在golang中使用空白标识符导入的用例

Question

The Go programming language specification states: "To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name."

For example:

import _ "foo/bar"

I am having difficulty imagining a use case for this construct. The accepted answer for Usage of the `import` statement mentions a use case where a database driver may be loaded without the need for the importing program to use any of the exported functions of that package, but it leaves out the details to reader's imagination.

Are there real life examples (with code snippets and explanation) that illustrate this use case?

Answer 1

I was writing an image scaler. I wanted it to be able to read images in different formats like JPEG, PNG & GIF and output them in JPEG.

So in addition to the image and image/jpeg packages, I also had to import image/png and image/gif only to register their respective decoders.

Had I not imported those, the scaler would only be able to read JPEG images.

package main

import(
  "image"
  "image/jpeg" // I wanted to export the images as JPEG
  _ "image/png"
  _ "image/gif"
)

// ...

The relevant documentation from the image package:

Decoding any particular image format requires the prior registration of a decoder function. Registration is typically automatic as a side effect of initializing that format's package so that, to decode a PNG image, it suffices to have:

import _ "image/png"