在Spring中如何实现加载Bean的

TestBean bean = factory.getBean("testBean");

@Override
public Object getBean(String name) throws BeansException {
  return doGetBean(name, null, null, false);
}

protected <T> T doGetBean(
    final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
    throws BeansException {
  // 提取beanName
  final String beanName = transformedBeanName(name);
  Object bean;

  // 试着从缓存或实例工厂中获取
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
    if (logger.isDebugEnabled()) {
      if (isSingletonCurrentlyInCreation(beanName)) {
        logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
            "' that is not fully initialized yet - a consequence of a circular reference");
      }
      else {
        logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
      }
    }
    // 返回实例，有时存在如BeanFactory这样情况时并不是直接返回实例本身而是返回指定方法返回的实例
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
  }

  else {
    // Fail if we're already creating this bean instance:
    // We're assumably within a circular reference.
    if (isPrototypeCurrentlyInCreation(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
    }

    // Check if bean definition exists in this factory.
    BeanFactory parentBeanFactory = getParentBeanFactory();
    // 如果在所有已经加载的类中没有beanName则会尝试从parentBeanFactory中检测
    if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
      // Not found -> check parent.
      String nameToLookup = originalBeanName(name);
      // 到BeanFactory查找
      if (args != null) {
        // Delegation to parent with explicit args.
        return (T) parentBeanFactory.getBean(nameToLookup, args);
      }
      else {
        // No args -> delegate to standard getBean method.
        return parentBeanFactory.getBean(nameToLookup, requiredType);
      }
    }
    // 如果不是只做类型检查则是创建bean
    if (!typeCheckOnly) {
      markBeanAsCreated(beanName);
    }

    try {
      // 将存储XML配置文件的GernericBeanDefinition转换成RootBeanDefinition，如果BeanName是子Bean的话会合并父类的相关属性
      final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      checkMergedBeanDefinition(mbd, beanName, args);

      // Guarantee initialization of beans that the current bean depends on.
      String[] dependsOn = mbd.getDependsOn();
      // 如果存在依赖的话要递归实例化依赖的bean
      if (dependsOn != null) {
        for (String dependsOnBean : dependsOn) {
          if (isDependent(beanName, dependsOnBean)) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
          }
          registerDependentBean(dependsOnBean, beanName);
          getBean(dependsOnBean);
        }
      }

      // Create bean instance.
      // 依赖的bean实例化完后就可以实例化mbd了
      if (mbd.isSingleton()) {
        sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
          @Override
          public Object getObject() throws BeansException {
            try {
              return createBean(beanName, mbd, args);
            }
            catch (BeansException ex) {
              // Explicitly remove instance from singleton cache: It might have been put there
              // eagerly by the creation process, to allow for circular reference resolution.
              // Also remove any beans that received a temporary reference to the bean.
              destroySingleton(beanName);
              throw ex;
            }
          }
        });
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      }

      else if (mbd.isPrototype()) {
        // It's a prototype -> create a new instance.
        Object prototypeInstance = null;
        try {
          beforePrototypeCreation(beanName);
          prototypeInstance = createBean(beanName, mbd, args);
        }
        finally {
          afterPrototypeCreation(beanName);
        }
        bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      }

      else {
        // 在对应的scope上实例化bean
        String scopeName = mbd.getScope();
        final Scope scope = this.scopes.get(scopeName);
        if (scope == null) {
          throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
        }
        try {
          Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
            @Override
            public Object getObject() throws BeansException {
              beforePrototypeCreation(beanName);
              try {
                return createBean(beanName, mbd, args);
              }
              finally {
                afterPrototypeCreation(beanName);
              }
            }
          });
          bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
        }
        catch (IllegalStateException ex) {
          throw new BeanCreationException(beanName,
              "Scope '" + scopeName + "' is not active for the current thread; " +
              "consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
              ex);
        }
      }
    }
    catch (BeansException ex) {
      cleanupAfterBeanCreationFailure(beanName);
      throw ex;
    }
  }

  // 检查需要的类型是否符合实例bean的实际类型
  if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
    try {
      return getTypeConverter().convertIfNecessary(bean, requiredType);
    }
    catch (TypeMismatchException ex) {
      if (logger.isDebugEnabled()) {
        logger.debug("Failed to convert bean '" + name + "' to required type [" +
            ClassUtils.getQualifiedName(requiredType) + "]", ex);
      }
      throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    }
  }
  return (T) bean;
}

