When do you use constructor, field, or setter injection in Spring?

Prefers constructor injection for required dependencies while explaining the limited roles and costs of setter and field injection.

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TL;DR

I use constructor injection for required dependencies because the object cannot be created in an invalid state, fields can be final, and tests can instantiate it without a Spring context. Setter injection fits a truly optional or reconfigurable dependency when the class has a sensible default. Field injection hides requirements, prevents final fields, and makes plain unit construction awkward, so I avoid it in application code. With one constructor, Spring does not require @Autowired on that constructor.

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Dependency visibility, immutability, lifecycle completeness, optional collaborators, testing, and circular dependency signals.

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Core answer first, then checkpoints toward a stronger answer.

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Core answer
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Overview
The injection style should express whether a dependency is required and when the object becomes valid.
I use constructor injection for required dependencies because the object cannot be created in an invalid state, fields can be final, and tests can instantiate it without a Spring context. Setter injection fits a truly optional or reconfigurable dependency when the class has a sensible default. Field injection hides requirements, prevents final fields, and makes plain unit construction awkward, so I avoid it in application code. With one constructor, Spring does not require @Autowired on that constructor.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
Constructor injection also exposes circular dependencies immediately, which is useful because the cycle usually indicates confused responsibilities. Setter injection can be appropriate for framework integration or a third-party type whose construction API is constrained, but optional should not mean silently nullable everywhere. If a dependency is required only for one operation, I still reconsider the class boundary before reaching for field injection or runtime container lookup.

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Details

Using field injection to hide a large dependency list.
Calling a required collaborator optional to justify a setter.
Adding @Autowired to every single constructor as a requirement.

When do you use constructor, field, or setter injection in Spring?

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