case): the important part is <Map<String,User>> which defines type to bind to.

Can this be accomplished in the Spring template? I took a glance at the code and it makes me thing not, but maybe I just don't know some trick.

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Solution

The ultimate solution, thanks to the helpful answers below, was to not send a List, but rather send a single object which simply extends a List, such as: class PojoList extends ArrayList<Pojo>. Spring can successfully marshal this Object, and it accomplishes the same thing as sending a List<Pojo>, though it be a little less clean of a solution. I also posted a JIRA in spring for them to address this shortcoming in their HttpMessageConverter interface.

Solution

If I read the docs for MappingJacksonHttpMessageConverter right, you will have to create and register a subclass of MappingJacksonHttpMessageConverter and override the getJavaType(Class<?>) method:

Returns the Jackson JavaType for the specific class. Default implementation returns TypeFactory.type(java.lang.reflect.Type), but this can be overridden in subclasses, to allow for custom generic collection handling. For instance:

protected JavaType getJavaType(Class<?> clazz) {
   if (List.class.isAssignableFrom(clazz)) {
     return TypeFactory.collectionType(ArrayList.class, MyBean.class);
   } else {
     return super.getJavaType(clazz);
   }
}

Answered By - Sean Patrick Floyd
Answer Checked By - Terry (JavaFixing Volunteer)