JSON.deserialize won't call default constructor

Question

One colleague and I spend a few mins wondering why a class constructor was not called when Salesforce was reconstructing the class using JSON.deserialize(jsonString, apexType). I could not find any documentation that specifies that, but I expected the no-arg constructor to be called.

The deserialized class was something like this:

public class MyType {

  public String config;
  public final String defaultConfig;

  public MyType() {
    defaultConfig = 'anyValue';
  }

  public void calculate() {
    if (String.isEmpty(config)) config = defaultConfig;
    ...
  }

}

So, is this expected behaviour, or is this an extraneous versioned behaviour?

Answer 1

Yes, that's the intended behavior. Constructors are not called when you use JSON.deserialize. As an alternative approach, you can use getters:

public class MyType {

  public String config {
    public get { return String.isNotEmpty(config)? config: defaultConfig; }
    public set;
  }
  public String defaultConfig { get { return 'anyValue'; } }

}

Answer 2

While it isn't documented, it is the behaviour we've observed.

I think it is fair to say that the assumption is, in deserializing the object, all required state should already be in place based on that deserialized data. Sure, things like transients will be in their default state, but these likely need specific methods to be invoked to calculate them from the inputs and any state of the object anyway.

As an aside, personally I would avoid the sort of structure you have expressed in your example; for me, defaultConfig should be initialized in its declaration.

Edit:

In addition, according to Jeferson Chaves's findings, neither initializing member declarations nor instance initializer blocks are executed (search for "Instance initialization") when JSON.deserialize is used.

See @sfdcfox's excellent alternative solution to your initialization issue.