• Springboot


    今天抽空在仔细看了下Springboot 关于 @Import 的处理过程, 记下来以后看。

    1. @Import

    先看Spring对它的注释 (文档贴过来的), 总结下来作用就是和xml配置的 <import />标签作用一样,允许通过它引入 @Configuration 注解的类 (java config), 引入ImportSelector接口(这个比较重要, 因为要通过它去判定要引入哪些@Configuration) 和 ImportBeanDefinitionRegistrar 接口的实现, 也包括 @Component注解的普通类。

    但是如果要引入另一个xml 文件形式配置的 bean, 则需要通过 @ImportResource 注解。

    /**
     * Indicates one or more {@link Configuration @Configuration} classes to import.
     *
     * <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
     * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
     * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
     * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
     *
     * <p>{@code @Bean} definitions declared in imported {@code @Configuration} classes should be
     * accessed by using {@link org.springframework.beans.factory.annotation.Autowired @Autowired}
     * injection. Either the bean itself can be autowired, or the configuration class instance
     * declaring the bean can be autowired. The latter approach allows for explicit, IDE-friendly
     * navigation between {@code @Configuration} class methods.
     *
     * <p>May be declared at the class level or as a meta-annotation.
     *
     * <p>If XML or other non-{@code @Configuration} bean definition resources need to be
     * imported, use the {@link ImportResource @ImportResource} annotation instead.
     *
     * @author Chris Beams
     * @author Juergen Hoeller
     * @since 3.0
     * @see Configuration
     * @see ImportSelector
     * @see ImportResource
     */
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Import {
    
        /**
         * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
         * or regular component classes to import.
         */
        Class<?>[] value();
    
    }
    View Code

    2. ImportSelector

    因为 @Import 的实现有很多时候需要借助 ImportSelector 接口, 所以我们再看下这个接口的描述, 总结下来就是需要通过这个接口的实现去决定要引入哪些 @Configuration。 它如果实现了以下四个Aware 接口, 那么spring保证会在调用它之前先调用Aware接口的方法。

    至于为什么要保证调用Aware, 我个人觉得应该是你可以通过这些Aware去感知系统里边所有的环境变量, 从而决定你具体的选择逻辑。

    /**
     * Interface to be implemented by types that determine which @{@link Configuration}
     * class(es) should be imported based on a given selection criteria, usually one or more
     * annotation attributes.
     *
     * <p>An {@link ImportSelector} may implement any of the following
     * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
     * methods will be called prior to {@link #selectImports}:
     * <ul>
     * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
     * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
     * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
     * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
     * </ul>
     *
     * <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
     * annotations, however, it is also possible to defer selection of imports until all
     * {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
     * for details).
     *
     * @author Chris Beams
     * @since 3.1
     * @see DeferredImportSelector
     * @see Import
     * @see ImportBeanDefinitionRegistrar
     * @see Configuration
     */
    public interface ImportSelector {
    
        /**
         * Select and return the names of which class(es) should be imported based on
         * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
         */
        String[] selectImports(AnnotationMetadata importingClassMetadata);
    
    }
    ImportSelector

    3. Springboot 对@Import注解的处理过程

    Springboot对注解的处理都发生在AbstractApplicationContext -> refresh() -> invokeBeanFactoryPostProcessors(beanFactory) -> ConfigurationClassPostProcessor -> postProcessBeanDefinitionRegistry()方法中。 

    (稍微说下也免得我自己忘了, springboot初始化的普通context(非web) 是AnnotationConfigApplicationContext, 在初始化的时候会初始化两个工具类, AnnotatedBeanDefinitionReader 和 ClassPathBeanDefinitionScanner 分别用来从 annotation driven 的配置和xml的配置中读取beanDefinition并向context注册, 那么在初始化 AnnotatedBeanDefinitionReader 的时候, 会向BeanFactory注册一个ConfigurationClassPostProcessor 用来处理所有的基于annotation的bean, 这个ConfigurationClassPostProcessor 是 BeanFactoryPostProcessor 的一个实现,springboot会保证在  invokeBeanFactoryPostProcessors(beanFactory) 方法中调用注册到它上边的所有的BeanFactoryPostProcessor)

