In my previous blog JSON deserialize generic types using Gson and Jackson
I talked about how you can deserialize JSON using Java Generics. Now in this blog post, we are going to learn how
to deserialize JSON into a Java class that doesn’t have a default constructor. For this blog, I am using the Jackson library.
Almost all frameworks require a default(no-argument) constructor in your class because these frameworks use reflection
to create objects by invoking the default constructor but if there is no default constructor present in class then
its hard to instantiate it using reflection. Let’s assume we have a class with a no-args constructor
(only a parameterized constructor present) and we want to deserialize it.
And a json file
There are two ways in Jackson to deserialize this type of class.
Custom deserializer
Mixin Annotations
1. Custom Deserializer
In Custom Deserializer you will create a class and extend it with com.fasterxml.jackson.databind.JsonDeserializer and override its abstract method deserialize(). This class gives you full control, you’ll get json in com.fasterxml.jackson.core.JsonParser. Now you can map json properties with class properties. In my case, I am creating UserProfileDeserializer.
Now we need to register above deserializer in com.fasterxml.jackson.databind.ObjectMapper. So that, while deserializing the UserProfile class it uses UserProfileDeserializer.
In above code we are adding our Deserializer in com.fasterxml.jackson.databind.module.SimpleModule and that module is registered in com.fasterxml.jackson.databind.ObjectMapper. Similarly, you can register deserializers for other classes too.
2. Mixin Annotations
Jackson provides another way of doing this by using Mixin Annotations. You can read about Jackson Mixin here. In our class UserProfile there is no default constructor, it has a parameterized constructor. Let’s check out how to create a mixin for our class.
That’s it !!
We have created a constructor same as in UserProfile class and marked it with com.fasterxml.jackson.annotation.JsonCreator annotation. This will tell com.fasterxml.jackson.databind.ObjectMapper this is how target class (UserProfile) constructor looks and constructor parameters are marked with com.fasterxml.jackson.annotation.JsonProperty annotation.
Now check out how to register this mixin class in com.fasterxml.jackson.databind.ObjectMapper
