Binding

Parsing request data is a crucial part of a web application. In Echo this is done with a process called binding. This is done with information passed by the client in the following parts of an HTTP request:

• URL Path parameter
• URL Query parameter
• Header
• Request body

Echo provides different ways to perform binding, each described in the sections below.

Struct Tag Binding

With struct binding you define a Go struct with tags specifying the data source and corresponding key. In your request handler you simply call Context#Bind(i interface{}) with a pointer to your struct. The tags tell the binder everything it needs to know to load data from the request.

In this example a struct type User tells the binder to bind the query string parameter id to its string field ID:

type User struct {
  ID string `query:"id"`
}

// in the handler for /users?id=<userID>
var user User
err := c.Bind(&user); if err != nil {
    return c.String(http.StatusBadRequest, "bad request")
}

Data Sources

Echo supports the following tags specifying data sources:

• query - query parameter
• param - path parameter (also called route)
• header - header parameter
• json - request body. Uses builtin Go json package for unmarshalling.
• xml - request body. Uses builtin Go xml package for unmarshalling.
• form - form data. Values are taken from query and request body. Uses Go standard library form parsing.

Data Types

When decoding the request body, the following data types are supported as specified by the Content-Type header:

• application/json
• application/xml
• application/x-www-form-urlencoded

When binding path parameter, query parameter, header, or form data, tags must be explicitly set on each struct field. However, JSON and XML binding is done on the struct field name if the tag is omitted. This is according to the behaviour of Go's json package.

For form data, Echo uses Go standard library form parsing. This parses form data from both the request URL and body if content type is not MIMEMultipartForm. See documentation for non-MIMEMultipartFormand MIMEMultipartForm

Multiple Sources

It is possible to specify multiple sources on the same field. In this case request data is bound in this order:

1. Path parameters
2. Query parameters (only for GET/DELETE methods)
3. Request body

type User struct {
  ID string `param:"id" query:"id" form:"id" json:"id" xml:"id"`
}

Note that binding at each stage will overwrite data bound in a previous stage. This means if your JSON request contains the query param name=query and body {"name": "body"} then the result will be User{Name: "body"}.

Direct Source

It is also possible to bind data directly from a specific source:

Request body:

err := (&DefaultBinder{}).BindBody(c, &payload)

Query parameters:

err := (&DefaultBinder{}).BindQueryParams(c, &payload)

Path parameters:

err := (&DefaultBinder{}).BindPathParams(c, &payload)

Header parameters:

err := (&DefaultBinder{}).BindHeaders(c, &payload)

Note that headers is not one of the included sources with Context#Bind. The only way to bind header data is by calling BindHeaders directly.

Security

To keep your application secure, avoid passing bound structs directly to other methods if these structs contain fields that should not be bindable. It is advisable to have a separate struct for binding and map it explicitly to your business struct.

Consider what will happen if your bound struct has an Exported field IsAdmin bool and the request body contains {IsAdmin: true, Name: "hacker"}.

Example

In this example we define a User struct type with field tags to bind from json, form, or query request data:

type User struct {
  Name  string `json:"name" form:"name" query:"name"`
  Email string `json:"email" form:"email" query:"email"`
}

type UserDTO struct {
  Name    string
  Email   string
  IsAdmin bool
}

And a handler at the POST /users route binds request data to the struct:

e.POST("/users", func(c echo.Context) (err error) {
  u := new(User)
  if err := c.Bind(u); err != nil {
    return c.String(http.StatusBadRequest, "bad request")
  }

  // Load into separate struct for security
  user := UserDTO{
    Name: u.Name,
    Email: u.Email,
    IsAdmin: false // avoids exposing field that should not be bound
  }

  executeSomeBusinessLogic(user)
  
  return c.JSON(http.StatusOK, u)
})

JSON Data

curl -X POST http://localhost:1323/users \
  -H 'Content-Type: application/json' \
  -d '{"name":"Joe","email":"joe@labstack"}'

Form Data

curl -X POST http://localhost:1323/users \
  -d 'name=Joe' \
  -d '[email protected]'

Query Parameters

curl -X GET 'http://localhost:1323/users?name=Joe&[email protected]'

Fluent Binding

Echo provides an interface to bind explicit data types from a specified source. It uses method chaining, also known as a Fluent Interface.

The following methods provide a handful of methods for binding to Go data type. These binders offer a fluent syntax and can be chained to configure & execute binding, and handle errors.

• echo.QueryParamsBinder(c) - binds query parameters (source URL)
• echo.PathParamsBinder(c) - binds path parameters (source URL)
• echo.FormFieldBinder(c) - binds form fields (source URL + body). See also Request.ParseForm.

Error Handling

A binder is usually completed by calling BindError() or BindErrors(). If any errors have occurred, BindError() returns the first error encountered, whileBindErrors() returns all bind errors. Any errors stored in the binder are also reset.

With FailFast(true) the binder can be configured to stop binding on the first error, or with FailFast(false) execute the entire binder call chain. Fail fast is enabled by default and should be disabled when using BindErrors().

Example

// url =  "/api/search?active=true&id=1&id=2&id=3&length=25"
var opts struct {
  IDs []int64
  Active bool
}
length := int64(50) // default length is 50

// creates query params binder that stops binding at first error
err := echo.QueryParamsBinder(c).
  Int64("length", &length).
  Int64s("ids", &opts.IDs).
  Bool("active", &opts.Active).
  BindError() // returns first binding error

Supported Data Types

Data Type	Notes
bool
float32
float64
int
int8
int16
int32
int64
uint
uint8/byte	Does not support bytes(). Use BindUnmarshaler/CustomFunc to convert value from base64 etc to []byte{}.
uint16
uint32
uint64
string
time
duration
BindUnmarshaler()	binds to a type implementing BindUnmarshaler interface
TextUnmarshaler()	binds to a type implementing encoding.TextUnmarshaler interface
JsonUnmarshaler()	binds to a type implementing json.Unmarshaler interface
UnixTime()	converts Unix time (integer) to time.Time
UnixTimeMilli()	converts Unix time with millisecond precision (integer) to time.Time
UnixTimeNano()	converts Unix time with nanosecond precision (integer) to time.Time
CustomFunc()	callback function for your custom conversion logic

Each supported type has the following methods:

• <Type>("param", &destination) - if parameter value exists then binds it to given destination of that type i.e Int64(...).
• Must<Type>("param", &destination) - parameter value is required to exist, binds it to given destination of that type i.e MustInt64(...).
• <Type>s("param", &destination) - (for slices) if parameter values exists then binds it to given destination of that type i.e Int64s(...).
• Must<Type>s("param", &destination) - (for slices) parameter value is required to exist, binds it to given destination of that type i.e MustInt64s(...).

For certain slice types BindWithDelimiter("param", &dest, ",") supports splitting parameter values before type conversion is done. For example binding an integer slice from the URL /api/search?id=1,2,3&id=1 will result in []int64{1,2,3,1}.

Custom Binding

A custom binder can be registered using Echo#Binder.

type CustomBinder struct {}

func (cb *CustomBinder) Bind(i interface{}, c echo.Context) (err error) {
  // You may use default binder
  db := new(echo.DefaultBinder)
  if err := db.Bind(i, c); err != echo.ErrUnsupportedMediaType {
    return
  }

  // Define your custom implementation here
  return
}