Full round replay — dependency injection

Pass: A final field must be assigned by the end of the constructor, and field injection assigns it after construction. The code does not compile — it is a Java rule, not a Spring one.

Fail: Says Spring does not support it, or that you can do it with reflection.

Pass: Nothing measurable. The cost is when problems surface: a missing dependency, a half-wired test and a cycle all appear later, as an NPE in whichever method touches the field first.

Fail: Claims it is slower, or that reflection makes it expensive.

Pass: Field injection runs after construction, so every injected field is null while the constructor body executes. Move it to @PostConstruct, or take the dependency as a constructor parameter so the mistake cannot be written.

Fail: Suspects a missing bean or component scanning.

Pass: The container instantiates both beans first and wires them in a second pass, so neither needs the other to exist yet. Constructor injection has no such phase and no valid order, so it fails immediately.

Fail: Thinks Spring resolves it by cleverness, or that constructor injection also works.

Pass: Yes — a cycle that the container can satisfy is still two classes that cannot be built, read or tested independently. Failing at startup moves the discovery to the cheapest moment.

Fail: Only knows the flag to turn it back on.

Pass: No. It injects a proxy that resolves on first use, so startup succeeds and the cycle survives — a startup failure becomes a runtime one. The fix is to extract the type both classes were reaching for.

Fail: Accepts it because the application starts.

Pass: Not since Spring 4.3, when the class has exactly one constructor. With two or more you must mark the one Spring should use.

Fail: Says it is always required, or always optional.

Pass: In test classes, where the container is present and nothing else constructs the object. Setter injection has a narrower case: a genuinely optional or reconfigurable dependency.

Fail: Treats it as always wrong, with no case where it is fine.

Pass: Nine collaborators means more than one responsibility. Split the class or introduce a facade that means something. Hiding the list removes the only signal that was working; @RequiredArgsConstructor removes the typing without removing the signal.

Fail: Switches to field injection to shorten it.