Spring循环依赖​​的解决方式详解

@Component
public class A {
    @Autowired
    private B b;
}
@Component
public class B {
    @Autowired
    private A a;
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    // 首先检查一级缓存
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
        synchronized (this.singletonObjects) {
            // 检查二级缓存
            singletonObject = this.earlySingletonObjects.get(beanName);
            if (singletonObject == null && allowEarlyReference) {
                // 检查三级缓存
                ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                if (singletonFactory != null) {
                    singletonObject = singletonFactory.getObject();
                    // 从三级缓存移到二级缓存
                    this.earlySingletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                }
            }
        }
    }
    return singletonObject;
}

@Component
public class A {
    private B b;
    @Autowired
    public A(B b) { this.b = b; }
}
@Component
public class B {
    private A a;
    @Autowired
    public B(A a) { this.a = a; }
}

@Component
public class A {
    @Lazy
    @Autowired
    private B b;
}