SpringBoot源码解析 -- AutoConfigure的实现原理

SpringBoot深入理解 -- @AliasFor注解的作用
SpringBoot源码解析 -- SpringBoot启动过程
SpringBoot源码解析 -- AutoConfigure的实现原理
SpringBoot源码解析 -- @ComponentScan的实现原理
SpringBoot源码解析 -- @Value,@Autowired实现原理
SpringBoot源码解析 -- Tomcat,SpringMVC启动
SpringBoot源码解析 -- Logging,Environment启动

源码分析基于spring boot 2.1

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,调用ImportBeanDefinitionRegistrar#registerBeanDefinitions方法

@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
#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应用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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