How to serialize Exception

Question

According to ruby-doc and apidock, you can serialize and deserialize an exception using to_json and json_create.

But after having wasted some time trying to use them, I still haven't found a way.

Calling exc.to_json gives me an empty hash, and Exception.json_create(hash) gives me this error: undefined method 'json_create' for Exception:Class

I guess I could easily recreate those functions since the source is available, but I'd rather understand what I'm doing wrong... Any idea?

Answer 1

The JSON module doesn't extend Exception by default. You have to require "json/add/exception". I'm not sure if this is documented anywhere:

require "json/add/exception"

begin
  nil.foo
rescue => exception
  ex = exception
end

puts ex.to_json
# => {"json_class":"NoMethodError","m":"undefined method `foo' for nil:NilClass","b":["prog.rb:5:in `<main>'"]}

Check out ext/json/lib/json/add in the Ruby source to see which classes work this way. If you do require "json/add/core" it will load JSON extensions for Date, DateTime, Exception, OpenStruct, Range, Regexp, Struct, Symbol, Time, and others.

Answer 2

The answer from Jordan is correct. If you have a case eg. that you need to serialize an Exception and send to to ActiveJob where you want to reconstruct it, then you need to use .as_json method.

require "json/add/exception"

begin
  nil.foo
rescue => exception
  ex = exception
end

puts ex.to_json.class
# => String

string = ex.to_json
puts Exception.json_create(string).message
# => m

puts ex.as_json.class
# => Hash

hash = ex.as_json
puts Exception.json_create(hash).message
# => undefined method `foo' for nil:NilClass

You need to read the source code to understand why Exception.json_create(string).messagereturns m :)

Answer 3

It's also important to note that the Exception.json_create example doesn't keep the error class.

Exception.json_create(JSON.parse(ArgumentError.new('error').to_json))
# => #<Exception: error>

Try instead:

require "json/add/exception"

def deserialize_exception(json)
  hash = JSON.parse(json)
  hash['json_class'].constantize.json_create(hash)
end