一文告诉你Spring是如何利用“三级缓存“巧妙解决Bean的循环依赖问题的【享学Spring】（上）

public class Main {
    public static void main(String[] args) throws Exception {
        System.out.println(new A());
    }
}
class A {
    public A() {
        new B();
    }
}
class B {
    public B() {
        new A();
    }
}

Exception in thread "main" java.lang.StackOverflowError

@Service
public class AServiceImpl implements AService {
    @Autowired
    private BService bService;
    ...
}
@Service
public class BServiceImpl implements BService {
    @Autowired
    private AService aService;
    ...
}

@Service
public class A {
    public A(B b) {
    }
}
@Service
public class B {
    public B(A a) {
    }
}

Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:339)
  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:215)
  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)

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

@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
@Service
public class A {
    @Autowired
    private B b;
}
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
@Service
public class B {
    @Autowired
    private A a;
}

// 在单例Bean内注入
    @Autowired
    private A a;

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mytest.TestSpringBean': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'a': Unsatisfied dependency expressed through field 'b'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'b': Unsatisfied dependency expressed through field 'a'; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
  at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:596)
  at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
  at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:374)

    @Lazy
    @Autowired
    private A a;

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
  ...
  // 从上至下 分表代表这“三级缓存”
  private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); //一级缓存
  private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); // 二级缓存
  private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 三级缓存
  ...
  /** Names of beans that are currently in creation. */
  // 这个缓存也十分重要：它表示bean创建过程中都会在里面呆着~
  // 它在Bean开始创建时放值，创建完成时会将其移出~
  private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));
  /** Names of beans that have already been created at least once. */
  // 当这个Bean被创建完成后，会标记为这个 注意：这里是set集合 不会重复
  // 至少被创建了一次的  都会放进这里~~~~
  private final Set<String> alreadyCreated = Collections.newSetFromMap(new ConcurrentHashMap<>(256));
}

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
  ...
  @Override
  @Nullable
  public Object getSingleton(String beanName) {
    return getSingleton(beanName, true);
  }
  @Nullable
  protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    Object singletonObject = this.singletonObjects.get(beanName);
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
      synchronized (this.singletonObjects) {
        singletonObject = this.earlySingletonObjects.get(beanName);
        if (singletonObject == null && allowEarlyReference) {
          ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
          if (singletonFactory != null) {
            singletonObject = singletonFactory.getObject();
            this.earlySingletonObjects.put(beanName, singletonObject);
            this.singletonFactories.remove(beanName);
          }
        }
      }
    }
    return singletonObject;
  }
  ...
  public boolean isSingletonCurrentlyInCreation(String beanName) {
    return this.singletonsCurrentlyInCreation.contains(beanName);
  }
  protected boolean isActuallyInCreation(String beanName) {
    return isSingletonCurrentlyInCreation(beanName);
  }
  ...
}