golang gin gonic content-type not setting to application/json with c.JSON

According to the official documentation, c.JSON of gin-gonic should set the response header to application/json, but when I call my API from Postman, the response header is set to text/plain; charset=utf-8

I don't understand what I am missing, any idea ?

Doc :
> func JSON

> JSON serializes the given struct as JSON into the response body. It
> also sets the Content-Type as "application/json".

Here is a sample of my code :

func postLogin(c *gin.Context) {
	var credentials DTO.Credentials
	if err := c.BindJSON(&credentials); err == nil {
		c.JSON(buildResponse(services.CheckUserCredentials(credentials)))
	} else {
		var apiErrors = DTO.ApiErrors{}
		for _, v := range err.(validator.ValidationErrors) {
			apiErrors.Errors = append(apiErrors.Errors, DTO.ApiError{Field: v.Field, Message: v.Field + " is " + v.Tag})
		}
		c.JSON(http.StatusBadRequest, apiErrors)
	}
}

EDIT

After investigation, log.Println(c.Writer.Header().Get("Content-Type")) doesn't print any thing, showing content-type is empty as it should be.

func writeContentType(w http.ResponseWriter, value []string) {
	header := w.Header()
	log.Println(header.Get("Content-Type")) // <=========== Nothing happen
	if val := header["Content-Type"]; len(val) == 0 {
		header["Content-Type"] = value
	}
}

I really don't want to have to add c.Writer.Header().Set("Content-Type", "application/json") to every route in my architecture...

EDIT 2

It seems like binding:"required" break the Content-Type Header

type Credentials struct {
	Email         string        `json:"email" binding:"required"`
	Password      string        `json:"password" binding:"required"`
}

If you expect all your requests to be JSON, add a middle ware instead.

func JSONMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        c.Writer.Header().Set("Content-Type", "application/json")
        c.Next()
    }
}

On your router add

router.Use(JSONMiddleware())

Use c.ShouldBindJSON(&credentials) instead of c.BindJSON.
> Gin README.md - Model binding and validation
>
> These methods use MustBindWith under the hood. If there is a binding
> error, the request is aborted with c.AbortWithError(400,
> err).SetType(ErrorTypeBind). This sets the response status code to 400
> and the Content-Type header is set to text/plain; charset=utf-8.

As Eutychus said in a comment: c.BindJSON uses MustBindWith under the hood. If there is a binding error, the request is aborted and the response status code is set to 400 and the Content-Type header is set to text/plain; charset=utf-8.

The ShouldBindJSON uses ShouldBindWith under the hood. If there is a binding error, the error is returned and it is the developer's responsibility to handle the request and error appropriately. That is why it is the better one in this case.

After looking at the source, it looks like it won't write the Content-Type header if it is already set.

c.JSON calls this function which calls the following code:

func writeContentType(w http.ResponseWriter, value []string) {
	header := w.Header()
	if val := header["Content-Type"]; len(val) == 0 {
		header["Content-Type"] = value
	}
}

Therefore your Content-Type must be set somewhere else.

This can also happen if your output is not actually valid JSON and fails marshal. I saw content-type being set to application/text when i was not returning in an error handler, and i was accidentally concatenating a string onto JSON as a result of my poor error handling.