ImportSelector与DeferredImportSelector的区别（spring4）

Posted 2022-06-20 程序员欣宸

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在使用@Import注解来注册bean的时候，Import注解的值可以是ImportSelector或者DeferredImportSelector的实现类，spring容器会实例化这个实现类，并执行其selectImports方法，那么问题来了：ImportSelector和DeferredImportSelector的区别在哪里，我们自定义Imort逻辑的时候该选择哪个呢？本文通过分析相关的spring源码来查找答案；

全文概览

本文由以下几部分组成：
1. 看官方文档；
2. 分析spring源码中对这两个接口的处理；
3. 实战验证；

看官方文档

先看官方文档看起，我选择了4.3.9版本在线文档（这是个Release版），地址：https://docs.spring.io/spring/docs/4.3.19.RELEASE/javadoc-api/
原文：
A variation of ImportSelector that runs after all @Configuration beans have been processed. This type of selector can be particularly useful when the selected imports are @Conditional.
Implementations can also extend the Ordered interface or use the Order annotation to indicate a precedence against other DeferredImportSelectors.
我的理解：
1. DeferredImportSelector是ImportSelector的一个扩展；
2. ImportSelector实例的selectImports方法的执行时机，是在@Configguration注解中的其他逻辑被处理之前，所谓的其他逻辑，包括对@ImportResource、@Bean这些注解的处理（注意，这里只是对@Bean修饰的方法的处理，并不是立即调用@Bean修饰的方法，这个区别很重要！）；
3. DeferredImportSelector实例的selectImports方法的执行时机，是在@Configguration注解中的其他逻辑被处理完毕之后，所谓的其他逻辑，包括对@ImportResource、@Bean这些注解的处理；
4. DeferredImportSelector的实现类可以用Order注解，或者实现Ordered接口来对selectImports的执行顺序排序；

分析spring源码中对这两个接口的处理

接下来看看源码：
在spring-framework-4.1.8.RELEASE工程中找到类ConfigurationClassParser.java，这里面有处理配置类的主要逻辑；
找到方法parse(Set<BeanDefinitionHolder> configCandidates)：

public void parse(Set<BeanDefinitionHolder> configCandidates) 
        this.deferredImportSelectors = new LinkedList<DeferredImportSelectorHolder>();
        //检查每个bean的定义
        for (BeanDefinitionHolder holder : configCandidates) 
            BeanDefinition bd = holder.getBeanDefinition();
            try 
                if (bd instanceof AnnotatedBeanDefinition) 
                    //对于每个有注解的类，都执行方法parse(AnnotationMetadata metadata, String beanName)
                    parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
                
                else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) 
                    parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
                
                else 
                    parse(bd.getBeanClassName(), holder.getBeanName());
                
            
            catch (BeanDefinitionStoreException ex) 
                throw ex;
            
            catch (Exception ex) 
                throw new BeanDefinitionStoreException(
                        "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
            
        

        //最后再处理DeferredImportSelector的实现类
        processDeferredImportSelectors();

由以上代码可以大致看出DeferredImportSelector的实现类被最后放在processDeferredImportSelectors方法中处理，那么前面的parse(AnnotationMetadata metadata, String beanName)做了些什么呢？继续看；
展开方法parse(AnnotationMetadata metadata, String beanName)里面，是执行processConfigurationClass方法;
再展开processConfigurationClass方法，看到核心逻辑是调用doProcessConfigurationClass方法，展开看看：

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass) throws IOException 
        //为了聚焦Import相关处理，此处略去部分不相关代码，不在这里展示了
        ...
        ...
        // 处理@Import注解
        processImports(configClass, sourceClass, getImports(sourceClass), true);

        // 处理@ImportResource注解
        if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) 
            AnnotationAttributes importResource = AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
            String[] resources = importResource.getStringArray("value");
            Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
            for (String resource : resources) 
                String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
                configClass.addImportedResource(resolvedResource, readerClass);
            
        

        // 处理@Bean注解，注意是处理注解，不是执行@Bean修饰的方法
        Set<MethodMetadata> beanMethods = sourceClass.getMetadata().getAnnotatedMethods(Bean.class.getName());
        for (MethodMetadata methodMetadata : beanMethods) 
            configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
        

        // 处理Configuration类的父类，外面在调用doProcessConfigurationClass方法的时有迭代处理，确保所有父类的注解都会被处理
        if (sourceClass.getMetadata().hasSuperClass()) 
            String superclass = sourceClass.getMetadata().getSuperClassName();
            if (!superclass.startsWith("java") && !this.knownSuperclasses.containsKey(superclass)) 
                this.knownSuperclasses.put(superclass, configClass);
                // Superclass found, return its annotation metadata and recurse
                return sourceClass.getSuperClass();
            
        

        // 再也没有父类了，返回null表示当前Configuration处理完毕
        return null;

