Constructor injection (dependencies passed as constructor arguments, enabling immutability via `final` fields), setter injection (dependencies set via public setter methods after construction, allowing optional dependencies), and field injection (dependencies assigned directly to fields via `@Autowired`, the most concise but least testable and explicit).

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#Constructor Injection#Setter Injection#Field Injection

What are the different types of Dependency Injection in Spring?

Why Field Injection Looks Convenient But Isn't Recommended

@Autowired on a field skips the constructor entirely, which means the class can be instantiated by reflection (as in a unit test using `new`) in a half-wired, broken state with silent NullPointerExceptions later — and it's impossible to make the dependency `final`, or to see at a glance what the class needs just from its constructor signature. Spring's own documentation recommends constructor injection as the default.

Constructor: required, immutable · Setter: optional, mutable · Field: concise but hides dependencies