GoLang: Initialization before the execution of init()

Problem Definition

I have this goLang package which reads a JSON config file and sets up a map. I didn't want to call a separate method to load the file and set up the map. I wanted this to happen in the beginning automatically.
So I created init() method which loads the file and initializes the map with required configuration.

Problem

I fell into problem when I wrote unit test.

For the test I wanted to load with mocked content, not the actual file content. I actually didn't want to load the file at all.

So I created a variable fileReader as an instance of ioutil.ReadFile() method. It allows me to inject mocked file reader as dependency and execute whatever I want.

But as I said earlier, I had implemented init() method to read the file. This init() executed in the beginning before I was able to inject my mocked file reader.

Solution

Clearly the solution was to find a way to inject the dependency before the init() method executes. Fortunately I found a way to do that. Its using the following:

        // executes becore init() method so that fileReader can be injected
        var _ = func() (_ struct{}) {
          fileReader = mockedFileReader
          return
        }()
      

Code Illustration

Packakge Implementation

Test Implementation

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