Spring读书笔记——bean创建（下）

我就是我 2024-03-27 19:03 78阅读 0赞

## Spring读书笔记——bean创建（下） ##

从缓存中加载单例

bean实例化

如何创建单例bean

创建bean

--------------------

本文章向大家介绍Spring读书笔记——bean创建（下），主要内容包括从缓存中加载单例、bean实例化、如何创建单例bean、创建bean、基本概念、基础应用、原理机制和需要注意的事项等，并结合实例形式分析了其使用技巧，希望通过本文能帮助到大家理解应用这部分内容。

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