SpringBoot源码解析 -- AutoConfigure的实现原理

源码分析基于spring boot 2.1

本文通过阅读源码，分析SpringBoot AutoConfigure功能的实现原理。

（源码解析类文章建议在PC端阅读）

SpringBoot中使用@EnableAutoConfiguration注解启动AutoConfigure功能

@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    ...
}

这里起作用的实际上是@Import和AutoConfigurationImportSelector。

@Import注解非常重要，它是SpringBoot中AutoConfiguration功能的基础。

前面解析SpringBoot启动过程的文章说过，SpringBoot启动时会注入ConfigurationClassPostProcessor，该PostProcessor正是处理@Import的类。

ConfigurationClassPostProcessor#postProcessBeanFactory -> ConfigurationClassPostProcessor#processConfigBeanDefinitions

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
    List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
    String[] candidateNames = registry.getBeanDefinitionNames();

    for (String beanName : candidateNames) {
        BeanDefinition beanDef = registry.getBeanDefinition(beanName);
        if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
            }
        }
        else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {    // #1
            configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
        }
    }
    if (configCandidates.isEmpty()) {
        return;
    }

    ...
    
    ConfigurationClassParser parser = new ConfigurationClassParser(
            this.metadataReaderFactory, this.problemReporter, this.environment,
            this.resourceLoader, this.componentScanBeanNameGenerator, registry);    //#2

    Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
    Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
    do {
        parser.parse(candidates);    //#3
        parser.validate();

        Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());    // #4
        configClasses.removeAll(alreadyParsed);

        if (this.reader == null) {
            this.reader = new ConfigurationClassBeanDefinitionReader(
                    registry, this.sourceExtractor, this.resourceLoader, this.environment,
                    this.importBeanNameGenerator, parser.getImportRegistry());
        }
        this.reader.loadBeanDefinitions(configClasses);    // #5
        alreadyParsed.addAll(configClasses);

        candidates.clear();
        if (registry.getBeanDefinitionCount() > candidateNames.length) {
            String[] newCandidateNames = registry.getBeanDefinitionNames();    // #6
            Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
            Set<String> alreadyParsedClasses = new HashSet<>();
            for (ConfigurationClass configurationClass : alreadyParsed) {
                alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
            }
            for (String candidateName : newCandidateNames) {
                if (!oldCandidateNames.contains(candidateName)) {
                    BeanDefinition bd = registry.getBeanDefinition(candidateName);    
                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
                            !alreadyParsedClasses.contains(bd.getBeanClassName())) {
                        candidates.add(new BeanDefinitionHolder(bd, candidateName));    // #7
                    }
                }
            }
            candidateNames = newCandidateNames;
        }
    }
    while (!candidates.isEmpty());    // #8

    ...
}

#1 检查bean是否为ConfigurationClass，这里主要是检查class注解信息（spring中将@Configuration标注的类归类为ConfigurationClass）

#2 构建ConfigurationClassParser

#3 解析ConfigurationClass

#4 获取结果，注意ConfigurationClassParser#getConfigurationClasses方法获取ConfigurationClassParser的处理结果

#5 获取ConfigurationClass引入的Class，将其转化为BeanDefinition，并注册到Spring上下文

最后构造bean，是在AbstractApplicationContext#refresh方法中，调用finishBeanFactoryInitialization，构建热加载的单例bean时完成。

#6 获取新的BeanDefinition列表

#7 如果前面的ConfigurationClass有引入了新的ConfigurationClass，添加到待处理集合

#8 循环处理，直到待处理集合为空

ConfigurationClassParser#parse -> ConfigurationClassParser#processConfigurationClass

protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
    if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {    // #1
        return;
    }

    ...

