Spring之@Configuration配置解析

1.简单的示例：

 @Configuration
 @EnableConfigurationProperties({DemoProperties.class})
 public class DemoConfiguration {

     @Bean
     public Book getBook(){
        return new Book();
     }
 }

Configuration

     @Autowired Book book;

     @Test
     public void testBook(){
         System.out.println(book.toString());
     }

单元测试

结果打印出book对象，证明Book已经被注入到Spring 容器中了。

2.@Configuration配置的bean是如何注入到Spring容器中的呢？

首先先来简单介绍下通过BeanDefinitionRegistry注入bean

     @Autowired
     public void registBeanDefinition(BeanFactory factory){
         if(factory instanceof BeanDefinitionRegistry) {
             BeanDefinitionRegistry registry = (BeanDefinitionRegistry)factory;
             BeanDefinition beanDefinition = BeanDefinitionBuilder.rootBeanDefinition(Book.class).getBeanDefinition();
             registry.registerBeanDefinition("mybook", beanDefinition);
         }
     }

     @Autowired @Qualifier("mybook") Book book2;
     @Test
     public void testBook2(){
         System.out.println(book2);
     }

单元测试

结果同样打印出book对象，这里，笔者将beanFactory强转为BenDefinitionRegistry,因为笔者的Demo中使用的是默认BeanFactory,----DefaultListableBeanFactory,他实现了BeanDefinitionRegistry接口。

3.入口，下图为ApplicaitonContext refresh方法的简化，只保留了BeandefinitionRegistry注册bean部分功能。

然，似乎并没什么用？此处会调用BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法。要解决的一个问题是，BeanDefinitionRegistryPostProcessor类型的bean是如何注入的。以SpringBoot初始化为例。

注意到，ApplicationContext的构造方法：

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

ApplicationContext构造

 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;
         this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
         AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
     }

     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);
                 //注解@Configuration处理
         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));
         }

         .......//省略部分代码

         return beanDefs;
     }

注意到对@Configuration的处理为ConfigurationClassPostProcessor。

注意到ConfigurationClassPostProcessor实现了BeanDefinitionRegistryPostProcessor接口，显然关键方法为postProcessBeanDefinitionRegistry。ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry会调用ConfirgurationClassParser的parse方法。会依次解析注解，我们一步一步查看对各个注解的解析。

（1）@PropertySources和@PropertySource　　

 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 public @interface PropertySources {

     PropertySource[] value();

 }

PropertySources定义

最终都是处理@PropertySource,@PropertySources仅仅只是包含多个@PropertySource,@PropertySource注解的主要功能是引入配置文件，将配置的属性键值对与环境变量中的配置合并。其中最关键的类为MutablePropertySources

 public class MutablePropertySources implements PropertySources {

     private final Log logger;

     private final List<PropertySource<?>> propertySourceList = new CopyOnWriteArrayList<PropertySource<?>>();
 ......
 }

显然MutablePropertySources中包含有一个PropertySource列表。MutablePropertySources仅仅是封装了迭代器功能。可以理解成PropertySources是PropertySource的集合，增加了常用的集合操作。

（2）@ComponentScan

定义自动扫描的包。简化的序列图如下：

其最关键的方法为doScan方法，会注册BeanDefinition到容器中。

     protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
         Assert.notEmpty(basePackages, "At least one base package must be specified");
         Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
         for (String basePackage : basePackages) {
             Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
             for (BeanDefinition candidate : candidates) {
                 ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                 candidate.setScope(scopeMetadata.getScopeName());
                 String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
                 if (candidate instanceof AbstractBeanDefinition) {
                     postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
                 }
                 if (candidate instanceof AnnotatedBeanDefinition) {
                     AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
                 }
                 if (checkCandidate(beanName, candidate)) {
                     BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                     definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                     beanDefinitions.add(definitionHolder);
                     registerBeanDefinition(definitionHolder, this.registry);
                 }
             }
         }
         return beanDefinitions;
     }

doScan方法

registerBeanDefinition(definitionHolder, this.registry);能说明一切。

（3）@Import

@Import注解可以配置需要引入的class(假设配置为A,可以是数组),有三种方式。其流程图如下：

如果A为ImportSelector的子类，调用selectImports()方法，返回class类名数组，循环解析每一个import的类，如果A为BeanDefinitionRegistrar则直接调用registerBeanDefinition直接注入bean到容器中。如果A为普通的类（非前面提到的两种类型），则将A当做@Configuration配置的类，重新解析Configuration.

（4）@ImportSource

主要功能为引入资源文件。

（5）@Bean，比较简单，童FactoryMethod一样

         // Process individual @Bean methods
         Set<MethodMetadata> beanMethods = sourceClass.getMetadata().getAnnotatedMethods(Bean.class.getName());
         for (MethodMetadata methodMetadata : beanMethods) {
             configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
         }

         // Process default methods on interfaces
         for (SourceClass ifc : sourceClass.getInterfaces()) {
             beanMethods = ifc.getMetadata().getAnnotatedMethods(Bean.class.getName());
             for (MethodMetadata methodMetadata : beanMethods) {
                 if (!methodMetadata.isAbstract()) {
                     // A default method or other concrete method on a Java 8+ interface...
                     configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
                 }
             }
         }

@Bean解析

最后真实加载beanDefinition是loadBeanDefinitionsForConfigurationClass方法：

     private void loadBeanDefinitionsForConfigurationClass(ConfigurationClass configClass,
             TrackedConditionEvaluator trackedConditionEvaluator) {

         if (trackedConditionEvaluator.shouldSkip(configClass)) {
             String beanName = configClass.getBeanName();
             if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
                 this.registry.removeBeanDefinition(beanName);
             }
             this.importRegistry.removeImportingClassFor(configClass.getMetadata().getClassName());
             return;
         }

         if (configClass.isImported()) {
             registerBeanDefinitionForImportedConfigurationClass(configClass);
         }
         for (BeanMethod beanMethod : configClass.getBeanMethods()) {
             loadBeanDefinitionsForBeanMethod(beanMethod);
         }
         loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
         loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
     }