Spring源码分析之IOC的三种常见用法及源码实现（一）

深藏阁楼爱情的钟 2023-06-08 10:52 31阅读 0赞

# 1.ioc核心功能bean的配置与获取api #

有以下四种

![format_png][]

(来自精通spring4.x的p175)

常用的是前三种

### 第一种方式 ###

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

<?xml version="1.0" encoding="GBK"?> 
    
    <beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    
        xmlns="http://www.springframework.org/schema/beans" 
    
        xsi:schemaLocation="http://www.springframework.org/schema/beans 
    
        http://www.springframework.org/schema/beans/spring-beans-4.0.xsd"> 
    
         
        <bean id="person" class="com.mao.gouzao.Person"> 
        </bean> 
    </beans> 
    
    public static void main( String[] args )
    
    {
        ClassPathXmlApplicationContext ctx = new  ClassPathXmlApplicationContext("beans.xml");
        System.out.println(ctx.getBean("person"));
    }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

### 第二种方式 ###

定义

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

@Service
    
    public class UserServiceImpl {
        public void test(){
            System.out.println(666);
        }
    }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

获取

@Autowired
    
    private static UserServiceImpl userService;

### 第三种方式 ###

定义

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

@Configuration
    
    public class MainConfig {
        @Bean
        public Person person(){
            return new Person();
        }
    }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

**注意****:** **通过****@Bean****的形式是使用的话，** **bean****的默认名称是方法名，若****@Bean(value="bean****的名称****")****那么****bean****的名称是指定的**

**去容器中读取****Bean****的信息（传入配置类）**

获取

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public static void main( String[] args )
    
    {
        AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(MainConfig.class);
        System.out.println(ctx.getBean("person"));
    }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

## 2.如何实现的 ##

### 1.基于Java类配置的实现方式源码分析 ###

1.就执行了这两句代码

AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(MainConfig.class);
    System.out.println(ctx.getBean("person"));

先看看，打开构造器源码：

public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
       this();
       register(annotatedClasses);
       refresh();
    }

2.首先不急，主角儿AnnotationConfigApplicationContext 它有父类，因为类初始化顺序的关系，会先初始化父类，所以得看父类，一直看到最后父类为DefaultResourceLoader

![format_png 1][]

DefaultResourceLoader，很明显是用来加载资源的，倒数第二个AbstractApplicationContext的无参构造中也是初始化一个加载资源相关：

Ok，再来看最后一个父类GenericApplicationContext了，父类默认初始化使用下面这个无参的

public GenericApplicationContext() {
       this.beanFactory = new DefaultListableBeanFactory();
    }

这里是创建了springioc体系中的一个重要的类DefaultListableBeanFactory，DefaultListableBeanFactory非常关键里面实现了ioc相关很多功能，可以提供给我们的AnnotationConfigApplicationContext使用（后面会看到）

3.ok父类都看完了，那就回到我们的主角儿AnnotationConfigApplicationContext了

我们来看它的构造器刚刚是什么代码（回归本身）：

public AnnotationConfigApplicationContext() {
       this.reader = new AnnotatedBeanDefinitionReader(this);
       this.scanner = new ClassPathBeanDefinitionScanner(this);
    }

就两行。

4.先看第一句this.reader = new AnnotatedBeanDefinitionReader(this);

进去

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
       this(registry, getOrCreateEnvironment(registry));
    }

它再次调用自己的构造器

这里还涉及了一个方法getOrCreateEnvironment(registry)

很明显这个方法从名字看就知道是有缓存的意思，获取或创建，如果是第一次则创建 第二次则就是获取了，第二次拿的第一次缓存的。如下：

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

private static Environment getOrCreateEnvironment(BeanDefinitionRegistry registry) {
            Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
            if (registry instanceof EnvironmentCapable) {
                return ((EnvironmentCapable) registry).getEnvironment();
            }
            return new StandardEnvironment();
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

ok，继续看，刚刚是从构造器到另一个构造器了：

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
            Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
            Assert.notNull(environment, "Environment must not be null");
            this.registry = registry;
            this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
            AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

ConditionEvaluator是计算conditon的解析器（后面会讲），这里很明显主要逻辑在 AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);

