spring bean 加载过程（spring）

以classpathXmlApplication为例

入口方法包含3个部分，

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {
        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

1.继承父类，没什么东西

2.设置配置文件

3.执行refresh方法（关键）。下面我们就这两个方法看看内部实现

设置配置文件，这块其实没什么东西，就是给context对象设置一个configLocations的数组，我们看一下context类的继承关系

下面看重点的refresh方法

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {//锁住一个对象，防止多线程同时执行初始化的操作
            // Prepare this context for refreshing.准备上下文，这里设置一下开始实现，准备propery等
            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) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

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

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

这个代码有点长，我们把每个方法干什么先研究一下，在深入看每个方法怎么实现的。

从7行开始看，这个方法是为了一个refreshBeanFactory。返回的类是ConfigurableListableBeanFactory，而这个factory是一个接口，我们待会看看这个接口有哪些实现，先看看这个方法内部实现

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

我们看到最后return的beanFactory就是ConfigurableListableBeanFactory。而这个接口只有一个default实现类DefaultListableBeanFactory。我们看看这个类主要有哪些成员变量

private boolean allowEagerClassLoading = true;

    /** Optional OrderComparator for dependency Lists and arrays */
    private Comparator<Object> dependencyComparator;

    /** Resolver to use for checking if a bean definition is an autowire candidate */
    private AutowireCandidateResolver autowireCandidateResolver = new SimpleAutowireCandidateResolver();

    /** Map from dependency type to corresponding autowired value */
    private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<Class<?>, Object>(16);

    /** Map of bean definition objects, keyed by bean name */
    private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(256);

    /** Map of singleton and non-singleton bean names, keyed by dependency type */
    private final Map<Class<?>, String[]> allBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64);

    /** Map of singleton-only bean names, keyed by dependency type */
    private final Map<Class<?>, String[]> singletonBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64);

    /** List of bean definition names, in registration order */
    private volatile List<String> beanDefinitionNames = new ArrayList<String>(256);

    /** List of names of manually registered singletons, in registration order */
    private volatile Set<String> manualSingletonNames = new LinkedHashSet<String>(16);

    /** Cached array of bean definition names in case of frozen configuration */
    private volatile String[] frozenBeanDefinitionNames;

    /** Whether bean definition metadata may be cached for all beans */
    private volatile boolean configurationFrozen = false;

这些成员变量有机会我们一个个分析下，我们知道每个bean 有一个beanDefinition对象来定义。在13行，我们看到有一个BeanDefinition的Map，我们随便调试一下，看看这个里面存了什么东西。

这里面竟然只有7个对象，分别是：ConfigurationClassPostProcessor、DefaultEventListenerFactory、MessageResolver、EventListenerMethodProcessor、AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、PropertiesFactoryBean、RequiredAnnotationBeanPostProcessor。这七个对象怎么来的呢，举个例子来看ConfigurationClassPostProcessor，这个类说明是，当配置文件中

Registered by default when using {@code <context:annotation-config/>} or
* {@code <context:component-scan/>}有这两个配置的时候，便会注入这么个类。好吧，那我们看看在哪里register的。我们猜测是在

refreshBeanFactory();这个方法里面。那看看这个方法

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);
        }
    }

我们看到第7行，创建了一个DefaultListableBeanFactory ，第10行有loadBeanDefinition。应该是在这里处理的。我们还是先调试下看看。执行7行代码，这个时候beanDefinitionMap是空的。执行完10行代码，我们的beanDefinitionMap有值了。这里面为了加深理解，我们在我们的spring-config.xml文件中加一个bean看看会怎么样。原来的spring-config.xml文件如下

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="
           http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-4.1.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-4.1.xsd
           http://www.springframework.org/schema/util
           http://www.springframework.org/schema/util/spring-util-4.1.xsd">

    <context:annotation-config/>
    
    <context:component-scan base-package="com.newbie.common"></context:component-scan>

    <util:properties id="systemProperties" location="classpath:conf/user.properties" />
    
    
</beans>

我们随便加一个bean，如下图所示，有多个一个beanDefinition。所以这个地就是定义所以beanDefinition的入口，而我们从autowire的bean是由相关processor处理的，这个今后如果有时间的话，我们在分析。我们继续往下看

我们回到原来的refresh方法里面。

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) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

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

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

我们现在知道第7行返回的这个ConfigurableListableBeanFactory 有我们所有的beanDefintion。这个方法是给ConfigurableListableBeanFactory 设置了若干预设变量，现在我们也不清楚这个变量是干嘛的，我们先跳过这个方法，等回头再分析他。

第14行postProcessBeanFactory（ConfigurableListableBeanFactory ）。我们看看这个代码是做什么事情，我们有一个关注点，我们@autowired的bean是怎么被处理的。这个代码，会根据不同context子类加入不同Processors，这个我们也暂时不分析他，等回头再看

第17行invokeBeanFactoryPostProcessors（ConfigurableListableBeanFactory ）。这个方法应该就是能看到bean是如何创建的。这个代码比较长，我们先把调用关系简单画出来

执行invokeBeanFactoryPostProcessors之后，这里主要是讲各个processors类注册进去，这里需要获得具体的bean了，根据beanName和BeanDefinition。如何获得bean，通过DefaultListableBeanFactory的getBean方法，这里判断bean是单利还是原型的，进入不同的getBean的逻辑，之后调用的AbstractAutowireCapableBeanFactory的createBean方法，之后通过反射来获得对应的bean。注意看一下doCreateBean方法里面的populateBean(）方法，这里能够完成对属性的注入。

上面这个方法，我们调用的时候发现就注册了一个ConfigurationClassPostProcessor，也不知道干嘛的。后来通过搜索了解到这个方法主要是用于处理spring的后置处理器的相关内容，参考https://www.cnblogs.com/sishang/p/6588542.html和https://www.cnblogs.com/sishang/p/6576665.html。