Alibaba Interview: Tackling Spring’s Circular Dependency “Chain of Questions”

Question 1: What is circular dependency?

Circular dependency occurs when multiple objects depend on each other forming a closed loop, such as two beans referencing each other, which can cause injection deadlock in Spring.

Question 2: What forms can Spring circular dependencies take?

Spring can encounter three shapes of circular dependency:

Mutual dependency – A depends on B and B depends on A.

Three‑bean cycle – A depends on B, B depends on C, and C depends on A.

Self‑dependency – a bean depends on itself.

Illustrations:

Question 3: What does Spring’s three‑level cache store?

Level 1 – singletonObjects: cache of fully initialized singleton bean instances.

Level 2 – earlySingletonObjects: cache of early‑exposed singleton instances.

Level 3 – singletonFactories: cache of ObjectFactory objects that can create bean instances.

Code snippets:

/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

Question 4: How does the three‑level cache resolve circular dependencies?

The resolution proceeds in four steps:

When getBean(A.class) is invoked, Spring cannot find A in level 1, creates the instance, places it in level 3, and exposes it.

During A’s construction, @Autowired triggers getBean(B.class); Spring repeats the process for B, adding B to level 3.

When B requires A, Spring finds A in level 3, creates a proxy if A is AOP‑enhanced, moves A to level 2, and completes B’s injection, then moves B from level 3 to level 1.

Finally Spring finishes A’s injection, moving A from level 2 to level 1.

Summary diagram:

Animated illustration (GIF) is also provided for deeper understanding.

Question 5: When can Spring not resolve circular dependencies?

Four situations are unsolvable:

Prototype‑scoped beans injected via setter.

Beans injected through constructor injection.

Singleton proxy beans injected via setter.

Beans marked with @DependsOn.

Understanding these mechanisms helps interviewees explain Spring’s internals clearly and demonstrates deep knowledge of the framework.