Is default no-args constructor mandatory for Gson?

Question

Gson user guide states that we should define default no-args constructor for any class to work with Gson properly. Even more, in the javadoc on Gson's InstanceCreator class said that exception will be thrown if we try to deserialize instance of class missing default constructor and we should use InstanceCreator in such cases. However, I've tried to test use Gson with class lacking default constructor and both serialization and deserialization work without any trouble.

Here is the piece of code for deserializaiton. A class without non-args constructor:

public class Mushroom {
    private String name;
    private double diameter;

    public Mushroom(String name, double diameter) {
        this.name = name;
        this.diameter = diameter;
    }

    //equals(), hashCode(), etc.
}

and a test:

@Test
public void deserializeMushroom() {
    assertEquals(
            new Mushroom("Fly agaric", 4.0),
            new Gson().fromJson(
                    "{name:\"Fly agaric\", diameter:4.0}", Mushroom.class));
}

which works fine.

So my question is: could I actually use Gson without need to have default constructor or there is any circumstances when it will not work?

Answer 1

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.

Answer 2

There is a good solution in the Jackson library as described here:

https://stackoverflow.com/a/11838468/2854723

The point is to tell the serializer via the Mix-Ins feature which JSON fields to use when using the constructor with arguments.

If that entity is part of an external library then you can "remote annotate" with the Creator feature.