根据上述代码分析，可以梳理出下图中的逻辑：

现在需要再看看processImports和processDeferredImportSelectors这两个方法的具体代码；
先看processImports方法：

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
            Collection<SourceClass> importCandidates, boolean checkForCircularImports) throws IOException 

        if (importCandidates.isEmpty()) 
            return;
        

        if (checkForCircularImports && this.importStack.contains(configClass)) 
            this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
        
        else 
            this.importStack.push(configClass);
            try 
                for (SourceClass candidate : importCandidates) 
                    //如果是ImportSelector接口的实现类，就在此处理
                    if (candidate.isAssignable(ImportSelector.class)) 
                        // Candidate class is an ImportSelector -> delegate to it to determine imports
                        Class<?> candidateClass = candidate.loadClass();
                        //实例化这些ImportSelector的实现类
                        ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                        //如果这实现类还实现了BeanFactoryAware、EnvironmentAware这些接口，就要先执行这些接口中声明的方法
                        invokeAwareMethods(selector);
                        //如果这个实现类也实现了DeferredImportSelector接口，就被加入到集合deferredImportSelectors中
                        if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) 
                            this.deferredImportSelectors.add(
                                    new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
                        
                        else 
                            //注意，这一行是关键代码！！！执行实现类的selectImports方法
                            String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                            Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                            processImports(configClass, currentSourceClass, importSourceClasses, false);
                        
                    
                    //此处略去的和ImportSelector不相关的逻辑代码
                    ...
                    ...
                    ...
                
            
            catch (BeanDefinitionStoreException ex) 
                throw ex;
            
            catch (Exception ex) 
                throw new BeanDefinitionStoreException("Failed to process import candidates for configuration class [" +
                        configClass.getMetadata().getClassName() + "]", ex);
            
            finally 
                this.importStack.pop();

以上代码有两个关键点：
第一、当前被处理的类，如果实现了DeferredImportSelector接口，就被加入到集合deferredImportSelectors中；
第二、当前被处理的类，如果没有实现DeferredImportSelector接口，但是实现了ImportSelector接口，就被执行selectImports方法；
接下来看看processDeferredImportSelectors方法的源码，提前推测应该是处理集合deferredImportSelectors中的所有类，这些类都实现了DeferredImportSelector接口：

private void processDeferredImportSelectors() 
        List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
        this.deferredImportSelectors = null;
        //按照Order注解或者Ordered接口进行排序
        Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);

        for (DeferredImportSelectorHolder deferredImport : deferredImports) 
            ConfigurationClass configClass = deferredImport.getConfigurationClass();
            try 
                //此处是关键代码，执行DeferredImportSelector实现类的selectImports方法
                String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());
                processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false);
            
            catch (BeanDefinitionStoreException ex) 
                throw ex;
            
            catch (Exception ex) 
                throw new BeanDefinitionStoreException("Failed to process import candidates for configuration class [" +
                        configClass.getMetadata().getClassName() + "]", ex);

至此，源码分析完毕了，从代码可以很清晰的看出ImportSelector与DeferredImportSelector的区别，就是selectImports方法执行时机有差别，这个差别期间，spring容器对此Configguration类做了些其他的逻辑：包括对@ImportResource、@Bean这些注解的处理（注意，这里只是对@Bean修饰的方法的处理，并不是立即调用@Bean修饰的方法，这个区别很重要！）；

实战验证

接下来到了实战验证的环节了，本次实战的内容是创建一个springboot工程，在里面自定义三个ImportSelector接口的实现类，如果您不想敲代码，也可以去github下载源码，地址和链接信息如下表所示：

名称	链接	备注
项目主页	https://github.com/zq2599/blog_demos	该项目在GitHub上的主页
git仓库地址(https)	https://github.com/zq2599/blog_demos.git	该项目源码的仓库地址，https协议
git仓库地址(ssh)	git@github.com:zq2599/blog_demos.git	该项目源码的仓库地址，ssh协议

这个git项目中有多个文件夹，本章源码在文件夹customizeimportselector下，如下图红框所示：

开始编码吧：
我们创建三个ImportSelector的实现类来检查其先后顺序，三个Selector类简介如下表，有两个是DeferredImportSelector的实现类，一个是ImportSelector的实现类，每个Selector负责向spring容器注册一种实例，：

类名	实现接口	Order注解	Import的类
CustomizeImportSelector1	DeferredImportSelector	102	CustomizeServiceImpl1
CustomizeImportSelector2	DeferredImportSelector	101	CustomizeServiceImpl2
CustomizeImportSelector3	ImportSelector	无	CustomizeServiceImpl3

基于maven创建springboot框架的web工程，pom.xml内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.bolingcavalry</groupId>
    <artifactId>customizeimportselector</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>customizeimportselector</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.9.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

创建三个接口CustomizeService1、CustomizeService2、CustomizeService3，第一个源码如下，另外两个除了类名，其余部分一样：

package com.bolingcavalry.customizeimportselector.service;

public interface CustomizeService1 
    void execute();

