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.
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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