[Java] SpringBean依赖和三级缓存的案例讲解

@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");
  }
}
 
//错误提示
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 {
 
  //从缓存中获取实例
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
    //省略代码
  } else {
    //省略代码
    createBean(beanName, mbd, args);
  }
  //将创建完成的bean放入一级缓存
  addSingleton(beanname，object)
  }
 
  protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
  //省略代码
  doCreateBean()
  }
 
  protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException {
  //根据构造方法创建对象实例
  BeanWrapper instanceWrapper = null;
  if (mbd.isSingleton()) {
    instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
  }
  if (instanceWrapper == null) {
    instanceWrapper = createBeanInstance(beanName, mbd, args);
  }
  final Object bean = instanceWrapper.getWrappedInstance();
 
  //将对象实例放入第三级缓存中
  addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
  //实例内部属性填充
  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;
 }