Is default no-args constructor mandatory for Gson?

Question

As of Gson 2.3.1.

Regardless of what the Gson documentation says, if your class doesn't have an no-args constructor and you have not registered any InstanceCreater objects, then it will create an ObjectConstructor (which constructs your Object) with an UnsafeAllocator which uses Reflection to get the allocateInstance method of the class sun.misc.Unsafe to create your class' instance.

This Unsafe class goes around the lack of no-args constructor and has many other dangerous uses. allocateInstance states

Allocate an instance but do not run any constructor. Initializes the class if it has not yet been.

So it doesn't actually need a constructor and will go around your two argument constructor. See some examples here.

If you do have a no-args constructor, Gson will use an ObjectConstructor which uses that default Constructor by calling

yourClassType.getDeclaredConstructor(); // ie. empty, no-args

My 2 cents: Follow what Gson says and create your classes with a no-arg constructor or register an InstanceCreator. You might find yourself in a bad position using Unsafe.