Spring读书笔记——bean创建（下）

有关Spring加载bean系列，今天这是最后一篇了，主要接上篇对于从Spring容器中获取Bean的一些细节实现的补充。

• 《Spring读书笔记——bean加载》——Spring如何加载消化一个xml配置文件
• 《Spring读书笔记——bean解析》——Spring如何将xml文件的各种标签转换为BeanDefinition并注册到Spring容器下
• 《Spring读书笔记——bean创建（上）》——概述Spring如何从容器中取出需要的那个Bean

从缓存中加载单例

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 != NULL_OBJECT ? singletonObject : null);
}

在看这段代码之前，我们先了解下Spring对于单例bean出现循环依赖的解决方法。如果出现上面我们介绍的A->B->C->A的情况，那是不是说Spring就无能为力了，显然Spring没有那么弱。那么Spring是怎么做的？ 鉴于单例bean的循环依赖问题，Spring创建bean的原则是不等bean创建完成就会将bean的ObjectFactory提前曝光加入到缓存中，一旦有某个bean创建时需要依赖这个bean了，那么就可以直接使用ObjectFactory。 简单说，创建bean的时候，就是打包快递发货，主管为了知道你今天要派发多少个包裹，为了节省大家时间以及以免统计漏掉的情况。你可以先拿出一个包裹箱子，上面写上要寄收件人、收货地址、联系方式等等，但是这时候还没有往里面打包真正的快递。 这里曝光的bean就相当于这个快递箱子。

好了，知道了这个原则之后，我们就好理解代码了。 首先从singletonObjects中获取实例，取不到则从earlySingletonObjects中获取，仍然取不到，我们还可以到singletonFactories中获取相应的ObjectFactory，在调用这个ObjectFactory的getObject方法来创建bean。 然后将其加入到earlySingletonObjects中，在将其从singletonFactories中删除。

想必，你已经被这些用来存储和删除的集合搞疯了，没关系，我们来理一下：

• singletonObjects /** Cache of singleton objects: bean name --> bean instance */ private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64); 用于保存BeanName和创建bean实例之间的关系。
• singletonFactories: /** Cache of singleton factories: bean name --> ObjectFactory */ private final Map<String, ObjectFactory> singletonFactories = new HashMap<String, ObjectFactory>(16); 用于保存BeanName和创建bean的工厂之间的关系
• earlySingletonObjects: /** Cache of early singleton objects: bean name --> bean instance */ private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16); 用于保存BeanName和创建bean的工厂之间的关系，与singletonObjects的区别是当一个bean被放入这个集合后，可以用于其他bean做循环依赖检查

bean实例化

我们从缓存中拿到bean之后，就需要根据bean的不同类型做不同的处理，返回相应的bean，实现这个功能的就是getObjectForBeanInstance方法

protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {

        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }

        // Now we have the bean instance, which may be a normal bean or a FactoryBean.
        // If it's a FactoryBean, we use it to create a bean instance, unless the
        // caller actually wants a reference to the factory.
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }

        Object object = null;
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);
        }
        if (object == null) {
            // Return bean instance from factory.
            FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
            // Caches object obtained from FactoryBean if it is a singleton.
            if (mbd == null && containsBeanDefinition(beanName)) {
                mbd = getMergedLocalBeanDefinition(beanName);
            }
            boolean synthetic = (mbd != null && mbd.isSynthetic());
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }

• 首先检测指定的beanName是否是工厂bean相关，如果既不是工厂bean又是以"&"开头，则校验失败，抛出异常
• 如果这个bean不是工厂Bean（FactoryBean），那么就直接返回bean实例
• 剩下代码就是处理FactoryBean，我们顺着这样的顺序依次来到getObjectForBeanInstance->getObjectFromFactoryBean->doGetObjectFromFactoryBean

private Object doGetObjectFromFactoryBean(
        final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
        throws BeanCreationException {

    Object object;
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
                    public Object run() throws Exception {
                            return factory.getObject();
                        }
                    }, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            object = factory.getObject();
        }
    }
    catch (FactoryBeanNotInitializedException ex) {
        throw new BeanCurrentlyInCreationException(beanName, ex.toString());
    }
    catch (Throwable ex) {
        throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
    }


    // Do not accept a null value for a FactoryBean that's not fully
    // initialized yet: Many FactoryBeans just return null then.
    if (object == null && isSingletonCurrentlyInCreation(beanName)) {
        throw new BeanCurrentlyInCreationException(
                beanName, "FactoryBean which is currently in creation returned null from getObject");
    }

    if (object != null && shouldPostProcess) {
        try {
            object = postProcessObjectFromFactoryBean(object, beanName);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
        }
    }

    return object;
}

这么长的代码，如果嫌累，就只看factory.getObject()这行就好，这诠释了FactoryBean的加载时通过factory.getObject的方式获取到对应的bean实例的。

如何创建单例bean

在上篇的doGetBean方法中，如果从缓存中加载不到，那么我们就需要老老实实的从头开始加载bean了，对于单例bean的加载就都在这里实现了

// Create bean instance.
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                    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);
            }

创建bean

我们从AbstractBeanFactory的createBean方法来到了AbstractAutowiredCapableBeanFactory的createbean方法，而真正的创建bean其实在doCreateBean方法中

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) {
      instanceWrapper = createBeanInstance(beanName, mbd, args);
   }
   final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
   Class beanType = (instanceWrapper != null ? 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;
      }
   }

   // Eagerly cache singletons to be able to resolve circular references
   // even when triggered by lifecycle interfaces like BeanFactoryAware.
   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");
      }
      addSingletonFactory(beanName, new ObjectFactory() {
         public Object getObject() throws BeansException {
            return getEarlyBeanReference(beanName, mbd, bean);
         }
      });
   }

   // Initialize the bean instance.
   Object exposedObject = bean;
   try {
      populateBean(beanName, mbd, instanceWrapper);
      if (exposedObject != null) {
         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);
      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) {
               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 {
      registerDisposableBeanIfNecessary(beanName, bean, mbd);
   }
   catch (BeanDefinitionValidationException ex) {
      throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
   }

   return exposedObject;
}

• 如果是单例Bean，那么首先是从factoryBeanInstanceCache中清除该beanName对应的记录
• 实例化bean，将BeanDefinition转换为BeanWrapper对象
• bean合并后的处理
• 解决循环依赖问题
• 属性填充，将所有属性填充到bean的实例中 这个方法，本身不算长，但是层层深入就会发现其下面包罗了创建bean的诸多繁杂的细节（这块自己看的也是云里雾里，就不往下延伸扩展了）。

虽然对于Spring加载bean，我只写了四篇，但是其内部实现远比我表述的要复杂的多。 看源码确实很煎熬，对于目前看不懂的地方要么多看几遍，要么先跳过。阅读代码的过程中要懂得取舍，对于非重点部分比如日志或者异常处理可以先忽略，沿着一条主线往下看，最主要是先弄懂代码的只要意图。 Spring的bean加载代码量虽然巨大，但是思路还是比较清晰的，我们知道Spring如何加载xml然后解析xml，再到如何把xml的元素转为自己的BeanDefinition，最后又是如何取出对应的beanName然后返回一个bean实例供容器使用的。

网上有一位大神用一张图就把整个过程画出来了

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