创建三个类，分别实现上面的三个接口，也是除了类名其余部分一样：

package com.bolingcavalry.customizeimportselector.service.impl;

import com.bolingcavalry.customizeimportselector.service.CustomizeService1;

public class CustomizeServiceImpl1 implements CustomizeService1 

    public CustomizeServiceImpl1() 
        System.out.println("construct : " + this.getClass().getSimpleName());
    

    @Override

    public void execute() 
        System.out.println("execute : " + this.getClass().getSimpleName());

创建CustomizeImportSelector1：

package com.bolingcavalry.customizeimportselector.selector;

import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.context.annotation.ImportSelector;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotationMetadata;

@Order(102)
public class CustomizeImportSelector1 implements DeferredImportSelector 
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) 
        System.out.println("selectImports : " + this.getClass().getSimpleName());
        return new String[]"com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl1";

创建CustomizeImportSelector2：

package com.bolingcavalry.customizeimportselector.selector;

import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotationMetadata;

@Order(101)
public class CustomizeImportSelector2 implements DeferredImportSelector 
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) 
        System.out.println("selectImports : " + this.getClass().getSimpleName());
        return new String[]"com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl2";

创建CustomizeImportSelector3，实现的是ImportSelector接口：

package com.bolingcavalry.customizeimportselector.selector;

import org.springframework.context.annotation.ImportSelector;
import org.springframework.core.annotation.Order;
import org.springframework.core.type.AnnotationMetadata;

public class CustomizeImportSelector3 implements ImportSelector 
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) 
        System.out.println("selectImports : " + this.getClass().getSimpleName());
        return new String[]"com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl3";

创建配置类，将CustomizeImportSelector1、CustomizeImportSelector2、CustomizeImportSelector3全部用Import注解引入：

package com.bolingcavalry.customizeimportselector;

import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector1;
import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector2;
import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector3;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

@Configuration
@Import(CustomizeImportSelector1.class, CustomizeImportSelector2.class, CustomizeImportSelector3.class)
public class SysConfig

创建启动类CustomizeimportselectorApplication.java：

package com.bolingcavalry.customizeimportselector;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class CustomizeimportselectorApplication 

    public static void main(String[] args) 
        SpringApplication.run(CustomizeimportselectorApplication.class, args);

启动应用，可见输入信息如下：

2018-09-09 15:43:45.790  INFO 15364 --- [           main] c.b.c.CustomizeimportselectorApplication : Starting CustomizeimportselectorApplication on DESKTOP-82CCEBN with PID 15364 (D:\\github\\blog_demos\\customizeimportselector\\target\\classes started by 12167 in D:\\github\\blog_demos\\customizeimportselector)
2018-09-09 15:43:45.791  INFO 15364 --- [           main] c.b.c.CustomizeimportselectorApplication : No active profile set, falling back to default profiles: default
2018-09-09 15:43:45.825  INFO 15364 --- [           main] ationConfigEmbeddedWebApplicationContext : Refreshing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@641147d0: startup date [Sun Sep 09 15:43:45 GMT+08:00 2018]; root of context hierarchy
selectImports : CustomizeImportSelector3
selectImports : CustomizeImportSelector2
selectImports : CustomizeImportSelector1
2018-09-09 15:43:46.425  INFO 15364 --- [           main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat initialized with port(s): 8080 (http)
2018-09-09 15:43:46.430  INFO 15364 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2018-09-09 15:43:46.431  INFO 15364 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/8.5.23
2018-09-09 15:43:46.493  INFO 15364 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2018-09-09 15:43:46.493  INFO 15364 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 670 ms
2018-09-09 15:43:46.569  INFO 15364 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Mapping servlet: dispatcherServlet to [/]
2018-09-09 15:43:46.572  INFO 15364 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: characterEncodingFilter to: [/*]
2018-09-09 15:43:46.572  INFO 15364 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: hiddenHttpMethodFilter to: [/*]
2018-09-09 15:43:46.572  INFO 15364 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: httpPutFormContentFilter to: [/*]
2018-09-09 15:43:46.572  INFO 15364 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: requestContextFilter to: [/*]
construct : CustomizeServiceImpl1
construct : CustomizeServiceImpl2
construct : CustomizeServiceImpl3

从上述信息可以看出：
首先、三个selector实现类的selectImports方法执行顺序符合预期：先执行ImportSelector实现类的，再执行DeferredImportSelector实现类的，并且DeferredImportSelector实现类的执行顺序会按照Order的设置从小到大执行；
其次、CustomizeServiceImpl1、CustomizeServiceImpl2、CustomizeServiceImpl3的实例化顺序并未受到影响；
至此，ImportSelector与DeferredImportSelector的区别已经分析和验证完毕，随着对Configuration初始化处理逻辑的深入了解，我们可以定制出更灵活强大的配置逻辑，以符合业务需求；

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