**接下来AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);是重中之重，终于开始有大段逻辑了！之前都是各种继承跳转！**

**registerAnnotationConfigProcessors源码实现如下**

![aHR0cHM6Ly9pbWFnZXMuY25ibG9ncy5jb20vT3V0bGluaW5nSW5kaWNhdG9ycy9FeHBhbmRlZEJsb2NrU3RhcnQuZ2lm][]

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
                BeanDefinitionRegistry registry, Object source) {
    
            DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
            if (beanFactory != null) {
                if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
                    beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
                }
                if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
                    beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
                }
            }
    
            Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<BeanDefinitionHolder>(4);
    
            if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
    
            if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
    
            if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
    
            // Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
            if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
    
            // Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
            if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition();
                try {
                    def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
                            AnnotationConfigUtils.class.getClassLoader()));
                }
                catch (ClassNotFoundException ex) {
                    throw new IllegalStateException(
                            "Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
                }
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
    
            if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
            }
            if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
            }
    
            return beanDefs;
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

挺长，先看第一部分：

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
            if (beanFactory != null) {
                if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
                    beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
                }
                if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
                    beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
                }
            }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

很明显这是从registry里拿DefaultListableBeanFactory ，还记得前面讲主角儿AnnotationConfigApplicationContext的父类GenericApplicationContext吗？在它的无参构造中就创建了DefaultListableBeanFactory。现在到这个registry里去拿，而这个registry就是前面用this传过来的主角儿AnnotationConfigApplicationContext自然就有DefaultListableBeanFactory了。后面的逻辑就是null判断和设置比较器和解析器到里面了。

![aHR0cHM6Ly9pbWFnZXMuY25ibG9ncy5jb20vT3V0bGluaW5nSW5kaWNhdG9ycy9Db250cmFjdGVkQmxvY2suZ2lm][]

创建了一个set集合保存BeanDefinitionHolder（BeanDefinition是）。后面就非常多的if判断。这都是判断啥呢？

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
            }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

判断registry中是否存在一个个的常量定义的东西。那这些常量是什么呢？打开一看：

public static final String CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME = "org.springframework.context.annotation.internalConfigurationAnnotationProcessor";

是个类的全路径，如果registry里面没有就执行最后一句（前两句都是给最后一句服务的）：

beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));

也就是判断有没有这个类，没有就添加这个类进去，这这个类是用来解析配置注解的处理器。

还有个 public static final String AUTOWIRED\_ANNOTATION\_PROCESSOR\_BEAN\_NAME = *"org.springframework.context.annotation.internalAutowiredAnnotationProcessor";*

**综合所见，这段代码后半部分其实就是注册spring支持的各种注解的解析器的逻辑**

这个对应是用来解析自动装配注解的

最后如下：

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
                BeanDefinitionRegistry registry, Object source) {
            
            //获取之前创建的DefaultListableBeanFactory，检查null并塞入相关组件
            DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
            if (beanFactory != null) {
                if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
                    beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
                }
                
                if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
                    beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
                }
            }
    
            Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<BeanDefinitionHolder>(8);
            
            //注册一个配置类@Configuration解析器的bean定义（ConfigurationClassPostProcessor）
            if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
            
            //设置AutoWired注解解析器的bean定义信息
            if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
            
            //注册解析@Required 注解的处理器
            if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
            
            //检查是否支持JSR250规范，如何支持注册 解析JSR250规范的注解
            // Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
            if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
            
            //检查是否支持jpa，若支持注册解析jpa规范的注解
            // Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
            if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition();
                try {
                    def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
                            AnnotationConfigUtils.class.getClassLoader()));
                }
                catch (ClassNotFoundException ex) {
                    throw new IllegalStateException(
                            "Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
                }
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
            }
            
            //注册解析@EventListener的注解解析器
            if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
            }
            
            if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
                RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
                def.setSource(source);
                beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
            }
            //最后带着这一堆spring支持的功能的解析器返回（实际上刚刚分析的过程中压根没用到这个返回值，那是为什么呢？值得注意的是 原来它是把这些搞到registry里去了，综上， 注入这堆注解解析器 到registry 也就是DefaultListableBeanFactory中！）
            return beanDefs;
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

好，以上这部分分析结束，回到主角儿AnnotationConfigApplicationContext，之前我们是从它的构造器：

public AnnotationConfigApplicationContext() {
            this.reader = new AnnotatedBeanDefinitionReader(this);
            this.scanner = new ClassPathBeanDefinitionScanner(this);
        }

