How to Handle JSON and Image Data in Multipart/Form-Data Requests with Gin in Go?

Receiving JSON and image data with Gin in Go

Brief

In this article, we'll examine how to design an HTTP request handler in Go using the Gin framework to accept both JSON data and an image file sent as part of a multipart/form-data request.

Setting up the request handler

We start by defining the request handler, UpdateProfile(), with a struct that defines the expected request body format:

type request struct {
    Username    string `json:"username" binding:"required,min=4,max=20"`
    Description string `json:"description" binding:"required,max=100"`
}

func (h *Handlers) UpdateProfile() gin.HandlerFunc {
    return func(c *gin.Context) {
        // ...
    }
}

Parsing JSON with c.BindJSON()

To extract JSON data from the request body, we use c.BindJSON(), providing a pointer to the request struct:

var updateRequest request

if err := c.BindJSON(&updateRequest); err != nil {
    // Handle JSON binding errors
    ...
}

Parsing image with c.FormFile()

To parse the image file, we utilize c.FormFile():

avatar, err := c.FormFile("avatar")
if err != nil {
    // Handle file parsing errors
    ...
}

Error: "invalid character '-' in numeric literal"

If we encounter an error like "invalid character '-' in numeric literal," it is likely caused by attempting to parse JSON while the request body also contains a boundary for multipart/form-data. We can specify the binding.FormMultipart mode explicitly instead:

// c.ShouldBind will choose binding.FormMultipart based on the Content-Type header.
// We call c.ShouldBindWith to make it explicitly.
if err := c.ShouldBindWith(&updateRequest, binding.FormMultipart); err != nil {
    // Handle binding errors
    ...
}

Parsing XML or YAML from multipart/form-data

If XML or YAML is expected alongside JSON in the multipart/form-data request, we can manually parse them, for example:

var event struct {
    At     time.Time `xml:"time" binding:"required"`
    Player string    `xml:"player" binding:"required"`
    Action string    `xml:"action" binding:"required"`
}

if err := binding.XML.BindBody([]byte(updateRequest.Event), &event); err != nil {
    // Handle binding errors
    ...
}

Conclusion

By following the techniques discussed, you can effectively parse both JSON data and uploaded images within a single multipart/form-data request in Go using Gin. Just remember to handle any potential binding errors appropriately.

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