@Override
public Object getSingleton(String beanName) {
  return getSingleton(beanName, true); // true表示允许早期依赖
}

protected Object getSingleton(String beanName, boolean allowEarlyReference) {
  // 尝试从缓存获取实例
  Object singletonObject = this.singletonObjects.get(beanName);
  if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
    synchronized (this.singletonObjects) {
      // 若该bean正在加载则不处理
      singletonObject = this.earlySingletonObjects.get(beanName);
      if (singletonObject == null && allowEarlyReference) {
        ObjectFactory<&#63;> singletonFactory = this.singletonFactories.get(beanName);
        if (singletonFactory != null) {
          singletonObject = singletonFactory.getObject();
          // 存入到缓存中
          this.earlySingletonObjects.put(beanName, singletonObject);
          this.singletonFactories.remove(beanName);
        }
      }
    }
  }
  return (singletonObject != NULL_OBJECT &#63; singletonObject : null);
}
/* 这两个方法在DefaultSingletonBeanRegistry类中 */

public Object getSingleton(String beanName, ObjectFactory<&#63;> singletonFactory) {
  Assert.notNull(beanName, "'beanName' must not be null");
  synchronized (this.singletonObjects) {
    // 先检查bean是否已经加载
    Object singletonObject = this.singletonObjects.get(beanName);
    // 如果空才进行singleton的bean的初始化
    if (singletonObject == null) {
      if (this.singletonsCurrentlyInDestruction) {
        throw new BeanCreationNotAllowedException(beanName,
            "Singleton bean creation not allowed while the singletons of this factory are in destruction " +
            "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
      }
      if (logger.isDebugEnabled()) {
        logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
      }
      beforeSingletonCreation(beanName);
      boolean newSingleton = false;
      boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
      if (recordSuppressedExceptions) {
        this.suppressedExceptions = new LinkedHashSet<Exception>();
      }
      try {
        // 初始化bean
        singletonObject = singletonFactory.getObject();
        newSingleton = true;
      }
      catch (IllegalStateException ex) {
        // Has the singleton object implicitly appeared in the meantime ->
        // if yes, proceed with it since the exception indicates that state.
        singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
          throw ex;
        }
      }
      catch (BeanCreationException ex) {
        if (recordSuppressedExceptions) {
          for (Exception suppressedException : this.suppressedExceptions) {
            ex.addRelatedCause(suppressedException);
          }
        }
        throw ex;
      }
      finally {
        if (recordSuppressedExceptions) {
          this.suppressedExceptions = null;
        }
        afterSingletonCreation(beanName);
      }
      if (newSingleton) {
        // 存入缓存
        addSingleton(beanName, singletonObject);
      }
    }
    return (singletonObject != NULL_OBJECT &#63; singletonObject : null);
  }
}

protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
    throws BeanCreationException {

  if (logger.isDebugEnabled()) {
    logger.debug("Creating instance of bean '" + beanName + "'");
  }
  // 锁定class，根据设置的class属性或根据className来解析Class
  resolveBeanClass(mbd, beanName);

  // 验证和准备覆盖的方法
  try {
    mbd.prepareMethodOverrides();
  }
  catch (BeanDefinitionValidationException ex) {
    throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
        beanName, "Validation of method overrides failed", ex);
  }

  try {
    // 用BeanPostProcessors返回代理来替代真正的实例
    Object bean = resolveBeforeInstantiation(beanName, mbd);
    if (bean != null) {
      return bean;
    }
  }
  catch (Throwable ex) {
    throw new BeanCreationException(mbd.getResourceDescription(), beanName,
        "BeanPostProcessor before instantiation of bean failed", ex);
  }

  Object beanInstance = doCreateBean(beanName, mbd, args);
  if (logger.isDebugEnabled()) {
    logger.debug("Finished creating instance of bean '" + beanName + "'");
  }
  return beanInstance;
}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
  // Instantiate the bean.
  BeanWrapper instanceWrapper = null;
  if (mbd.isSingleton()) {
    instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
  }
  if (instanceWrapper == null) {
    // 根据指定bean使用相应策略创建实例
    instanceWrapper = createBeanInstance(beanName, mbd, args);
  }
  final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
  Class<&#63;> beanType = (instanceWrapper != null &#63; instanceWrapper.getWrappedClass() : null);

  // Allow post-processors to modify the merged bean definition.
  synchronized (mbd.postProcessingLock) {
    if (!mbd.postProcessed) {
      applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
      mbd.postProcessed = true;
    }
  }

  // 检测循环依赖，是否需要提早曝光
  boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
      isSingletonCurrentlyInCreation(beanName));
  if (earlySingletonExposure) {
    if (logger.isDebugEnabled()) {
      logger.debug("Eagerly caching bean '" + beanName +
          "' to allow for resolving potential circular references");
    }
    // 避免循环依赖，可以在bean初始化完成前将创建实例的ObjectFactory加入工厂
    addSingletonFactory(beanName, new ObjectFactory<Object>() {
      @Override
      public Object getObject() throws BeansException {
        // 对bean再次依赖引用
        // AOP也是在这里将advice动态织入bean中，若没有则直接返回bean，不做处理
        return getEarlyBeanReference(beanName, mbd, bean);
      }
    });
  }