    如下代码显示是通过 ConfigurationClassParser 类来转换的

    // Parse each @Configuration class
            ConfigurationClassParser parser = new ConfigurationClassParser(
                    this.metadataReaderFactory, this.problemReporter, this.environment,
                    this.resourceLoader, this.componentScanBeanNameGenerator, registry);

    那么在 ConfigurationClassParser -> processConfigurationClass() -> doProcessConfigurationClass() 方法中我们找到了 (这里边的流程还是很清楚的, 分别按次序处理了@PropertySource, @ComponentScan, @Import, @ImportResource, 在处理这些注解的时候是通过递归处理来保证所有的都被处理了)

    // Process any @Import annotations
            processImports(configClass, sourceClass, getImports(sourceClass), true);

    那接下来就看它到底是怎么做的 . 流程依然清晰 :

      首先, 判断如果被import的是 ImportSelector.class 接口的实现, 那么初始化这个被Import的类, 然后调用它的selectImports方法去获得所需要的引入的configuration, 然后递归处理

      其次, 判断如果被import的是 ImportBeanDefinitionRegistrar 接口的实现, 那么初始化后将对当前对象的处理委托给这个ImportBeanDefinitionRegistrar (不是特别明白, 只是我的猜测)

      最后, 将import引入的类作为一个正常的类来处理 ( 调用最外层的doProcessConfigurationClass())

    所以, 从这里我们知道, 如果你引入的是一个正常的component, 那么会作为@Compoent或者@Configuration来处理, 这样在BeanFactory里边可以通过getBean拿到, 但如果你是 ImportSelector 或者 ImportBeanDefinitionRegistrar 接口的实现, 那么spring并不会将他们注册到beanFactory中,而只是调用他们的方法。

    private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
                Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
    
            if (importCandidates.isEmpty()) {
                return;
            }
    
            if (checkForCircularImports && isChainedImportOnStack(configClass)) {
                this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
            }
            else {
                this.importStack.push(configClass);
                try {
                    for (SourceClass candidate : importCandidates) {
                        if (candidate.isAssignable(ImportSelector.class)) {
                            // Candidate class is an ImportSelector -> delegate to it to determine imports
                            Class<?> candidateClass = candidate.loadClass();
                            ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                            ParserStrategyUtils.invokeAwareMethods(
                                    selector, this.environment, this.resourceLoader, this.registry);
                            if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
                                this.deferredImportSelectors.add(
                                        new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
                            }
                            else {
                                String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                                Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                                processImports(configClass, currentSourceClass, importSourceClasses, false);
                            }
                        }
                        else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                            // Candidate class is an ImportBeanDefinitionRegistrar ->
                            // delegate to it to register additional bean definitions
                            Class<?> candidateClass = candidate.loadClass();
                            ImportBeanDefinitionRegistrar registrar =
                                    BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
                            ParserStrategyUtils.invokeAwareMethods(
                                    registrar, this.environment, this.resourceLoader, this.registry);
                            configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                        }
                        else {
                            // Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
                            // process it as an @Configuration class
                            this.importStack.registerImport(
                                    currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                            processConfigurationClass(candidate.asConfigClass(configClass));
                        }
                    }
                }
                catch (BeanDefinitionStoreException ex) {
                    throw ex;
                }
                catch (Throwable ex) {
                    throw new BeanDefinitionStoreException(
                            "Failed to process import candidates for configuration class [" +
                            configClass.getMetadata().getClassName() + "]", ex);
                }
                finally {
                    this.importStack.pop();
                }
            }
        }
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  • 原文地址:https://www.cnblogs.com/hermanlife/p/10019473.html
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