    SourceClass sourceClass = asSourceClass(configClass);    // #2
    do {
        sourceClass = doProcessConfigurationClass(configClass, sourceClass);    // #3
    }
    while (sourceClass != null);    // #4

    this.configurationClasses.put(configClass, configClass);    // #5
}

#1 检查ConfigurationClass是否存在@Conditional注解，如果存在，取注解中Condition条件判断类进行判断

#2 将ConfigurationClass转化为SourceClass

SourceClass对Class元数据进行封装，可以兼容处理JVM加载的Class和ASM读取的元数据，获取元数据中注解，方法等信息

#3 doProcessConfigurationClass方法很关键，处理@Component，@PropertySources，@ComponentScans，@Import，@ImportResource，带@Bean的方法，接口及父类。

#4 如果ConfigurationClass存在父类，doProcessConfigurationClass返回父类，这里递归处理父类数据

#5 将该ConfigurationClass加入configurationClasses，以便ConfigurationClassPostProcessor#processConfigBeanDefinitions方法 #5 步骤使用

该方法是处理ConfigurationClass的入口，doProcessConfigurationClass中引入了新的ConfigurationClass，也会调用该方法处理。

ConfigurationClassParser#doProcessConfigurationClass

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
        throws IOException {
    if (configClass.getMetadata().isAnnotated(Component.class.getName())) {    
        processMemberClasses(configClass, sourceClass);    // #1
    }

    ...

    processImports(configClass, sourceClass, getImports(sourceClass), true);    // #2

    AnnotationAttributes importResource =
            AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);    // #3
    if (importResource != null) {
        String[] resources = importResource.getStringArray("locations");
        Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
        for (String resource : resources) {
            String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
            configClass.addImportedResource(resolvedResource, readerClass);
        }
    }

    Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);    // #4
    for (MethodMetadata methodMetadata : beanMethods) {
        configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
    }

    processInterfaces(configClass, sourceClass);    // #5

    if (sourceClass.getMetadata().hasSuperClass()) {    // #6
        String superclass = sourceClass.getMetadata().getSuperClassName();
        if (superclass != null && !superclass.startsWith("java") &&
                !this.knownSuperclasses.containsKey(superclass)) {
            this.knownSuperclasses.put(superclass, configClass);
            return sourceClass.getSuperClass();
        }
    }

    return null;
}

这里只关注该方法AutoConfigure功能相关的代码

#1 如果Class存在@Component注解，会查询Class的内部类，如果内部类也是ConfigurationClass，会调用processConfigurationClass方法处理内部类（注意，@Configuration注解上标识了@Component注解）

#2 处理@Import注解

#3 处理@ImportResource，添加引入资源信息到ConfigurationClass#importedResources，ConfigurationClassPostProcessor#processConfigBeanDefinitions方法 #5 步骤会处理

#4 选择Class中存在@Bean标注的方法，加入到ConfigurationClass#beanMethods中

#5 选择接口中存在@Bean标注的方法，同样加入到ConfigurationClass#beanMethods中

#6 如果存在父类，返回父类到ConfigurationClassParser#processConfigurationClass中，递归处理父类。

ConfigurationClassParser#processImports

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)) {    // #1
                    Class<?> candidateClass = candidate.loadClass();
                    ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                    ParserStrategyUtils.invokeAwareMethods(
                            selector, this.environment, this.resourceLoader, this.registry);    
                    if (selector instanceof DeferredImportSelector) {    // #2
                        this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
                    }
                    else {
                        String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());    // #3
                        Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);    // #4
                        processImports(configClass, currentSourceClass, importSourceClasses, false);    //#5
                    }
                }
                else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {    // #6
                    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());    //#7
                }
                else {    
                    this.importStack.registerImport(
                            currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                    processConfigurationClass(candidate.asConfigClass(configClass));    //#8
                }
            }
        }
        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();
        }
    }
}

#1 @Import引入的类是ImportSelector实现类

#2 DeferredImportSelector接口需要延迟处理，加入到deferredImportSelectorHandler中

ConfigurationClassParser#parse方法最后会调用deferredImportSelectorHandler#process方法处理DeferredImportSelector接口

#3 调用ImportSelector#selectImports方法

#4 使用ImportSelector#selectImports返回Class Name数组，加载对应的SourceClass

#5 使用processImports方法继续处理这些SourceClass

#6 @Import引入的类是ImportBeanDefinitionRegistrar实现类

#7 将该类加入到ConfigurationClass#importBeanDefinitionRegistrars中，ConfigurationClassPostProcessor#processConfigBeanDefinitions方法 #5 步骤会处理

