Lowercase JSON key names with JSON Marshal in Go

json go serialization tags marshalling

ANisus · Jul 27, 2012 · Viewed 43.9k times · Source

I wish to use the "encoding/json" package to marshal a struct declared in one of the imported packages of my application.

Eg.:

type T struct {
    Foo int
}

Because it is imported, all available (exported) fields in the struct begins with an upper case letter. But I wish to have lower case key names:

out, err := json.Marshal(&T{Foo: 42})

will result in

{"Foo":42}

but I wish to get

{"foo":42}

Is it possible to get around the problem in some easy way?

Answer

Have a look at the docs for encoding/json.Marshal. It discusses using struct field tags to determine how the generated json is formatted.

For example:

type T struct {
    FieldA int    `json:"field_a"`
    FieldB string `json:"field_b,omitempty"`
}

This will generate JSON as follows:

{
    "field_a": 1234,
    "field_b": "foobar"
}