应用框架：Spring IOC基于注解启动的分析

浅浅的花香味﹌ 2022-03-12 15:50 176阅读 0赞

牛旦教育IT课堂 2019-03-05 15:25:54

> 原文： http://www.jianshu.com/p/573bdae020e9

**Spring 基于注解启动**

主要有两个Class实现注解启动

*  AnnotationConfigApplicationContext
 *  AnnotationConfigWebApplicationContext

我们以AnnotationConfigApplicationContext 为研究对象

![应用框架：Spring IOC基于注解启动的分析][Spring IOC]

AnnotationConfigApplicationContext.png

引入Spring 最小依赖

<dependency>
     <groupId>org.springframework</groupId>
     <artifactId>spring-context</artifactId>
     <version>${spring.version}</version>
     </dependency>

编写器启动代码

public static void main(String[] args) {
     AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
     applicationContext.register(BeanConfig.class);
     applicationContext.refresh();
     Date date = applicationContext.getBean("date",Date.class);
     System.out.println(date);
     }

**AnnotationConfigApplicationContext 构造函数**

public AnnotationConfigApplicationContext() {
     //负责注册Class ，读取器
     this.reader = new AnnotatedBeanDefinitionReader(this);
     //负责扫描指定类路径下的Class，注册bean
     this.scanner = new ClassPathBeanDefinitionScanner(this);
     }

**AnnotatedBeanDefinitionReader 构造方法**

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
     this(registry, getOrCreateEnvironment(registry));
     }
     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;
     //初始化ConditionEvaluator
     this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
     
     /** 在給定的注册表中註冊所有相关的post processors
     * 判断容器是否已经存在给定注册表的bean，如果没有注册bean，并将bean放入容器中
     * 把所有的处理处理器列出来
     * ConfigurationClassPostProcessor 內部管理的配置注解处理器
     * AutowiredAnnotationBeanPostProcessor 内部管理@Autowired 的处理器
     * RequiredAnnotationBeanPostProcessor @Required的处理器
     * CommonAnnotationBeanPostProcessor JSR-250注解处理器 ，先判断是否支持jsr，如果支持注册
     * PersistenceAnnotationBeanPostProcessor JPA管理 先使用类加载器查找是否存在，如果有这个包则注册
     * EventListenerMethodProcessor @EventListener的处理器
     * DefaultEventListenerFactory 管理EventListenerFactory处理器
     */
     AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
     }

ConditionEvaluator这个对象干什么,点击进去

public ConditionEvaluator(@Nullable BeanDefinitionRegistry registry,
     @Nullable Environment environment, @Nullable ResourceLoader resourceLoader) {
     this.context = new ConditionContextImpl(registry, environment, resourceLoader); 
     }
     //ConditionContextImpl 实现了ConditionContext接口，ConditionEvaluator静态内部类
     public ConditionContextImpl(@Nullable BeanDefinitionRegistry registry,
     @Nullable Environment environment, @Nullable ResourceLoader resourceLoader) {
     this.registry = registry;
     this.beanFactory = deduceBeanFactory(registry);
     this.environment = (environment != null ? environment : deduceEnvironment(registry));
     this.resourceLoader = (resourceLoader != null ? resourceLoader : deduceResourceLoader(registry));
     this.classLoader = deduceClassLoader(resourceLoader, this.beanFactory);
     }

可以知道 ConditionEvaluator 使用外部传参的方法初始化了Spring容器顶级对象

BeanFactory , Environment , ResourceLoader , ClassLoader 。再将这些传给ConditionContextImpl为接下来的解析 @Conditional 注解做好准备

**ClassPathBeanDefinitionScanner构造函数**

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

绕了地球几圈了，其实就是将Spring 顶级接口 Environment , ResourceLoader 赋值，使用默认注解过滤器，首先将@Component加入List中，判断当前环境是否支持 **JSR-250 **， **JSR-330 **，相应加入过滤器中。也就是这个扫描器默认只扫描 @Component 或者 **JSR-250 **， **JSR-330 **的标记的Class。

**applicationContext.register(BeanConfig.class)**

public void register(Class<?>... annotatedClasses) {
     Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
     this.reader.register(annotatedClasses); //调用 刚刚初始化读取器
     }
     |
    ============================AnnotatedBeanDefinitionReader 读取器代码======================================================================================================
     public void register(Class<?>... annotatedClasses) {
     for (Class<?> annotatedClass : annotatedClasses) {
     registerBean(annotatedClass);
     }
     }
     public void registerBean(Class<?> annotatedClass) {
     doRegisterBean(annotatedClass, null, null, null);
     }
     
     /**
     *从给定的bean解析Class给定的注解，执行相应的初始化，保存到Spring容器中
     */
     <T> void doRegisterBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
     @Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {
     //根据Class的Annotated 得出元数据 AnnotationMetadata
     AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
     /**
     * 判断注册的Class 是否包含@Conditional注解，如果有获取全部value，放入List中
     * 排序后，遍历所有的Conditiion的实现，使用反射获取对象，执行matches方法，
     * 如果发现有返回false，中断循环直接返回true，
     */
     if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) { //如果 @Conditional条件不满足，不进行注册
     return;
     }
     abd.setInstanceSupplier(instanceSupplier);
     //解析Class是否有@Scope,解析@Scope注解返回ScopeMetadata对象，没有直接返回空
     ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
     abd.setScope(scopeMetadata.getScopeName());
     //判断注解上Value是否有值，有就使用这个作为BeanName，没有则取类名 
     String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
     //继续解析AnnotationMetadata的@Lazy,@Primary，@DependsOn，@Role，@Description的注解，放入结果放入对象的属性中
     AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
     //这个类只是BeanDefinition 包装类
     BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
     //是否需要代理类，如果是则修改内部属性，重新生成BeanDefinition 对象
     definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
     //调用DefaultListableBeanFactory.registerBeanDefinition的方法，做一些安全性校验再，将definitionHolder 放入register容器中
     BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
     }

