Understanding Spring Circular Dependency and Its Resolution Mechanism

This article discusses Spring's approach to handling circular dependencies in object-oriented programming. It explains the problem through a code example involving two interdependent classes A and B, where each class references the other.

The solution involves Spring's three-level caching system (singletonObjects, earlySingletonObjects, singletonFactories) to manage object creation states. The process includes object instantiation, property initialization, and caching strategies to prevent infinite loops and null reference exceptions.

Key steps include: creating half-finished objects (earlySingletonObjects), finalizing them into completed objects (singletonObjects), and using lambda expressions (singletonFactories) for deferred initialization. The article also covers source code analysis of Spring's dependency resolution process, including method calls like getBean(), createBean(), and populateBean().

Code snippets demonstrate class definitions, XML configuration, and Java code for dependency injection. The explanation includes visual diagrams of object creation cycles and caching states.

The conclusion summarizes why three-level caching is necessary over simpler two-level approaches, emphasizing its role in handling complex scenarios like AOP integration and dynamic proxies.