After using this library in production for a year, I made some
conclusions about library API and really useful features. I don't want
to make breaking changes and have a v3 version. It's not that popular.
I put old and new ideas in goava/di.
Dependency injection is one form of the broader technique of inversion of control. It is used to increase modularity of the program and make it extensible.
go get -u github.com/defval/inject/v2This library follows SemVer strictly.
Let's learn to use Inject by example. We will code a simple application that processes HTTP requests.
The full tutorial code is available here
To start, we will need to create two fundamental types: http.Server
and http.ServeMux. Let's create a simple constructors that initialize
it:
// NewServer creates a http server with provided mux as handler.
func NewServer(mux *http.ServeMux) *http.Server {
return &http.Server{
Handler: mux,
}
}
// NewServeMux creates a new http serve mux.
func NewServeMux() *http.ServeMux {
return &http.ServeMux{}
}Supported constructor signature:
func([dep1, dep2, depN]) (result, [cleanup, error])
Now let's teach a container to build these types.
container := inject.New(
// provide http server
inject.Provide(NewServer),
// provide http serve mux
inject.Provide(NewServeMux)
)The function inject.New() parse our constructors, compile dependency
graph and return *inject.Container type for interaction. Container
panics if it could not compile.
I think that panic at the initialization of the application and not in runtime is usual.
We can extract the built server from the container. For this, define the
variable of extracted type and pass variable pointer to Extract
function.
If extracted type not found or the process of building instance cause error,
Extractreturn error.
If no error occurred, we can use the variable as if we had built it yourself.
// declare type variable
var server *http.Server
// extracting
err := container.Extract(&server)
if err != nil {
// check extraction error
}
server.ListenAndServe()Note that by default, the container creates instances as a singleton. But you can change this behaviour. See Prototypes.
As an alternative to extraction we can use Invoke() function. It
resolves function dependencies and call the function. Invoke function
may return optional error.
// StartServer starts the server.
func StartServer(server *http.Server) error {
return server.ListenAndServe()
}
container.Invoke(StartServer)Result dependencies will be lazy-loaded. If no one requires a type from the container it will not be constructed.
Inject make possible to provide implementation as an interface.
// NewServer creates a http server with provided mux as handler.
func NewServer(handler http.Handler) *http.Server {
return &http.Server{
Handler: handler,
}
}For a container to know that as an implementation of http.Handler is
necessary to use, we use the option inject.As(). The arguments of this
option must be a pointer(s) to an interface like new(Endpoint).
This syntax may seem strange, but I have not found a better way to specify the interface.
Updated container initialization code:
container := inject.New(
// provide http server
inject.Provide(NewServer),
// provide http serve mux as http.Handler interface
inject.Provide(NewServeMux, inject.As(new(http.Handler)))
)Now container uses provide *http.ServeMux as http.Handler in server
constructor. Using interfaces contributes to writing more testable code.
Container automatically groups all implementations of interface to
[]<interface> group. For example, provide with
inject.As(new(http.Handler) automatically creates a group
[]http.Handler.
Let's add some http controllers using this feature. Controllers have typical behavior. It is registering routes. At first, will create an interface for it.
// Controller is an interface that can register its routes.
type Controller interface {
RegisterRoutes(mux *http.ServeMux)
}Now we will write controllers and implement Controller interface.
// OrderController is a http controller for orders.
type OrderController struct {}
// NewOrderController creates a auth http controller.
func NewOrderController() *OrderController {
return &OrderController{}
}
// RegisterRoutes is a Controller interface implementation.
func (a *OrderController) RegisterRoutes(mux *http.ServeMux) {
mux.HandleFunc("/orders", a.RetrieveOrders)
}
// Retrieve loads orders and writes it to the writer.
func (a *OrderController) RetrieveOrders(writer http.ResponseWriter, request *http.Request) {
// implementation
}// UserController is a http endpoint for a user.
type UserController struct {}
// NewUserController creates a user http endpoint.
func NewUserController() *UserController {
return &UserController{}
}
// RegisterRoutes is a Controller interface implementation.