这个方法就是将注册的Bean，解析Class上的注解，初始化注解数据，做相应处理，转化成BeanDefinition ,放入Spring 容器中保存起来。

我们看下 BeanDefinition 是怎么实现注册到Spring的容器中，主要由 DefaultListableBeanFactory.registerBeanDefinition 来实现

public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
     throws BeanDefinitionStoreException {
     Assert.hasText(beanName, "Bean name must not be empty");
     Assert.notNull(beanDefinition, "BeanDefinition must not be null");
     if (beanDefinition instanceof AbstractBeanDefinition) {
     try {
     //对beanDefinition 进行校验判断MethodOverrides不能为空，必须拥有工厂方法
     ((AbstractBeanDefinition) beanDefinition).validate();
     }
     catch (BeanDefinitionValidationException ex) {
     throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
     "Validation of bean definition failed", ex);
     }
     }
     BeanDefinition oldBeanDefinition;
     oldBeanDefinition = this.beanDefinitionMap.get(beanName); 
     if (oldBeanDefinition != null) {
     //这个方法是判断是否允许出现重名bean，并且是不同的定义bean，是否可以覆盖前者
     if (!isAllowBeanDefinitionOverriding()) {
     throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
     "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
     "': There is already [" + oldBeanDefinition + "] bound.");
     }
     else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
     // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
     if (this.logger.isWarnEnabled()) {
     this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
     "' with a framework-generated bean definition: replacing [" +
     oldBeanDefinition + "] with [" + beanDefinition + "]");
     }
     }
     else if (!beanDefinition.equals(oldBeanDefinition)) {
     if (this.logger.isInfoEnabled()) {
     this.logger.info("Overriding bean definition for bean '" + beanName +
     "' with a different definition: replacing [" + oldBeanDefinition +
     "] with [" + beanDefinition + "]");
     }
     }
     else {
     if (this.logger.isDebugEnabled()) {
     this.logger.debug("Overriding bean definition for bean '" + beanName +
     "' with an equivalent definition: replacing [" + oldBeanDefinition +
     "] with [" + beanDefinition + "]");
     }
     }
     this.beanDefinitionMap.put(beanName, beanDefinition);
     }
     else {
     //调用alreadyCreated.isEmpty()，alreadyCreated Set对象，保存已经创建beanName
     //文档中表示created，跟这里注册应该不是同一个行为，这个要看到后面才知道什么意思
     if (hasBeanCreationStarted()) {
     synchronized (this.beanDefinitionMap) {//更新数据
     this.beanDefinitionMap.put(beanName, beanDefinition);
     List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
     updatedDefinitions.addAll(this.beanDefinitionNames);
     updatedDefinitions.add(beanName);
     this.beanDefinitionNames = updatedDefinitions;
     if (this.manualSingletonNames.contains(beanName)) {
     Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
     updatedSingletons.remove(beanName);
     this.manualSingletonNames = updatedSingletons;
     }
     }
     }
     else { 
     //Spring beanDefinition 容器，一个Map转载
     this.beanDefinitionMap.put(beanName, beanDefinition);
     //保存beanName，主要用于记录每个bean注册顺序
     this.beanDefinitionNames.add(beanName);
     //删除单例，注册成一个普通bean
     this.manualSingletonNames.remove(beanName);
     }
     this.frozenBeanDefinitionNames = null;
     }
     if (oldBeanDefinition != null || containsSingleton(beanName)) { 
     //更新Spring容器里beanName
     resetBeanDefinition(beanName);
     }
     }

将beanDefinition注册到Spring容器中，并没有太多复杂的逻辑，只是做一些安全性的检查。

**BeanDefinition**

一个BeanDefinition描述了一个bean的实例，包括属性值，构造方法参数值和继承自它的类的更多信息。BeanDefinition仅仅是一个最简单的接口，主要功能是允许BeanFactoryPostProcessor 例如PropertyPlaceHolderConfigure 能够检索并修改属性值和别的bean的元数据（译注）

Spring 容器 **beanDefinition** 主要分为 RootBeanDefinition , AnnotatedGenericBeanDefinition 这两种

*  RootBeanDefinition Spring Factory中的特定bean
 *  AnnotatedGenericBeanDefinition 用户自定义bean

**Spring 启动流程总结**

![应用框架：Spring IOC基于注解启动的分析][Spring IOC 1]

AnnotationConfigApplicationContext 初始化.png

这些BeanDefinition只是放入到Spirng 容器中，并没有进行任何初始化对象的操作，真正的IOC操作都在 **refresh() **,这个方法有空再进行分析。

[Spring IOC]: http://p1.pstatp.com/large/pgc-image/d29b2c350074454781b849c2c6af5fc4
[Spring IOC 1]: /images/20220312/278a872c5db542f2955ee8c8d35aabe9.png