Spring之@Configuration配置解析

Posted 2020-08-13

1.简单的示例：

1 @Configuration
2 @EnableConfigurationProperties({DemoProperties.class})
3 public class DemoConfiguration {
4 
5     @Bean
6     public Book getBook(){
7        return new Book();
8     }
9 }

Configuration

1     @Autowired Book book;
2 
3     @Test
4     public void testBook(){
5         System.out.println(book.toString());
6     }

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。

[email protected]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

1     @Autowired
2     public void registBeanDefinition(BeanFactory factory){
3         if(factory instanceof BeanDefinitionRegistry) {
4             BeanDefinitionRegistry registry = (BeanDefinitionRegistry)factory;
5             BeanDefinition beanDefinition = BeanDefinitionBuilder.rootBeanDefinition(Book.class).getBeanDefinition();
6             registry.registerBeanDefinition("mybook", beanDefinition);
7         }
8     }

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1     @Autowired @Qualifier("mybook") Book book2;
2     @Test
3     public void testBook2(){
4         System.out.println(book2);
5     }

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：

1 public AnnotationConfigEmbeddedWebApplicationContext() {
2         this.reader = new AnnotatedBeanDefinitionReader(this);
3         this.scanner = new ClassPathBeanDefinitionScanner(this);
4     }

ApplicationContext构造

1 public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
2         this(registry, getOrCreateEnvironment(registry));
3     }

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1     public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
2         Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
3         Assert.notNull(environment, "Environment must not be null");
4         this.registry = registry;
5         this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
6         AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
7     }

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 1     public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
 2             BeanDefinitionRegistry registry, Object source) {
 3 
 4         DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
 5         if (beanFactory != null) {
 6             if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
 7                 beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
 8             }
 9             if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
10                 beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
11             }
12         }
13 
14         Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<BeanDefinitionHolder>(4);
15                 //注解@Configuration处理    
16         if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
17             RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
18             def.setSource(source);
19             beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
20         }
21 
22         .......//省略部分代码
23 
24         return beanDefs;
25     }

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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　　

1 @Target(ElementType.TYPE)
2 @Retention(RetentionPolicy.RUNTIME)
3 @Documented
4 public @interface PropertySources {
5 
6     PropertySource[] value();
7 
8 }

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

1 public class MutablePropertySources implements PropertySources {
2 
3     private final Log logger;
4 
5     private final List<PropertySource<?>> propertySourceList = new CopyOnWriteArrayList<PropertySource<?>>();
6 ......
7 }

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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。

 1     protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
 2         Assert.notEmpty(basePackages, "At least one base package must be specified");
 3         Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
 4         for (String basePackage : basePackages) {
 5             Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
 6             for (BeanDefinition candidate : candidates) {
 7                 ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
 8                 candidate.setScope(scopeMetadata.getScopeName());
 9                 String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
10                 if (candidate instanceof AbstractBeanDefinition) {
11                     postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
12                 }
13                 if (candidate instanceof AnnotatedBeanDefinition) {
14                     AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
15                 }
16                 if (checkCandidate(beanName, candidate)) {
17                     BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
18                     definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
19                     beanDefinitions.add(definitionHolder);
20                     registerBeanDefinition(definitionHolder, this.registry);
21                 }
22             }
23         }
24         return beanDefinitions;
25     }

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

 1         // Process individual @Bean methods
 2         Set<MethodMetadata> beanMethods = sourceClass.getMetadata().getAnnotatedMethods(Bean.class.getName());
 3         for (MethodMetadata methodMetadata : beanMethods) {
 4             configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
 5         }
 6 
 7         // Process default methods on interfaces
 8         for (SourceClass ifc : sourceClass.getInterfaces()) {
 9             beanMethods = ifc.getMetadata().getAnnotatedMethods(Bean.class.getName());
10             for (MethodMetadata methodMetadata : beanMethods) {
11                 if (!methodMetadata.isAbstract()) {
12                     // A default method or other concrete method on a Java 8+ interface...
13                     configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
14                 }
15             }
16         }

@Bean解析

最后真实加载beanDefinition是loadBeanDefinitionsForConfigurationClass方法：

 1     private void loadBeanDefinitionsForConfigurationClass(ConfigurationClass configClass,
 2             TrackedConditionEvaluator trackedConditionEvaluator) {
 3 
 4         if (trackedConditionEvaluator.shouldSkip(configClass)) {
 5             String beanName = configClass.getBeanName();
 6             if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
 7                 this.registry.removeBeanDefinition(beanName);
 8             }
 9             this.importRegistry.removeImportingClassFor(configClass.getMetadata().getClassName());
10             return;
11         }
12 
13         if (configClass.isImported()) {
14             registerBeanDefinitionForImportedConfigurationClass(configClass);
15         }
16         for (BeanMethod beanMethod : configClass.getBeanMethods()) {
17             loadBeanDefinitionsForBeanMethod(beanMethod);
18         }
19         loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
20         loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
21     }

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