ImportSelector接口是至spring中導入外部配置的核心接口，在SpringBoot的自動化配置和@EnableXXX(功能性註解)都有它的存在，關於SpringBoot的分析可以參考:深入理解SpringBoot的自動裝配。

一、關於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);

}

　　該接口文檔上說的明明白白，其主要作用是收集需要導入的配置類，如果該接口的實現類同時實現EnvironmentAware， BeanFactoryAware ，BeanClassLoaderAware或者ResourceLoaderAware，那麽在調用其selectImports方法之前先調用上述接口中對應的方法，如果需要在所有的@Configuration處理完在導入時可以實現DeferredImportSelector接口。

在這裏我舉個Spring中的實例來看一下：

@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;

}

此註解是開啟聲明式事務的註解，那麽它的@Import所導入的類為TransactionManagementConfigurationSelector，那麽我們看一下其類圖：

由此可知該類實現類ImportSelector接口

二、自定義@EnableXXX註解

　　在這裏我們先準備兩個Spring的項目工程:spring-project與ssm-project，其中spring-project裏我們先創建好如下結構目錄：

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;
    }
}

　　在這裏我們實現ImportSelector接口和BeanFactoryAware接口，重寫selectImports方法，最後我們返回的是AppConfig的類名，同時打印出相關的註解元數據與BeanFactory

自定義@EnableSpringStudy註解

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 {
}

在這裏我們仿照@EnableTransactionManagement來實現自定義註解，註意使用@Import導入我們剛才寫的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;
    }
}

當都完成以後我們打個jar包，準備引入至其他工程：

完成ssm-project工程中的AppConfig配置類

　　1) 首先我們將剛才的spring.jar導入到ssm-project工程裏

　　2) 在對應的配置類上添加上spring-project中定義的@EnableSpringStudy註解

@Configuration //表明此類是配置類
@ComponentScan // 掃描自定義的組件(repository service component controller)
@PropertySource("classpath:application.properties") // 讀取application.properties
@MapperScan("com.bdqn.lyrk.ssm.study.app.mapper") //掃描Mybatis的Mapper接口
@EnableTransactionManagement //開啟事務管理
@EnableSpringStudy
public class AppConfig {

  //....省略配置代碼      
}

　　3）編寫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());
    }

　　運行後輸出結果：

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

　　從這裏我們可以看到ImportSelector接口中的方法參數，可以獲取ssm-project項目下AppConfig的所有註解，並且能夠獲取當前BeanFactory所有配置的Bean

三、ImportSelector源碼分析

　　這個接口在哪裏調用呢？我們可以來看一下ConfigurationClassParser這個類的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) {
　　　　　　　　　　　　//對ImportSelector的處理
                    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) {
　　　　　　　　　　　　　　　　//如果為延遲導入處理則加入集合當中
                            this.deferredImportSelectors.add(
                                    new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
                        }
                        else {
　　　　　　　　　　　　　　　　//根據ImportSelector方法的返回值來進行遞歸操作
                            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 {
　　　　　　　　　　　　　　// 如果當前的類既不是ImportSelector也不是ImportBeanDefinitionRegistar就進行@Configuration的解析處理
                        // 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();
            }
        }
    }

　　在這裏我們可以看到ImportSelector接口的返回值會遞歸進行解析，把解析到的類全名按照@Configuration進行處理

深入理解Spring的ImportSelector接口