How do inversion of control and dependency injection work in Spring?

Explains container ownership, dependency declarations, bean creation, and why dependency injection is a specific form of inversion of control.

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Inversion of control means application code gives control of object creation and wiring to a container instead of constructing every collaborator itself. Dependency injection is the mechanism Spring uses: a bean declares dependencies through its constructor, a factory method, or a setter, and the container supplies matching beans. The class can focus on its behavior rather than finding implementations. This also makes required dependencies visible and lets a unit test construct the class with controlled collaborators.

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The Spring container mental model, explicit dependencies, testability, and separation from service-location patterns.

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Application objects declare what they need; the container builds and connects the managed object graph.
Inversion of control means application code gives control of object creation and wiring to a container instead of constructing every collaborator itself. Dependency injection is the mechanism Spring uses: a bean declares dependencies through its constructor, a factory method, or a setter, and the container supplies matching beans. The class can focus on its behavior rather than finding implementations. This also makes required dependencies visible and lets a unit test construct the class with controlled collaborators.
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Spring reads configuration metadata from component scanning, @Bean methods, imports, and auto-configuration, turns that into bean definitions, then resolves the dependency graph when creating beans. If requires a , the service asks for that contract in its constructor; it does not call the to fetch one. Multiple candidates require an intentional selection rule such as a qualifier or primary bean. Missing or ambiguous dependencies normally fail during context creation for eager singletons.

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Defining dependency injection as annotation use.
Looking up ordinary collaborators from the container.
Assuming interfaces alone create loose coupling.

How do inversion of control and dependency injection work in Spring?

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