一、prepareContext()方法

　　前面我们介绍了SpringBoot 启动流程run()方法的前三步，本章，我们将用一个章节介绍：第四步：刷新应用上下文前的准备阶段。也就是prepareContext()方法。

　　首先看prepareContext()方法。

 1 private void prepareContext(ConfigurableApplicationContext context,
 2                             ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
 3                             ApplicationArguments applicationArguments, Banner printedBanner) {
 4     //设置容器环境
 5     context.setEnvironment(environment);
 6     //执行容器后置处理
 7     postProcessApplicationContext(context);
 8     //执行容器中的 ApplicationContextInitializer 包括spring.factories和通过三种方式自定义的
 9     applyInitializers(context);
10     //向各个监听器发送容器已经准备好的事件
11     listeners.contextPrepared(context);
12     if (this.logStartupInfo) {
13         logStartupInfo(context.getParent() == null);
14         logStartupProfileInfo(context);
15     }
16 
17     // Add boot specific singleton beans
18     //将main函数中的args参数封装成单例Bean，注册进容器
19     context.getBeanFactory().registerSingleton("springApplicationArguments",
20             applicationArguments);
21     //将 printedBanner 也封装成单例，注册进容器
22     if (printedBanner != null) {
23         context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
24     }
25 
26     // Load the sources
27     Set<Object> sources = getAllSources();
28     Assert.notEmpty(sources, "Sources must not be empty");
29     //加载我们的启动类，将启动类注入容器
30     load(context, sources.toArray(new Object[0]));
31     //发布容器已加载事件
32     listeners.contextLoaded(context);
33 }

　　首先看这行 Set<Object> sources = getAllSources(); 在getAllSources()中拿到了我们的启动类。　　

　　我们本文重点讲解这行 load(context, sources.toArray(new Object[0])); ，其他的方法请参阅注释。

　　跟进load()方法，看源码

 1 protected void load(ApplicationContext context, Object[] sources) {
 2     if (logger.isDebugEnabled()) {
 3         logger.debug(
 4                 "Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
 5     }
 6     //创建 BeanDefinitionLoader 
 7     BeanDefinitionLoader loader = createBeanDefinitionLoader(
 8             getBeanDefinitionRegistry(context), sources);
 9     if (this.beanNameGenerator != null) {
10         loader.setBeanNameGenerator(this.beanNameGenerator);
11     }
12     if (this.resourceLoader != null) {
13         loader.setResourceLoader(this.resourceLoader);
14     }
15     if (this.environment != null) {
16         loader.setEnvironment(this.environment);
17     }
18     loader.load();
19 }

1.1、getBeanDefinitionRegistry()

　　继续看getBeanDefinitionRegistry()方法的源码

1 private BeanDefinitionRegistry getBeanDefinitionRegistry(ApplicationContext context) {
2     if (context instanceof BeanDefinitionRegistry) {
3         return (BeanDefinitionRegistry) context;
4     }
5     ...
6 }

　　这里将我们前文创建的上下文强转为BeanDefinitionRegistry，是不是很熟悉，前面的文章中咱们也介绍过，他们之间是有继承关系的。BeanDefinitionRegistry定义了很重要的方法registerBeanDefinition()，该方法将BeanDefinition注册进DefaultListableBeanFactory容器的beanDefinitionMap中。

1.2、createBeanDefinitionLoader()

　　继续看createBeanDefinitionLoader()方法，最终进入了BeanDefinitionLoader类的构造方法，如下

 1 BeanDefinitionLoader(BeanDefinitionRegistry registry, Object... sources) {
 2     Assert.notNull(registry, "Registry must not be null");
 3     Assert.notEmpty(sources, "Sources must not be empty");
 4     this.sources = sources;
 5     //注解形式的Bean定义读取器 比如：@Configuration @Bean @Component @Controller @Service等等
 6     this.annotatedReader = new AnnotatedBeanDefinitionReader(registry);
 7     //XML形式的Bean定义读取器
 8     this.xmlReader = new XmlBeanDefinitionReader(registry);
 9     if (isGroovyPresent()) {
10         this.groovyReader = new GroovyBeanDefinitionReader(registry);
11     }
12     //类路径扫描器
13     this.scanner = new ClassPathBeanDefinitionScanner(registry);
14     //扫描器添加排除过滤器
15     this.scanner.addExcludeFilter(new ClassExcludeFilter(sources));
16 }

　　先记住上面的三个属性，具体有什么用，先看看注释。前面的文章，我们说过，IoC容器的初始化分为三个步骤，上面三个属性在，BeanDefinition的Resource定位，和BeanDefinition的注册中起到了很重要的作用。

1.3、loader.load();

　　跟进load()方法

 1 private int load(Object source) {
 2     Assert.notNull(source, "Source must not be null");
 3     // 从Class加载
 4     if (source instanceof Class<?>) {
 5         return load((Class<?>) source);
 6     }
 7     // 从Resource加载
 8     if (source instanceof Resource) {
 9         return load((Resource) source);
10     }
11     // 从Package加载
12     if (source instanceof Package) {
13         return load((Package) source);
14     }
15     // 从 CharSequence 加载 ？？？
16     if (source instanceof CharSequence) {
17         return load((CharSequence) source);
18     }
19     throw new IllegalArgumentException("Invalid source type " + source.getClass());
20 }