  // Initialize the bean instance.
  Object exposedObject = bean;
  try {
    // 填充bean，注入属性值，如果存在依赖于其他bean的属性，会递归初始化
    populateBean(beanName, mbd, instanceWrapper);
    if (exposedObject != null) {
      // 调用初始化方法
      exposedObject = initializeBean(beanName, exposedObject, mbd);
    }
  }
  catch (Throwable ex) {
    if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
      throw (BeanCreationException) ex;
    }
    else {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
    }
  }

  if (earlySingletonExposure) {
    Object earlySingletonReference = getSingleton(beanName, false);
    // earlySingletonReference只有在检测到有循环依赖的情况下才会不为空
    if (earlySingletonReference != null) {
      if (exposedObject == bean) {
        exposedObject = earlySingletonReference;
      }
      else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
        String[] dependentBeans = getDependentBeans(beanName);
        Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
        for (String dependentBean : dependentBeans) {
          // 检测依赖
          if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
            actualDependentBeans.add(dependentBean);
          }
        }
        if (!actualDependentBeans.isEmpty()) {
          throw new BeanCurrentlyInCreationException(beanName,
              "Bean with name '" + beanName + "' has been injected into other beans [" +
              StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
              "] in its raw version as part of a circular reference, but has eventually been " +
              "wrapped. This means that said other beans do not use the final version of the " +
              "bean. This is often the result of over-eager type matching - consider using " +
              "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
        }
      }
    }
  }

  // Register bean as disposable.
  try {
    // 依据scopse注册bean
    registerDisposableBeanIfNecessary(beanName, bean, mbd);
  }
  catch (BeanDefinitionValidationException ex) {
    throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
  }

  return exposedObject;
}

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
  // 解析class
  Class<&#63;> beanClass = resolveBeanClass(mbd, beanName);

  if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
    throw new BeanCreationException(mbd.getResourceDescription(), beanName,
        "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
  }

  // 若工厂方法不为空则使用工厂方法初始化
  if (mbd.getFactoryMethodName() != null) {
    return instantiateUsingFactoryMethod(beanName, mbd, args);
  }

  // Shortcut when re-creating the same bean...
  boolean resolved = false;
  boolean autowireNecessary = false;
  if (args == null) {
    synchronized (mbd.constructorArgumentLock) {
      if (mbd.resolvedConstructorOrFactoryMethod != null) {
        resolved = true;
        autowireNecessary = mbd.constructorArgumentsResolved;
      }
    }
  }
  // 如果已经解析过则使用解析好的构造方法不需要再次锁定
  if (resolved) {
    if (autowireNecessary) {
      // 构造方法自动注入
      return autowireConstructor(beanName, mbd, null, null);
    }
    else {
      // 使用默认构造方法
      return instantiateBean(beanName, mbd);
    }
  }

  // 根据参数解析构造方法
  Constructor<&#63;>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
  if (ctors != null ||
      mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
      mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
    return autowireConstructor(beanName, mbd, ctors, args);
  }

  // No special handling: simply use no-arg constructor.
  return instantiateBean(beanName, mbd);
}

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
  PropertyValues pvs = mbd.getPropertyValues();

  if (bw == null) {
    if (!pvs.isEmpty()) {
      throw new BeanCreationException(
          mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
    }
    else {
      // Skip property population phase for null instance.
      return;
    }
  }

  // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
  // state of the bean before properties are set. This can be used, for example,
  // to support styles of field injection.
  boolean continueWithPropertyPopulation = true;

  if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
      if (bp instanceof InstantiationAwareBeanPostProcessor) {
        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
        if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
          continueWithPropertyPopulation = false;
          break;
        }
      }
    }
  }

  if (!continueWithPropertyPopulation) {
    return;
  }

  if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
      mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
    MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

    // Add property values based on autowire by name if applicable.
    if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
      autowireByName(beanName, mbd, bw, newPvs);
    }

    // Add property values based on autowire by type if applicable.
    if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
      autowireByType(beanName, mbd, bw, newPvs);
    }

    pvs = newPvs;
  }

  boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
  boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

  if (hasInstAwareBpps || needsDepCheck) {
    PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
    if (hasInstAwareBpps) {
      for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
          InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
          pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
          if (pvs == null) {
            return;
          }
        }
      }
    }
    if (needsDepCheck) {
      checkDependencies(beanName, mbd, filteredPds, pvs);
    }
  }

  applyPropertyValues(beanName, mbd, bw, pvs);
}

protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
  if (System.getSecurityManager() != null) {
    AccessController.doPrivileged(new PrivilegedAction<Object>() {
      @Override
      public Object run() {
        invokeAwareMethods(beanName, bean);
        return null;
      }
    }, getAccessControlContext());
  }
  else {
    // 特殊bean处理
    invokeAwareMethods(beanName, bean);
  }

  Object wrappedBean = bean;
  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
  }

  try {
    invokeInitMethods(beanName, wrappedBean, mbd);
  }
  catch (Throwable ex) {
    throw new BeanCreationException(
        (mbd != null &#63; mbd.getResourceDescription() : null),
        beanName, "Invocation of init method failed", ex);
  }

  if (mbd == null || !mbd.isSynthetic()) {
    wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
  }
  return wrappedBean;
}