Anshul Goyal I love to code and use new technologies.

Gin binding in Go: A tutorial with examples

4 min read 1164

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Over the past few years, Go has become very popular for microservices. Gin is a web framework for Go that focuses on performance and productivity and features Martini-like APIs.

In this tutorial, we’ll show you how to use Gin’s binding. We’ll walk you through request payload validation, writing custom validation using reflection and the validator module, and building custom bindings for various formats, such as TOML, etc.

What is Gin binding?

Gin binding is an awesome serialization library. It supports JSON, XML, query parameter, and more out of the box and comes with a built-in validation framework.

Gin bindings are used to serialize JSON, XML, path parameters, form data, etc. to structs and maps. It also has a baked-in validation framework with complex validations.

Gin supports various formats by providing struct tags. For example, the uri tag is used to serialize path parameters:

package main

import (
   "fmt"
   "github.com/gin-gonic/gin"
   "net/http"
)

type Body struct {
  // json tag to serialize json body
   Name string `json:"name"`
}

func main() {
   engine:=gin.New()
   engine.POST("/test", func(context *gin.Context) {
      body:=Body{}
      // using BindJson method to serialize body with struct
      if err:=context.BindJSON(&body);err!=nil{
         context.AbortWithError(http.StatusBadRequest,err)
         return
      }
      fmt.Println(body)
      context.JSON(http.StatusAccepted,&body)
   })
   engine.Run(":3000")
}

BindJSON reads the body buffer to serialize it to a struct. BindJSON cannot be called on the same context twice because it flushes the body buffer.

If you want to serialize the body to two different structs, use ShouldBindBodyWith to copy the body buffer and add it to context.

if err:=context.ShouldBindBodyWith(&body,binding.JSON);err!=nil{
   context.AbortWithError(http.StatusBadRequest,err)
   return
}

In the same way, the XML body and path parameters are mapped to structs.

package main

import (
   "fmt"
   "github.com/gin-gonic/gin"
   "net/http"
)

// path paramter with name details will mapped to Details
type URI struct {
   Details string `json:"name" uri:"details"`
}

func main() {
   engine:=gin.New()
// adding path params to router
   engine.GET("/test/:details", func(context *gin.Context) {
      uri:=URI{}
      // binding to URI
      if err:=context.BindUri(&uri);err!=nil{
         context.AbortWithError(http.StatusBadRequest,err)
         return
      }
      fmt.Println(uri)
      context.JSON(http.StatusAccepted,&uri)
   })
   engine.Run(":3000")
}

The example above is a basic use case for binding to serialize the body, query, and path parameters.

Basic validation using Gin

Gin uses the validator package internally for validations. This package validator provides an extensive set of inbuilt validations, including required, type validation, and string validation.

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Validations are added to structs:

type URI struct {
   Details string `json:"name" uri:"details" binding:"required"`
}

The validator package also supports more complex validation, such as len ,max, and min. Nested structs and arrays are also validated recursively.

type User struct {
   Name string `json:"name" binding:"required,min=3"`
   Age uint `json:"age" binding:"required,min=18"`
   Comments []*Comment `json:"comments" binding:"required"`
}

type Comment struct {
   Text string `json:"text" binding:"required,max=255"`
   Type string `json:"type" binding:"required,oneof=post nested"`
}

Gin comes with many inbuilt validations; you can find an exhaustive list on GitHub.

Writing custom validations

Not all use cases are well-suited to built-in Gin validations. For this reason, Gin provides methods to add custom validations.

The reflect package is used during the validation process to figure out types and the value of struct fields at runtime.

To create a new binding, you have to register a validation with a function that performs the validation.

 // getting the validation engine and type casting it.
if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
   // registering validation for nontoneof
   v.RegisterValidation("notoneof", func(fl validator.FieldLevel) bool {
     // split values using ` `. eg. notoneof=bob rob job
      match:=strings.Split(fl.Param()," ")
     // convert field value to string
      value:=fl.Field().String()
      for _,s:=range match {
       // match value with struct filed tag
         if s==value {
            return false
         }
      }
      return true
   })
}

You can access the validation engine using the binding package for adding custom validators. The Validator variable is exported. Validator provides the Engine method, which returns the validation engine.

The RegisterValidation method on the engine takes a name and function that returns whether the field is valid or not.

You can access arameters passed to the validator using the Param method.

The Field method returns the value of the field in a struct. The value can be typecast to various data types.

validatoe.FieldLevel has access to a whole struct. You can also access different keys of a parent struct.

Accessing other struct fields

FieldLevel has a Top method that returns a reflect.Value type of the struct. That can be used to access the field in a struct.

For example, you can create a validation where two fields can’t have the same value using reflect.Value.

v.RegisterValidation("unique", func(fl validator.FieldLevel) bool {
  // get the fields which need to be unique
   match:=strings.Split(fl.Param()," ")
  // value of the field
   value:=fl.Field().String()
   for _,s:=range match {
     // access to struct and getting value by field name
      fs:=fl.Top().FieldByName(s)
      // check only for string validation
      if fs.Kind() == reflect.String {
          // check value of both fields
         if value==fs.String() {
            return false
         }
      }
   }
   return true
})

The above example only checks for string values, but you can easily modify it for all data types:

type ExampleStruct struct {
   Name string `json:"name" uri:"name" binding:"notoneof=bob rob job"`
   LastName string `json:"last_name" binding:"unique=Name"`
}

Writing custom Gin bindings

In some cases, the client and server use different formats to interchange data. For example, instead of JSON or XML, TOML might be used as the body for a request.

For cases like this, Gin provides a plug-and-play method for changing the body parser.

Every binding needs to implement this interface. The Name method returns a binding name and the Bind methods parse the request body:

type Binding interface {
   Name() string
   Bind(*http.Request, interface{}) error
}

Here’s an example of binding:

type Toml struct {
}

// return the name of binding
func (t Toml) Name() string {
   return "toml"
}

// parse request
func (t Toml) Bind(request *http.Request, i interface{}) error {
// using go-toml package 
   tD:= toml.NewDecoder(request.Body)
// decoding the interface
   return tD.Decode(i)
}

Usage example:

engine.POST("/Toml", func(context *gin.Context) {
   uri:= URI{}

   if err:=context.MustBindWith(&uri, Toml{});err!=nil{
      context.AbortWithError(http.StatusBadRequest,err)
      return
   }
   context.JSON(200,uri)
})

Implementing BindBody to use ShouldBindBodyWith:

func (t Toml) BindBody(bytes []byte, i interface{}) error {
   return toml.Unmarshal(bytes,i)
}

Usage example:

engine.POST("/Toml", func(context *gin.Context) {
   uri:= URI{}

   if err:=context.ShouldBindBodyWith(&uri, Toml{});err!=nil{
      context.AbortWithError(http.StatusBadRequest,err)
      return
   }
   context.JSON(200,uri)
})

Conclusion

In this tutorial, we covered Gin binding, various built-in validators, and some more advanced use cases. We also covered how to build a custom binding using various interfaces provided by the Gin library. Finally, we build some custom validators using advanced reflection and the validator package. You can use these building blocks to build various HTTP body parsers.

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Anshul Goyal I love to code and use new technologies.

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