• 深入理解Spring的ImportSelector接口


      ImportSelector接口是至spring中导入外部配置的核心接口,在SpringBoot的自动化配置和@EnableXXX(功能性注解)都有它的存在,关于SpringBoot的分析可以参考:深入理解SpringBoot的自动装配

    一、关于ImportSelector接口

     

    package org.springframework.context.annotation;
    
    import org.springframework.core.type.AnnotationMetadata;
    
    /**
     * 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);
    
    }

      该接口文档上说的明明白白,其主要作用是收集需要导入的配置类,如果该接口的实现类同时实现EnvironmentAware, BeanFactoryAware ,BeanClassLoaderAware或者ResourceLoaderAware,那么在调用其selectImports方法之前先调用上述接口中对应的方法,如果需要在所有的@Configuration处理完在导入时可以实现DeferredImportSelector接口。

    在这里我举个Spring中的实例来看一下:

    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Import(TransactionManagementConfigurationSelector.class)
    public @interface EnableTransactionManagement {
    
        /**
         * Indicate whether subclass-based (CGLIB) proxies are to be created ({@code true}) as
         * opposed to standard Java interface-based proxies ({@code false}). The default is
         * {@code false}. <strong>Applicable only if {@link #mode()} is set to
         * {@link AdviceMode#PROXY}</strong>.
         * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
         * Spring-managed beans requiring proxying, not just those marked with
         * {@code @Transactional}. For example, other beans marked with Spring's
         * {@code @Async} annotation will be upgraded to subclass proxying at the same
         * time. This approach has no negative impact in practice unless one is explicitly
         * expecting one type of proxy vs another, e.g. in tests.
         */
        boolean proxyTargetClass() default false;
    
        /**
         * Indicate how transactional advice should be applied. The default is
         * {@link AdviceMode#PROXY}.
         * @see AdviceMode
         */
        AdviceMode mode() default AdviceMode.PROXY;
    
        /**
         * Indicate the ordering of the execution of the transaction advisor
         * when multiple advices are applied at a specific joinpoint.
         * The default is {@link Ordered#LOWEST_PRECEDENCE}.
         */
        int order() default Ordered.LOWEST_PRECEDENCE;
    
    }

    此注解是开启声明式事务的注解,那么它的@Import所导入的类为TransactionManagementConfigurationSelector,那么我们看一下其类图:

    由此可知该类实现类ImportSelector接口

    二、自定义@EnableXXX注解

      在这里我们先准备两个Spring的项目工程:spring-project与ssm-project,其中spring-project里我们先创建好如下结构目录:

    SpringStudySelector

    package org.hzgj.spring.study.config;
    
    import org.springframework.beans.BeansException;
    import org.springframework.beans.factory.BeanFactory;
    import org.springframework.beans.factory.BeanFactoryAware;
    import org.springframework.context.annotation.ImportSelector;
    import org.springframework.core.type.AnnotationMetadata;
    
    public class SpringStudySelector implements ImportSelector, BeanFactoryAware {
        private BeanFactory beanFactory;
    
        @Override
        public String[] selectImports(AnnotationMetadata importingClassMetadata) {
            importingClassMetadata.getAnnotationTypes().forEach(System.out::println);
            System.out.println(beanFactory);
            return new String[]{AppConfig.class.getName()};
        }
    
        @Override
        public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
            this.beanFactory = beanFactory;
        }
    }

      在这里我们实现ImportSelector接口和BeanFactoryAware接口,重写selectImports方法,最后我们返回的是AppConfig的类名,同时打印出相关的注解元数据与BeanFactory

    自定义@EnableSpringStudy注解

    package org.hzgj.spring.study.annotation;
    
    import org.hzgj.spring.study.config.SpringStudySelector;
    import org.springframework.context.annotation.Import;
    
    import java.lang.annotation.*;
    
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Target(ElementType.TYPE)
    @Import(SpringStudySelector.class)
    public @interface EnableSpringStudy {
    }

    在这里我们仿照@EnableTransactionManagement来实现自定义注解,注意使用@Import导入我们刚才写的SpringStudySelector

    AppConfig

    package org.hzgj.spring.study.config;
    
    import org.hzgj.spring.study.bean.StudentBean;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    public class AppConfig {
    
        @Bean
        public StudentBean studentBean() {
            StudentBean studentBean = new StudentBean();
            studentBean.setId(19);
            studentBean.setName("admin");
            return studentBean;
        }
    }

    当都完成以后我们打个jar包,准备引入至其他工程:

     完成ssm-project工程中的AppConfig配置类

      1) 首先我们将刚才的spring.jar导入到ssm-project工程里

      2) 在对应的配置类上添加上spring-project中定义的@EnableSpringStudy注解

      

    @Configuration //表明此类是配置类
    @ComponentScan // 扫描自定义的组件(repository service component controller)
    @PropertySource("classpath:application.properties") // 读取application.properties
    @MapperScan("com.bdqn.lyrk.ssm.study.app.mapper") //扫描Mybatis的Mapper接口
    @EnableTransactionManagement //开启事务管理
    @EnableSpringStudy
    public class AppConfig {
    
      //....省略配置代码      
    }

      3)编写Main方法

     public static void main(String[] args) throws IOException {
    
    
            AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
            StudentBean studentBean = applicationContext.getBean(StudentBean.class);
            System.out.println(studentBean.getName());
        }

      运行后输出结果:

    org.springframework.context.annotation.Configuration
    org.springframework.context.annotation.ComponentScan
    org.springframework.context.annotation.PropertySource
    org.mybatis.spring.annotation.MapperScan
    org.springframework.transaction.annotation.EnableTransactionManagement
    org.hzgj.spring.study.annotation.EnableSpringStudy
    org.springframework.beans.factory.support.DefaultListableBeanFactory@4b9e13df: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,org.springframework.context.event.internalEventListenerProcessor,org.springframework.context.event.internalEventListenerFactory,appConfig,propertiesConfig,logAspect,studentService]; root of factory hierarchy
    admin

      从这里我们可以看到ImportSelector接口中的方法参数,可以获取ssm-project项目下AppConfig的所有注解,并且能够获取当前BeanFactory所有配置的Bean

    三、ImportSelector源码分析

      这个接口在哪里调用呢?我们可以来看一下ConfigurationClassParser这个类的processImports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) {
                //对ImportSelector的处理
    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) {
                    //如果为延迟导入处理则加入集合当中
    this.deferredImportSelectors.add( new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector)); } else {
                    //根据ImportSelector方法的返回值来进行递归操作 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 {
                  // 如果当前的类既不是ImportSelector也不是ImportBeanDefinitionRegistar就进行@Configuration的解析处理
    // 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(); } } }

      在这里我们可以看到ImportSelector接口的返回值会递归进行解析,把解析到的类全名按照@Configuration进行处理

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  • 原文地址:https://www.cnblogs.com/niechen/p/9262452.html
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