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
1 public void refresh() throws BeansException, IllegalStateException {
2 synchronized (this.startupShutdownMonitor) {//锁住一个对象，防止多线程同时执行初始化的操作
3 // Prepare this context for refreshing.准备上下文，这里设置一下开始实现，准备propery等
4 prepareRefresh();
5
6 // Tell the subclass to refresh the internal bean factory.
7 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
8
9 // Prepare the bean factory for use in this context.
10 prepareBeanFactory(beanFactory);
11
12 try {
13 // Allows post-processing of the bean factory in context subclasses.
14 postProcessBeanFactory(beanFactory);
15
16 // Invoke factory processors registered as beans in the context.
17 invokeBeanFactoryPostProcessors(beanFactory);
18
19 // Register bean processors that intercept bean creation.
20 registerBeanPostProcessors(beanFactory);
21
22 // Initialize message source for this context.
23 initMessageSource();
24
25 // Initialize event multicaster for this context.
26 initApplicationEventMulticaster();
27
28 // Initialize other special beans in specific context subclasses.
29 onRefresh();
30
31 // Check for listener beans and register them.
32 registerListeners();
33
34 // Instantiate all remaining (non-lazy-init) singletons.
35 finishBeanFactoryInitialization(beanFactory);
36
37 // Last step: publish corresponding event.
38 finishRefresh();
39 }
40
41 catch (BeansException ex) {
42 if (logger.isWarnEnabled()) {
43 logger.warn("Exception encountered during context initialization - " +
44 "cancelling refresh attempt: " + ex);
45 }
46
47 // Destroy already created singletons to avoid dangling resources.
48 destroyBeans();
49
50 // Reset ‘active‘ flag.
51 cancelRefresh(ex);
52
53 // Propagate exception to caller.
54 throw ex;
55 }
56
57 finally {
58 // Reset common introspection caches in Spring‘s core, since we
59 // might not ever need metadata for singleton beans anymore...
60 resetCommonCaches();
61 }
62 }
63 }
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
1 private boolean allowEagerClassLoading = true; 2 3 /** Optional OrderComparator for dependency Lists and arrays */ 4 private Comparator<Object> dependencyComparator; 5 6 /** Resolver to use for checking if a bean definition is an autowire candidate */ 7 private AutowireCandidateResolver autowireCandidateResolver = new SimpleAutowireCandidateResolver(); 8 9 /** Map from dependency type to corresponding autowired value */ 10 private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<Class<?>, Object>(16); 11 12 /** Map of bean definition objects, keyed by bean name */ 13 private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(256); 14 15 /** Map of singleton and non-singleton bean names, keyed by dependency type */ 16 private final Map<Class<?>, String[]> allBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64); 17 18 /** Map of singleton-only bean names, keyed by dependency type */ 19 private final Map<Class<?>, String[]> singletonBeanNamesByType = new ConcurrentHashMap<Class<?>, String[]>(64); 20 21 /** List of bean definition names, in registration order */ 22 private volatile List<String> beanDefinitionNames = new ArrayList<String>(256); 23 24 /** List of names of manually registered singletons, in registration order */ 25 private volatile Set<String> manualSingletonNames = new LinkedHashSet<String>(16); 26 27 /** Cached array of bean definition names in case of frozen configuration */ 28 private volatile String[] frozenBeanDefinitionNames; 29 30 /** Whether bean definition metadata may be cached for all beans */ 31 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
1 protected final void refreshBeanFactory() throws BeansException {
2 if (hasBeanFactory()) {
3 destroyBeans();
4 closeBeanFactory();
5 }
6 try {
7 DefaultListableBeanFactory beanFactory = createBeanFactory();
8 beanFactory.setSerializationId(getId());
9 customizeBeanFactory(beanFactory);
10 loadBeanDefinitions(beanFactory);
11 synchronized (this.beanFactoryMonitor) {
12 this.beanFactory = beanFactory;
13 }
14 }
15 catch (IOException ex) {
16 throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
17 }
18 }
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
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns="http://www.springframework.org/schema/beans" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xmlns:context="http://www.springframework.org/schema/context" 5 xmlns:util="http://www.springframework.org/schema/util" 6 xsi:schemaLocation=" 7 http://www.springframework.org/schema/beans 8 http://www.springframework.org/schema/beans/spring-beans-4.1.xsd 9 http://www.springframework.org/schema/context 10 http://www.springframework.org/schema/context/spring-context-4.1.xsd 11 http://www.springframework.org/schema/util 12 http://www.springframework.org/schema/util/spring-util-4.1.xsd"> 13 14 <context:annotation-config/> 15 16 <context:component-scan base-package="com.newbie.common"></context:component-scan> 17 18 <util:properties id="systemProperties" location="classpath:conf/user.properties" /> 19 20 21 </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。
1 public void refresh() throws BeansException, IllegalStateException {
2 synchronized (this.startupShutdownMonitor) {
3 // Prepare this context for refreshing.
4 prepareRefresh();
5
6 // Tell the subclass to refresh the internal bean factory.
7 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
8
9 // Prepare the bean factory for use in this context.
10 prepareBeanFactory(beanFactory);
11
12 try {
13 // Allows post-processing of the bean factory in context subclasses.
14 postProcessBeanFactory(beanFactory);
15
16 // Invoke factory processors registered as beans in the context.
17 invokeBeanFactoryPostProcessors(beanFactory);
18
19 // Register bean processors that intercept bean creation.
20 registerBeanPostProcessors(beanFactory);
21
22 // Initialize message source for this context.
23 initMessageSource();
24
25 // Initialize event multicaster for this context.
26 initApplicationEventMulticaster();
27
28 // Initialize other special beans in specific context subclasses.
29 onRefresh();
30
31 // Check for listener beans and register them.
32 registerListeners();
33
34 // Instantiate all remaining (non-lazy-init) singletons.
35 finishBeanFactoryInitialization(beanFactory);
36
37 // Last step: publish corresponding event.
38 finishRefresh();
39 }
40
41 catch (BeansException ex) {
42 if (logger.isWarnEnabled()) {
43 logger.warn("Exception encountered during context initialization - " +
44 "cancelling refresh attempt: " + ex);
45 }
46
47 // Destroy already created singletons to avoid dangling resources.
48 destroyBeans();
49
50 // Reset ‘active‘ flag.
51 cancelRefresh(ex);
52
53 // Propagate exception to caller.
54 throw ex;
55 }
56
57 finally {
58 // Reset common introspection caches in Spring‘s core, since we
59 // might not ever need metadata for singleton beans anymore...
60 resetCommonCaches();
61 }
62 }
63 }
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。
原文地址：https://www.cnblogs.com/snail-tomorrow/p/10199240.html