Deserializing JSON Array of Object Using Restlet/Jackson

Question

I am using Restlet to call an API that returns a JSON Array of objects. When making a similar call to pull back a single object, everything gets mapped correctly to the custom class, but when I pull back the array the Object is getting mapped as a LinkedHashMap instead of this custom object. I'm sure I just need to define how to deserialize the array correctly, but I haven't figured out exactly how to do that.

@Get("json")
public Trait getTrait();

@Get("json")
public HashSet<Trait> getTraits();

The former works fine, but the latter isn't work. Ultimately when I try to iterate through the HashSet I can this error: java.util.LinkedHashMap cannot be cast to com.test.traits.Trait.

Any help would be appreciated.

StaxMan · Accepted Answer · 2012-10-21 01:06:36Z

2

This is generally because type is passed as class (like HashSet) instead of generic type (HashSet<Trait>). If so, this equivalent to HashSet<Object>, and Jackson has no choice but to bind contents to simplest Object type available that matches which is just java.util.Collection of some kind.

It sounds like Restlet is not passing generic type information correctly for some reason.

There are work-arounds to this:

Use a wrapper object, so that Lists and Maps are always properties of a Java object, never root-level values. This ensures that types work, even if Restlet did not pass generic type for root value
Sub-class HashSet so that you have non-generic type; Jackson can then figure out generic types for HashSet

For second case, you just do something like:

public class MyTraits extends HashSet<Trait> { }

and

@Get("json")
public MyTraits getTraits();

And it will work because MyTraits has no generic type of its own; but its super-type declaration has generic parameters that are available to Jackson.

answered Oct 21, 2012 at 1:06

StaxMan

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1 Comment

Nick Over a year ago

The latter worked. I had actually tried that, but my new collection also had generics. As soon as I took that out everything worked.

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