　　当前我们的主类会按Class加载。

　　继续跟进load()方法。

 1 private int load(Class<?> source) {
 2     if (isGroovyPresent()
 3             && GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
 4         // Any GroovyLoaders added in beans{} DSL can contribute beans here
 5         GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source,
 6                 GroovyBeanDefinitionSource.class);
 7         load(loader);
 8     }
 9     if (isComponent(source)) {
10         //将 启动类的 BeanDefinition注册进 beanDefinitionMap
11         this.annotatedReader.register(source);
12         return 1;
13     }
14     return 0;
15 }

　　isComponent(source)判断主类是不是存在@Component注解，主类@SpringBootApplication是一个组合注解（后面讲解自动装配会讲解<SpringBoot启动流程分析（五）：SpringBoot自动装配原理实现>），包含@Component。

　　this.annotatedReader.register(source);跟进register()方法，最终进到AnnotatedBeanDefinitionReader类的doRegisterBean()方法。

 1 <T> void doRegisterBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
 2         @Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {
 3 
 4     //将指定的类 封装为AnnotatedGenericBeanDefinition
 5     AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
 6     if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
 7         return;
 8     }
 9 
10     abd.setInstanceSupplier(instanceSupplier);
11     // 获取该类的 scope 属性
12     ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
13     abd.setScope(scopeMetadata.getScopeName());
14     String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
15 
16     AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
17     if (qualifiers != null) {
18         for (Class<? extends Annotation> qualifier : qualifiers) {
19             if (Primary.class == qualifier) {
20                 abd.setPrimary(true);
21             }
22             else if (Lazy.class == qualifier) {
23                 abd.setLazyInit(true);
24             }
25             else {
26                 abd.addQualifier(new AutowireCandidateQualifier(qualifier));
27             }
28         }
29     }
30     for (BeanDefinitionCustomizer customizer : definitionCustomizers) {
31         customizer.customize(abd);
32     }
33 
34     BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
35     definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
36     // 将该BeanDefinition注册到IoC容器的beanDefinitionMap中
37     BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
38 }

　　在该方法中将主类封装成AnnotatedGenericBeanDefinition

　　BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);方法将BeanDefinition注册进beanDefinitionMap

 1 public static void registerBeanDefinition(
 2         BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
 3         throws BeanDefinitionStoreException {
 4     // Register bean definition under primary name.
 5     // primary name 其实就是id吧
 6     String beanName = definitionHolder.getBeanName();
 7     registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
 8     // Register aliases for bean name, if any.
 9     // 然后就是注册别名
10     String[] aliases = definitionHolder.getAliases();
11     if (aliases != null) {
12         for (String alias : aliases) {
13             registry.registerAlias(beanName, alias);
14         }
15     }
16 }

　　继续跟进registerBeanDefinition()方法。

 1 @Override
 2 public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
 3         throws BeanDefinitionStoreException {
 4 
 5     Assert.hasText(beanName, "Bean name must not be empty");
 6     Assert.notNull(beanDefinition, "BeanDefinition must not be null");
 7 
 8     if (beanDefinition instanceof AbstractBeanDefinition) {
 9         try {
10             // 最后一次校验了
11             // 对bean的Overrides进行校验，还不知道会在哪处理这些overrides
12             ((AbstractBeanDefinition) beanDefinition).validate();
13         } catch (BeanDefinitionValidationException ex) {
14             throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
15                     "Validation of bean definition failed", ex);
16         }
17     }
18     // 判断是否存在重复名字的bean，之后看允不允许override
19     // 以前使用synchronized实现互斥访问，现在采用ConcurrentHashMap
20     BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
21     if (existingDefinition != null) {
22         //如果该类不允许 Overriding 直接抛出异常
23         if (!isAllowBeanDefinitionOverriding()) {
24             throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
25                     "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
26                             "': There is already [" + existingDefinition + "] bound.");
27         } else if (existingDefinition.getRole() < beanDefinition.getRole()) {
28             // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
29             if (logger.isWarnEnabled()) {
30                 logger.warn("Overriding user-defined bean definition for bean '" + beanName +
31                         "' with a framework-generated bean definition: replacing [" +
32                         existingDefinition + "] with [" + beanDefinition + "]");
33             }
34         } else if (!beanDefinition.equals(existingDefinition)) {
35             if (logger.isInfoEnabled()) {
36                 logger.info("Overriding bean definition for bean '" + beanName +
37                         "' with a different definition: replacing [" + existingDefinition +
38                         "] with [" + beanDefinition + "]");
39             }
40         } else {
41             if (logger.isDebugEnabled()) {
42                 logger.debug("Overriding bean definition for bean '" + beanName +
43                         "' with an equivalent definition: replacing [" + existingDefinition +
44                         "] with [" + beanDefinition + "]");
45             }
46         }
47         //注册进beanDefinitionMap
48         this.beanDefinitionMap.put(beanName, beanDefinition);
49     } else {
50         if (hasBeanCreationStarted()) {
51             // Cannot modify startup-time collection elements anymore (for stable iteration)
52             synchronized (this.beanDefinitionMap) {
53                 this.beanDefinitionMap.put(beanName, beanDefinition);
54                 List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
55                 updatedDefinitions.addAll(this.beanDefinitionNames);
56                 updatedDefinitions.add(beanName);
57                 this.beanDefinitionNames = updatedDefinitions;
58                 if (this.manualSingletonNames.contains(beanName)) {
59                     Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
60                     updatedSingletons.remove(beanName);
61                     this.manualSingletonNames = updatedSingletons;
62                 }
63             }
64         } else {
65             // Still in startup registration phase
66             //如果仍处于启动注册阶段，注册进beanDefinitionMap
67             this.beanDefinitionMap.put(beanName, beanDefinition);
68             this.beanDefinitionNames.add(beanName);
69             this.manualSingletonNames.remove(beanName);
70         }
71         this.frozenBeanDefinitionNames = null;
72     }
73 
74     if (existingDefinition != null || containsSingleton(beanName)) {
75         resetBeanDefinition(beanName);
76     }
77 }