The best classes in any application are the ones that do stuff: the
BarcodeDecoder, the KoopaPhysicsEngine, and the AudioStreamer. These
classes have dependencies; perhaps a BarcodeCameraFinder,
DefaultPhysicsEngine, and an HttpStreamer.
To contrast, the worst classes in any application are the ones that take up
space without doing much at all: the BarcodeDecoderFactory, the
CameraServiceLoader, and the MutableContextWrapper. These classes are the
clumsy duct tape that wires the interesting stuff together.
Dagger is a replacement for these Factory classes that implements the
dependency injection design pattern without the burden of writing the
boilerplate. It allows you to focus on the interesting classes. Declare
dependencies, specify how to satisfy them, and ship your app.
By building on standard javax.inject annotations (JSR 330), each class is
easy to test. You don’t need a bunch of boilerplate just to swap the
RpcCreditCardService out for a FakeCreditCardService.
Dependency injection isn’t just for testing. It also makes it easy to create
reusable, interchangeable modules. You can share the same
AuthenticationModule across all of your apps. And you can run
DevLoggingModule during development and ProdLoggingModule in production to
get the right behavior in each situation.
Why Dagger 2 is Different
Dependency injection frameworks have existed for years with a whole variety of APIs for configuring and injecting. So, why reinvent the wheel? Dagger 2 is the first to implement the full stack with generated code. The guiding principle is to generate code that mimics the code that a user might have hand-written to ensure that dependency injection is as simple, traceable and performant as it can be. For more background on the design, watch this talk (slides) by Gregory Kick.
License
Copyright 2012 The Dagger Authors
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
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.