NOTE
Spring Circular Dependencies
Constructor versus setter/field cycles, early references, proxy complications, and why redesign is usually preferable.
This is a historical learning note and may contain outdated or incomplete understanding.
A circular dependency exists when A requires B while B requires A.
Constructor cycles cannot be resolved by simply creating one fully initialized object first. Historical Spring singleton creation can resolve some setter/field cycles through early references, but that mechanism interacts with proxies and is an implementation detail that should not be used as a design technique.
Circular dependencies usually signal unclear responsibilities. Prefer extracting a third abstraction, reversing a dependency, using events/callbacks where semantically correct, or otherwise breaking the cycle explicitly.