Spring IOC基于注解启动示例详析

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

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。在将这些传给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

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。

总结

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