concept DI Container in category dependency injection

This is an excerpt from Manning's book Dependency Injection. Login to get full access to this book.

Closely associated with the previous misconception is the notion that DI requires a DI Container. If you held the previous, mistaken belief that DI involves a Service Locator, then it’s easy to conclude that a DI Container can take on the responsibility of the Service Locator. This might be the case, but it’s not at all how you should use a DI Container.

A DI Container is an optional library that makes it easier to compose classes when you wire up an application, but it’s in no way required. When you compose applications without a DI Container, it’s called Pure DI. It might take a little more work, but other than that, you don’t have to compromise on any DI principles.

A DI Container is a software library that provides DI functionality and automates many of the tasks involved in Object Composition, Interception, and Lifetime Management. It’s an engine that resolves and manages object graphs.

Figure 12.2 Simplified workflow for Auto-Wiring. A DI Container uses its configuration to find the appropriate concrete class that matches the requested type. It then uses reflection to examine the class’s constructor.

As shown, a DI Container finds the concrete type for a requested Abstraction. If the constructor of the concrete type requires arguments, a recursive process starts where the DI Container repeats the process for each argument type until all constructor arguments are satisfied. When this is complete, the container constructs the concrete type while injecting the recursively resolved Dependencies.