Crate runtime_injector[][src]

Expand description

Runtime dependency injection.

By default, services provided by the Injector use thread-safe pointers. This is because Arc<T> is used to hold instances of the services. This can be changed to Rc<T> by disabling default features and enabling the “rc” feature:

version = "*" # Replace with the version you want to use
default-features = false
features = ["rc"]

Getting started

If you are unfamiliar with dependency injection, then you may want to check about how a container can help simplify your application. Otherwise, check out the getting started guide

Dependency injection at runtime (rather than compile-time)

Runtime dependency injection allows for advanced configuration of services during runtime rather than needing to decide what services your application will use at compile time. This means you can read a config when your application starts, decide what implementations you want to use for your interfaces, and assign those at runtime. This is also slightly slower than compile-time dependency injection, so if pointer indirection, dynamic dispatch, or heap allocations are a concern, then a compile-time dependency injection library might work better instead. However, in asynchronous, I/O-based applications like a web server, the additional overhead is probably insignificant compared to the additional flexibility you get with runtime_injector.


Using interfaces allows you to write your services without worrying about how its dependencies are implemented. You can think of them like generic type parameters for your service, except rather than needing to add a new type parameter, you use a service pointer to the interface for your dependency. This makes your code easier to read and faster to write, and keeps your services decoupled from their dependencies and dependents.

Interfaces are implemented as trait objects in runtime_injector. For instance, you may define a trait UserDatabase and implement it for several different types. Svc<dyn UserDatabase> is a reference-counted service pointer to an implementation of your trait. Similarly, dyn UserDatabase is your interface. You can read more about how interfaces work and how they’re created in the type-level docs.

Service lifetimes

Lifetimes of services created by the Injector are controlled by the Provider used to construct those lifetimes. Currently, there are three built-in service provider types:

  • Transient: A service is created each time it is requested. This will never return the same instance of a service more than once.
  • Singleton: A service is created only the first time it is requested, then that single instance is reused for each future request.
  • Constant: Used for services that are not created using a service factory and instead can have their instance provided to the container directly. This behaves similar to singleton in that the same instance is provided each time the service is requested.

Custom service providers can also be created by implementing either the TypedProvider or Provider trait.

Fallible service factories

Not all types can always be successfully created. Sometimes, creating an instance of a service might fail. Rather than panicking on error, it’s possible to instead return a Result<T, E> from your constructors and inject the result as a Svc<T>. Read more in the docs for IntoFallible.

Owned service pointers

In general, providers need to be able to provide their services via reference-counted service pointers, or Svc<T>. The issue with this is that you cannot get mutable or owned access to the contents of those pointers since they are shared pointers. As a result, you may need to clone some dependencies in your constructors if you want to be able to own them.

If your dependency is a transient service, then it might make more sense to inject it as a Box<T> than clone it from a reference-counted service pointer. In these cases, you can request a Box<T> directly from the injector and avoid needing to clone your dependency entirely!

Custom target-specific arguments

Sometimes it’s useful to be able to pass a specific value into your services. For example, if you’re writing a database service and you need a connection string, you could define a new ConnectionString struct as a newtype for String, but that would be a bit excessive for passing in a single value. If you had several arguments you needed to pass in this way, then that would mean you would need a new type for each one.

Rather than creating a bunch of newtypes, you can use Arg<T> to pass in pre-defined values directly to your services. For example, you can use Arg<String> to pass in your connection string, plus you can use Arg<usize> to set the max size of your connection pool, and another Arg<String> in your logging service to set your logging format without needing to worry about accidentally using your connection string as your logging format!


use runtime_injector::{
    define_module, Module, interface, Injector, Svc, IntoSingleton,
    TypedProvider, IntoTransient, constant
use std::error::Error;

// Some type that represents a user
struct User;

// This is our interface. In practice, multiple structs can implement this
// trait, and we don't care what the concrete type is most of the time in
// our other services as long as it implements this trait. Because of this,
// we're going to use dynamic dispatch later so that we can determine the
// concrete type at runtime (vs. generics, which are determined instead at
// compile time).
// The `Send` and `Sync` supertrait requirements are only necessary when
// compiling with the "arc" feature to allow for service pointer
// downcasting.
trait DataService: Send + Sync {
    fn get_user(&self, user_id: &str) -> Option<User>;

// We can use a data service which connects to a SQL database.
struct SqlDataService;
impl DataService for SqlDataService {
    fn get_user(&self, _user_id: &str) -> Option<User> { todo!() }

// ... Or we can mock out the data service entirely!
struct MockDataService;
impl DataService for MockDataService {
    fn get_user(&self, _user_id: &str) -> Option<User> { Some(User) }

// Specify which types implement the DataService interface. This does not
// determine the actual implementation used. It only registers the types as
// possible implementations of the DataService interface.
interface!(dyn DataService = [SqlDataService, MockDataService]);

