Inject

Dependency injection library for Go (golang)

Why use Inject?

Like most other golang injection frameworks, Inject allows auto-resolving dependencies by type.

Unlike most other golang injection frameworks, Inject uses constructor functions and argument pointers to allow resolving any object, without it having to be designed for injection.

This has several advantages:

• Constructor arguments can be resolved automatically by type OR manually by unresolved pointer.
• Public access to the struct is not required, if they implement a public interface.
• No coordination is required between implementations of a common interface.
• Pointers can be used to distinguish between different implementations of the same interface.
• Pointers can even be used to distinguish between different instances of the same struct.

Use Cases

Most small applications don't need dependency injection. If you're going to use all the instances you need to construct, then you might as well just construct them and use them imperatively.

The cases where DI becomes useful usually have one or more of the following characteristics.

1. DRYing up dependency construction
2. Multiple consumers or controllers with overlapping, but not identical, dependency sets
3. Test code that uses the same dependency setup as the non-test code
4. Replacing a subset of the dependencies with Mocks, without having to rewrite the non-mock setup

Examples

1. Example Server - Example web server with dependency injected controllers & repositories

Usage

package yours

import (
  "path/to/your/pkgA"
  "path/to/your/pkgB"
  "github.com/karlkfi/inject"
)

func main() {
	graph := inject.NewGraph()

	var (
		primitive = "some string"
		a    pkgA.InterfaceA
		b    *pkgB.StructB
	)

	// define how to construct a (dependent on b)
	graph.Define(&a, inject.NewProvider(pkgA.NewA, &b))

	// define how to construct b (not dependent on the graph)
	graph.Define(&b, inject.NewProvider(pkgB.NewB, &primitive))

	// resolve a and all its (transitive) dependencies
	graph.Resolve(&a)

    // a is now usable
	a.DoStuff()
}

The simple example above makes use of the normal providers, which require the user to specify all constructor arguments.

The even simpler example below demonstrates auto-providers, which attempt to resolve constructor arguments by only their type. This method is simpler to use, but has the drawback that all auto-resolved types must have one and only one pointer in the graph that is assignable to that type. Auto-provider usage can be mixed with normal provider usage for increased flexibility.

...

func main() {
	graph := inject.NewGraph()

	var (
		primitive = "some string"
		a    pkgA.InterfaceA
		b    *pkgB.StructB
	)

	// define how to construct a (dependent on anything that implements InterfaceB)
	graph.Define(&a, inject.NewAutoProvider(pkgA.NewA))

	// define how to construct b (no dependencies)
	graph.Define(&b, inject.NewProvider(pkgB.NewB))

	// resolve a and its dependency b (because *pkgB.StructB is assignable to InterfaceB)
	graph.Resolve(&a)

    // a is now usable
	a.DoStuff()
}

You CAN resolve everything in the graph, using graph.ResolveAll(), but you can also share a graph between multiple code paths (like a controller with multiple endpoints, or a command with multiple sub-commands) and only resolve the dependencies you need using graph.Resolve(&ptr).

Because the definitions are uniquely keyed by pointer, you can also share code that produces a general graph, and override individual definitions with more specific providers (like tests that replace a few concrete impls with mocks).

Alternate Usage

Because the API is flexible, there are several ways to do the same thing. Here's another way to define the dependency graph:

func main() {
	var (
		primitive = "some string"
		a    pkgA.InterfaceA
		b    *pkgB.StructB
	)

    // define the pointer-provider relationships during graph construction
	graph := inject.NewGraph(
		inject.NewDefinition(&a, inject.NewProvider(pkgA.NewA, &b)),
		inject.NewDefinition(&b, inject.NewProvider(pkgB.NewB, &primitive)),
	)

	// resolve a and all its (transitive) dependencies
	graph.Resolve(&a)

    // a is now usable
	a.DoStuff()
}

Object Lifecycle

Definitions that point to structs (or struct pointers or interfaces) that implement a lifcycle interface (Initializable or Finalizable) have special behavior.

When the definition is first resolved, after the provider is called to return the value, the resolver will also call the Initialize() method on the value, if it has one.

When the definition is later obscured (on graph.Finalize()), after the defined pointer is zeroed, the obscurer will also call the Finalize() method on the resolved value, if it has one.

These lifecycle methods are optional, but may be useful for opening/closing objects or starting/stopping goroutines.

Resolving/initializing is lazily performed, either when the user calls graph.Resolve() or when another resolution causes transitive resolution of its dependencies (ex: provider arguments).

Obscuring/finalizing is performed on all resolved definitions when the user calls graph.Finalize(). If you use any Finalizable objects, you will need to make sure that graph.Finalize() is called before the program exits.

Installation

To install Inject, use go get:

go get github.com/karlkfi/inject

Updating

To update Inject, use go get -u:

go get -u github.com/karlkfi/inject

Dependencies

Inject has no runtime dependencies. Tests depend on Gomega.

Testing

Tests depend on Gomega.

To install Gomega, use go get:

go get github.com/onsi/gomega

To run Inject tests, use go test:

go test github.com/karlkfi/inject/test

License

Copyright 2015 Karl Isenberg

Licensed under the Apache License Version 2.0 (the "License"); you may not use this project except in compliance with the License.

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.