#8 @Import引入的类是其他类，转发为ConfigurationClass，使用processConfigurationClass方法处理

这里对应了@Import注解的三种用法，引入ImportSelector，ImportBeanDefinitionRegistrar或者具体的ConfigurationClass。

@Import最后都要processConfigurationClass处理它引入的ConfigurationClass

回到ConfigurationClassPostProcessor#processConfigBeanDefinitions方法 #5 步骤，

ConfigurationClassBeanDefinitionReader#loadBeanDefinitions -> loadBeanDefinitionsForConfigurationClass

private void loadBeanDefinitionsForConfigurationClass(
        ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {

    ...

    if (configClass.isImported()) {
        registerBeanDefinitionForImportedConfigurationClass(configClass);    // #1
    }
    for (BeanMethod beanMethod : configClass.getBeanMethods()) {
        loadBeanDefinitionsForBeanMethod(beanMethod);    // #2
    }

    loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());    // #3
    loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());    // #4
}

#1 注册ConfigurationClass自身的BeanDefinition

#2 注册@Bean注解标识方法引入的bean

#3 从@ImportResource引入的资源中读取BeanDefinition

#4 处理@Import引入的ImportBeanDefinitionRegistrar

@EnableAutoConfiguration注解引入的AutoConfigurationImportSelector，实现的是DeferredImportSelector接口

AutoConfigurationImportSelector#selectImports -> getAutoConfigurationEntry

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
        AnnotationMetadata annotationMetadata) {
    if (!isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    }
    AnnotationAttributes attributes = getAttributes(annotationMetadata);
    List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);    // #1
    configurations = removeDuplicates(configurations);    // #2
    Set<String> exclusions = getExclusions(annotationMetadata, attributes);    
    checkExcludedClasses(configurations, exclusions);
    configurations.removeAll(exclusions);    // #3
    configurations = filter(configurations, autoConfigurationMetadata);    // #4
    fireAutoConfigurationImportEvents(configurations, exclusions);    // #5
    return new AutoConfigurationEntry(configurations, exclusions);
}

#1 从spring.factories文件中获取@EnableAutoConfiguration对应的ConfigurationClass

#2 ConfigurationClass除重

#3 排除spring.autoconfigure.exclude配置的ConfigurationClass

#4 使用spring.factories中配置的AutoConfigurationImportFilter的实现类（OnBeanCondition，OnClassCondition，OnWebApplicationCondition）过滤部分ConfigurationClass，这里处理@ConditionalOnBean，@ConditionalOnClass，@ConditionalOnMissingClass等注解。

OnClassCondition可以判断当前Java环境中存在或者不存在某一个class，SpringBoot AutoConfigure功能可以实现当我们引入某个框架jar后，自动配置完成该框架的配置，正是通过该条件判断类实现。

来看一个例子，RedisAutoConfiguration

@Configuration
@ConditionalOnClass(RedisOperations.class)
@EnableConfigurationProperties(RedisProperties.class)
@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class })
public class RedisAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(name = "redisTemplate")
    public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory)
            throws UnknownHostException {
        RedisTemplate<Object, Object> template = new RedisTemplate<>();
        template.setConnectionFactory(redisConnectionFactory);
        return template;
    }
    ...
}

RedisAutoConfiguration是一个ConfigurationClass，他使用@Bean标识方法引入其他bean。

（RedisAutoConfiguration在spring-boot-autoconfigure这个jar下的spring.factories文件中已经被配置为@EnableAutoConfiguration的ConfigurationClass）

@ConditionalOnClass表明classpath只有存在RedisOperations这个类，RedisAutoConfiguration的配置才生效

（引入spring-data-redis的jar后有这个类了，RedisAutoConfiguration也就生效了）

@Import引入的 LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class用于与redis建立连接，并生成RedisConnectionFactory。

同样，引入 Lettuce相关jar后，LettuceConnectionConfiguration生效，引入Jedis相关jar后，JedisConnectionConfiguration生效。

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