@Service
public class ServiceA { 
  private ServiceB serviceB; 
  public ServiceA(ServiceB serviceB) {
    this.serviceB = serviceB;
  } 
  public void methodA(){
    System.out.println("a");
  }
}
 
@Service
public class ServiceB { 
  private ServiceA serviceA; 
  public ServiceB(ServiceA serviceA) {
    this.serviceA = serviceA;
  } 
  public void methodB(){
    System.out.println("b");
  }
}
 
//錯(cuò)誤提示
Description: 
The dependencies of some of the beans in the application context form a cycle:
 
┌─────┐
| serviceA defined in file [C:\demo\target\classes\com\example\demo\ServiceA.class]
↑   ↓
| serviceB defined in file [C:\demo\target\classes\com\example\demo\ServiceB.class]
└─────┘

@Service
public class ServiceA { 
  @Autowired
  private ServiceB serviceB;
 
  public void methodA(){
    System.out.println("a");
  }
}
 
@Service
public class ServiceB { 
  @Autowired
  private ServiceA serviceA;
 
  public void methodB(){
    System.out.println("b");
  }
}

@Service
@Scope("prototype")
public class ServiceA { 
  @Autowired
  private ServiceB serviceB;
 
  public void methodA(){
    System.out.println("a");
  }
}
 
@Service
@Scope("prototype")
public class ServiceB { 
  @Autowired
  private ServiceA serviceA;
 
  public void methodB(){
    System.out.println("b");
  }
}
 

public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
  }
 
  doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
 
    //從緩存中獲取實(shí)例
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
      //省略代碼
    } else {
      //省略代碼
      createBean(beanName, mbd, args);
    }
    //將創(chuàng)建完成的bean放入一級(jí)緩存
    addSingleton(beanname，object)
  }
 
  protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
    //省略代碼
    doCreateBean()
  }
 
  protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {
    //根據(jù)構(gòu)造方法創(chuàng)建對(duì)象實(shí)例
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
      instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
      instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
 
    //將對(duì)象實(shí)例放入第三級(jí)緩存中
    addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    //實(shí)例內(nèi)部屬性填充
    populateBean(beanName, mbd, instanceWrapper);
 
    //初始化bean
    exposedObject = initializeBean(beanName, exposedObject, mbd);
 
    return exposedObject;
  }
 
  protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
    Object exposedObject = bean;
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
          SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
          exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
        }
      }
    }
    return exposedObject;
  }

/** Cache of singleton objects: bean name to bean instance. */
 private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
 
 /** Cache of singleton factories: bean name to ObjectFactory. */
 private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
 
 /** Cache of early singleton objects: bean name to bean instance. */
 private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

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;
 }