func (e *UserController) RegisterRoutes(mux *http.ServeMux) {
mux.HandleFunc("/users", e.RetrieveUsers)
}
// Retrieve loads users and writes it using the writer.
func (e *UserController) RetrieveUsers(writer http.ResponseWriter, request *http.Request) {
// implementation
}Just like in the example with interfaces, we will use inject.As()
provide option.
container := inject.New(
inject.Provide(NewServer), // provide http server
inject.Provide(NewServeMux), // provide http serve mux
// endpoints
inject.Provide(NewOrderController, inject.As(new(Controller))), // provide order controller
inject.Provide(NewUserController, inject.As(new(Controller))), // provide user controller
)Now, we can use []Controller group in our mux. See updated code:
// NewServeMux creates a new http serve mux.
func NewServeMux(controllers []Controller) *http.ServeMux {
mux := &http.ServeMux{}
for _, controller := range controllers {
controller.RegisterRoutes(mux)
}
return mux
}In some cases you have more than one instance of one type. For example two instances of database: master - for writing, slave - for reading.
First way is a wrapping types:
// MasterDatabase provide write database access.
type MasterDatabase struct {
*Database
}
// SlaveDatabase provide read database access.
type SlaveDatabase struct {
*Database
}Second way is a using named definitions with inject.WithName() provide
option:
// provide master database
inject.Provide(NewMasterDatabase, inject.WithName("master"))
// provide slave database
inject.Provide(NewSlaveDatabase, inject.WithName("slave"))If you need to extract it from container use inject.Name() extract
option.
var db *Database
container.Extract(&db, inject.Name("master"))If you need to provide named definition in other constructor use
di.Parameter with embedding.
// ServiceParameters
type ServiceParameters struct {
di.Parameter
// use `di` tag for the container to know that field need to be injected.
MasterDatabase *Database `di:"master"`
SlaveDatabase *Database `di:"slave"`
}
// NewService creates new service with provided parameters.
func NewService(parameters ServiceParameters) *Service {
return &Service{
MasterDatabase: parameters.MasterDatabase,
SlaveDatabase: parameters.SlaveDatabase,
}
}Also di.Parameter provide ability to skip dependency if it not exists
in container.
// ServiceParameter
type ServiceParameter struct {
di.Parameter
Logger *Logger `di:"optional"`
}Constructors that declare dependencies as optional must handle the case of those dependencies being absent.
You can use naming and optional together.
// ServiceParameter
type ServiceParameter struct {
di.Parameter
StdOutLogger *Logger `di:"stdout"`
FileLogger *Logger `di:"file,optional"`
}If you need to specify some parameters on definition level you can use
inject.ParameterBag provide option. This is a map[string]interface{}
that transforms to di.ParameterBag type.
// Provide server with parameter bag
inject.Provide(NewServer, inject.ParameterBag{
"addr": ":8080",
})
// NewServer create a server with provided parameter bag. Note: use di.ParameterBag type.
// Not inject.ParameterBag.
func NewServer(pb di.ParameterBag) *http.Server {
return &http.Server{
Addr: pb.RequireString("addr"),
}
}If you want to create a new instance on each extraction use
inject.Prototype() provide option.
inject.Provide(NewRequestContext, inject.Prototype())todo: real use case
If a provider creates a value that needs to be cleaned up, then it can return a closure to clean up the resource.
func NewFile(log Logger, path Path) (*os.File, func(), error) {
f, err := os.Open(string(path))
if err != nil {
return nil, nil, err
}
cleanup := func() {
if err := f.Close(); err != nil {
log.Log(err)
}
}
return f, cleanup, nil
}After container.Cleanup() call, it iterate over instances and call
cleanup function if it exists.
container := inject.New(
// ...
inject.Provide(NewFile),
)
// do something
container.Cleanup() // file was closedCleanup now work incorrectly with prototype providers.
Dependency graph may be presented via (Graphviz). For it, load string representation:
var graph *di.Graph
if err = container.Extract(&graph); err != nil {
// handle err
}
dotGraph := graph.String() // use string representationAnd paste it to graphviz online tool:
I will be glad if you contribute to this library. I don't know much English, so contributing to the documentation is very meaningful to me.
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