// Here's another service our application uses. This service depends on our
// data service, however it doesn't care how that service is actually
// implemented as long as it works. Because of that, we're using dynamic
// dispatch to allow the implementation to be determined at runtime.
struct UserService {
    data_service: Svc<dyn DataService>,

impl UserService {
    // This is just a normal constructor. The only requirement is that each
    // parameter is a valid injectable dependency.
    pub fn new(data_service: Svc<dyn DataService>) -> Self {
        UserService { data_service }

    pub fn get_user(&self, user_id: &str) -> Option<User> {
        // UserService doesn't care how the user is actually retrieved

fn main() -> Result<(), Box<dyn Error>> {
    // This is where we register our services. Each call to `.provide` adds
    // a new service provider to our container, however nothing is actually
    // created until it is requested. This means we can add providers for
    // types we aren't actually going to use without worrying about
    // constructing instances of those types that we aren't actually using.
    let mut builder = Injector::builder();

    // We can manually add providers to our builder

    struct Foo(Svc<dyn DataService>);
    // Alternatively, modules can be used to group providers and
    // configurations together, and can be defined via the
    // define_module! macro
    let module = define_module! {
        services = [
            // Simple tuple structs can be registered as services directly without
            // defining any additional constructors
            // Note that we can register closures as providers as well
            (|_: Svc<dyn DataService>| "Hello, world!").singleton(),
            (|_: Option<Svc<i32>>| 120.9).transient(),

            // Since we know our dependency is transient, we can request an
            // owned pointer to it rather than a reference-counted pointer
            (|value: Box<f32>| format!("{}", value)).transient(),

            // We can also provide constant values directly to our services
        interfaces = {
            // Let's choose to use the MockDataService as our data service
            dyn DataService = [MockDataService::default.singleton()],

    // You can easily add a module to your builder

    // Now that we've registered all our providers and implementations, we
    // can start relying on our container to create our services for us!
    let injector =;
    let user_service: Svc<UserService> = injector.get()?;
    let _user = user_service.get_user("john");



Additional documentation modules.



Defines a new module using a domain specific language.


Marks a trait as being an interface for many other types. This means that a request for the given trait can resolve to any of the types indicated by this macro invocation.



Allows custom pre-defined values to be passed as arguments to services.


A TypedProvider which conditionally provides its service. If the condition is not met, then the provider is skipped during resolution.


A provider which returns a constant, predetermined value. Note that this is technically a singleton service in that it does not recreate the value each time it is requested.


Lazy request factory allowing requests to be made outside of service creation.


A service factory that may fail during service creation with a custom error type. During activation failure, an instance of InjectError::ActivationFailed is returned as an error.


A runtime dependency injection container. This holds all the bindings between service types and their providers, as well as all the mappings from interfaces to their implementations (if they differ).


A builder for an Injector.


Provides a service as an implementation of an interface. See TypedProvider::with_interface() for more information.


A collection of providers that can be added all at once to an InjectorBuilder. Modules can be used to group together related services and configure the injector in pieces rather than all at once.


An iterator over all the implementations of an interface. Each service is activated on demand.


Information about an active request.


Type information about a service.


A collection of all the providers for a particular interface.


An iterator over all the implementations of an interface. Each service is activated on demand.


A service provider that only creates a single instance of the service. The service is created only during its first request. Any subsequent requests return service pointers to the same service.


A service provider that creates an instance of the service each time it is requested. This will never return two service pointers to the same instance of a service.



An error occurred while injecting an instance of Arg<T>.


An error that has occurred during creation of a service.



Defines a conversion for a type into an dyn Any trait object.


Indicates functionality that can be implemented.


Marker trait that indicates that a type is an interface for another type.


Defines a conversion into a fallible service factory. This trait is automatically implemented for all service factories that return a Result<T, E> with an error type that implements Error and Service.


Defines a conversion into a singleton provider. This trait is automatically implemented for all service factories.


Defines a conversion into a transient provider. This trait is automatically implemented for all service factories.


Weakly typed service provider.


A request to an injector.


A parameter for configuring requested services.


Implemented automatically on types that are capable of being a service.


A factory for creating instances of a service. All functions of arity 12 or less are automatically service factories if the arguments to that function are valid service requests and the return value is a valid service type.


A strongly-typed service provider.


Allows defining pre-defined arguments to services.


Defines a conversion into a conditional provider. This trait is automatically implemented for all types that implement TypedProvider.



Create a provider from a constant value.

Type Definitions


A reference-counted service pointer holding an instance of dyn Any.


A result from attempting to inject dependencies into a service and construct an instance of it.


An owned service pointer holding an instance of dyn Any.


A reference-counted pointer holding a service. The pointer type is determined by the feature flags passed to this crate.