中分析的第一句终于把这个reader完毕了，接下来分析第二句咯。

名字可以看出是类路径下的扫描器，开始查看源码！

public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry) {
            this(registry, true);
        }

继续跟进

public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters) {
            this(registry, useDefaultFilters, getOrCreateEnvironment(registry));
        }

第一个registry 还是之前说的DefaultListableBeanFactory传的this,第二个也就是 传了个true，是否使用默认的filter，选择了是。第三个之前讲过了，有则用没有则创建，缓存。继续跟

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
                Environment environment, ResourceLoader resourceLoader) {
    
            Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
            this.registry = registry;
    
            if (useDefaultFilters) {
                registerDefaultFilters();
            }
            setEnvironment(environment);
            setResourceLoader(resourceLoader);
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

这部分代码前面就是赋值、注册默认的filters、设置从registry拿的环境设置、最后设置ResourceLoader

一部分一部分的看，先看registerDefaultFilters();

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

/**
         * Register the default filter for {@link Component @Component}.
         * <p>This will implicitly register all annotations that have the
         * {@link Component @Component} meta-annotation including the
         * {@link Repository @Repository}, {@link Service @Service}, and
         * {@link Controller @Controller} stereotype annotations.
         * <p>Also supports Java EE 6's {@link javax.annotation.ManagedBean} and
         * JSR-330's {@link javax.inject.Named} annotations, if available.
         *
         */
        @SuppressWarnings("unchecked")
        protected void registerDefaultFilters() {
            this.includeFilters.add(new AnnotationTypeFilter(Component.class));
            ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
            try {
                this.includeFilters.add(new AnnotationTypeFilter(
                        ((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));
                logger.debug("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning");
            }
            catch (ClassNotFoundException ex) {
                // JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.
            }
            try {
                this.includeFilters.add(new AnnotationTypeFilter(
                        ((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));
                logger.debug("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");
            }
            catch (ClassNotFoundException ex) {
                // JSR-330 API not available - simply skip.
            }
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

方法注释可以看到 意思是，注册这些默认的filter，@Component、@Repository、@Controller才会起作用

然后方法内部就注入添加Component.class进去了，让@Component起作用，而@Repository、@Controller本身都被打了@Component 是子注解，所以也连带起作用

后面部分就是判断jsr250 330相关注解了

到此看完了，回到上层上层去，至此

public AnnotationConfigApplicationContext(DefaultListableBeanFactory beanFactory) {
            super(beanFactory);
            this.reader = new AnnotatedBeanDefinitionReader(this);
            this.scanner = new ClassPathBeanDefinitionScanner(this);
        }

中的第二句我们也大致看完了，至此一个构造器看完了。大功告成.......了，一半。。。发现这个构造器是无参的，而我们之前用的两句代码：

AnnotationConfigApplicationContext annotationConfigApplication = new AnnotationConfigApplicationContext (MainConfig.class);
    Person person2 = (Person)annotationConfigApplication.getBean("person2");

中主角AnnotationConfigApplicationContext用的带class的构造器啊，看来没完，原来这个构造器还有上层调用，在这：

public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
            this();
            register(annotatedClasses);
            refresh();
        }

这里才是最开始那两行代码的第一次调用处。我们把this()；看完了！

接下来看第二句register(annotatedClasses);，不断跟进

public void register(Class<?>... annotatedClasses) {
            Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
            this.reader.register(annotatedClasses);
        }

不断跟进到这

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

public void registerBean(Class<?> annotatedClass, String name, Class<? extends Annotation>... qualifiers) {
            AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
            if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
                return;
            }
    
            ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
            abd.setScope(scopeMetadata.getScopeName());
            String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
            AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
            if (qualifiers != null) {
                for (Class<? extends Annotation> qualifier : qualifiers) {
                    if (Primary.class == qualifier) {
                        abd.setPrimary(true);
                    }
                    else if (Lazy.class == qualifier) {
                        abd.setLazyInit(true);
                    }
                    else {
                        abd.addQualifier(new AutowireCandidateQualifier(qualifier));
                    }
                }
            }
    
            BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
            definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
            BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
        }

![复制代码][aHR0cHM6Ly9jb21tb24uY25ibG9ncy5jb20vaW1hZ2VzL2NvcHljb2RlLmdpZg]

原来这句代码是把传入的配置类里的bean全给注册了。至此第二句完毕。至于最后第三句核心，留给下篇文章。一起加油！

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