How to call json.Unmarshal inside UnmarshalJSON without causing stack overflow?

Question

How can I create a method UnmarshalJSON inside a struct, use json.Unmarshal inside without causing stack overflow?

package xapo

type Xapo struct {}

func (x Xapo) UnmarshalJSON(data []byte) error {
    err := json.Unmarshal(data, &x)
    if err != nil {
        return err
    }
    fmt.Println("done!")
    return nil
}

Can someone explain me why the stack overflow happens? Can it be fixed?

Thanks in advance.

Answer 1

It looks like you may be trying to do custom unmarshaling by using the default unmarshaler and then post-processing the data. However, as you've discovered, the obvious way of trying this causes an infinite loop!

The usual workaround is to create a new type using your type, use the default unmarshaler on an instance of the new type, post-process the data, and then finally cast to the original type and assign back to the target instance. Note that you'll want to implement UnmarshalJSON on a pointer type.

For example:

func (x *Xapo) UnmarshalJSON(data []byte) error {
  // Create a new type from the target type to avoid recursion.
  type Xapo2 Xapo

  // Unmarshal into an instance of the new type.
  var x2 Xapo2
  err := json.Unmarshal(data, &x2)
  if err != nil {
    return err
  }

  // Perform post-processing here.
  // TODO

  // Cast the new type instance to the original type and assign.
  *x = Xapo(x2)
  return nil
}