Spring IoC源码执行流程解析

摘要：这篇文章是我学习分析其源码时总结的，其目的并非是完全理解源码信息，但是根据容器如何从文件到的实例初始化进行了一个流程的梳理，希望可以对大家理解源码有一定的帮助。

这篇文章是我学习spring IoC 分析其源码时总结的，其目的并非是完全理解源码信息，但是根据spring IoC 容器如何从xml文件到bean的实例初始化进行了一个流程的梳理，希望可以对大家理解源码有一定的帮助。话不多少，我们直接来看。

这里，我是用ApplicationContext来进行介绍，毕竟项目中大部分还是使用的这个容器。容器启动的入口我们可以从AbstractApplicationContext类的refresh方法看起。
Spring IoC容器对Bean定义资源的载入是从refresh()函数开始的，refresh()是一个模板方法，refresh()方法的作用是：在创建IoC容器前，如果已经有容器存在，则需要把已有的容器销毁和关闭，以保证在refresh之后使用的是新建立起来的IoC容器。refresh的作用类似于对IoC容器的重启，在新建立好的容器中对容器进行初始化，对Bean定义资源进行载入。

refresh

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset "active" flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }
        }
    }

上边有两个很重要的方法：

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
这个方法就是根据Xml配置文件解析Bean的信息生成BeanDifinition，然后注册到BeanDefinitionRegistry中，即bean信息的装载注册阶段

beanFactory.preInstantiateSingletons();
这个方法看其注释我们就可以理解，它是实例化所有非懒加载的单例bean。

AbstractApplicationContext的obtainFreshBeanFactory()方法调用子类容器的refreshBeanFactory()方法，启动容器载入Bean定义资源文件的过程，代码如下：

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {  
        //这里使用了委派设计模式，父类定义了抽象的refreshBeanFactory()方法，具体实现调用子类容器的refreshBeanFactory()方法
         refreshBeanFactory();  
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();  
        if (logger.isDebugEnabled()) {  
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);  
        }  
        return beanFactory;  
    } 

AbstractApplicationContext类中只抽象定义了refreshBeanFactory()方法，容器真正调用的是其子类AbstractRefreshableApplicationContext实现的 refreshBeanFactory()方法。代码如下：

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

在这个方法中，先判断BeanFactory是否存在，如果存在则先销毁beans并关闭beanFactory，接着创建DefaultListableBeanFactory，并调用loadBeanDefinitions(beanFactory)装载bean定义。

我们在其子类AbstractXmlApplicationContext最终可以看到下面这个方法，此方法就是真正从XML文件中读取并解析Bean的信息，放在对应的BeanDefinition中

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context"s
        // resource loading environment.
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        initBeanDefinitionReader(beanDefinitionReader);
        loadBeanDefinitions(beanDefinitionReader);
    }

再看preInstantiateSingletons()

public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isInfoEnabled()) {
            this.logger.info("Pre-instantiating singletons in " + this);
        }
        List beanNames;
        synchronized (this.beanDefinitionMap) {
            // Iterate over a copy to allow for init methods which in turn register new bean definitions.
            // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
            beanNames = new ArrayList(this.beanDefinitionNames);
        }
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                if (isFactoryBean(beanName)) {
                    final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged(new PrivilegedAction() {
                            public Boolean run() {
                                return ((SmartFactoryBean) factory).isEagerInit();
                            }
                        }, getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean) factory).isEagerInit());
                    }
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
                else {
                    getBean(beanName);
                }
            }
        }
    }

其中我们需要关注getBean(beanName)这个方法，此方法中进行了bean的实例化和初始化;我们可以进入这个方法中，看到doGetBean() 这个方法

protected  T doGetBean(
            final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
            throws BeansException {
            ... ...  
            //这一部分是先从缓存中获取对应的bean；如果有的话，返回缓存中的单例bean；
            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 + """);
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
        
        else {
        
        ... ...
        /**
            * Return a merged RootBeanDefinition, traversing the parent bean definition
            * if the specified bean corresponds to a child bean definition.
            */
        final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
        
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    getBean(dependsOnBean);
                    registerDependentBean(dependsOnBean, beanName);
                }
            }

            // Create bean instance.
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory