SpringBoot源碼解析 -- AutoConfigure的實現原理

時間 2020-05-17

標籤 springboot 源碼解析 autoconfigure 實現原理欄目 Spring 简体版

原文原文鏈接

源碼分析基於spring boot 2.1java

本文經過閱讀源碼，分析SpringBoot AutoConfigure功能的實現原理。
（源碼解析類文章建議在PC端閱讀）redis

SpringBoot中使用@EnableAutoConfiguration註解啓動AutoConfigure功能spring

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

這裏起做用的其實是@Import和AutoConfigurationImportSelector。
@Import註解很是重要，它是SpringBoot中AutoConfiguration功能的基礎。數組

前面解析SpringBoot啓動過程的文章說過，SpringBoot啓動時會注入ConfigurationClassPostProcessor，該PostProcessor正是處理@Import的類。微信

ConfigurationClassPostProcessor#postProcessBeanFactory -> ConfigurationClassPostProcessor#processConfigBeanDefinitions框架

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 檢查bean是否爲ConfigurationClass，這裏主要是檢查class註解信息（spring中將@Configuration標註的類歸類爲ConfigurationClass）
#2 構建ConfigurationClassParser
#3 解析ConfigurationClass
#4 獲取結果，注意ConfigurationClassParser#getConfigurationClasses方法獲取ConfigurationClassParser的處理結果
#5 獲取ConfigurationClass引入的Class，將其轉化爲BeanDefinition，並註冊到Spring上下文
最後構造bean，是在AbstractApplicationContext#refresh方法中，調用finishBeanFactoryInitialization，構建熱加載的單例bean時完成。
#6 獲取新的BeanDefinition列表
#7 若是前面的ConfigurationClass有引入了新的ConfigurationClass，添加到待處理集合
#8 循環處理，直到待處理集合爲空spring-boot

ConfigurationClassParser#parse -> ConfigurationClassParser#processConfigurationClass源碼分析

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 檢查ConfigurationClass是否存在@Conditional註解，若是存在，取註解中Condition條件判斷類進行判斷
#2 將ConfigurationClass轉化爲SourceClass
SourceClass對Class元數據進行封裝，能夠兼容處理JVM加載的Class和ASM讀取的元數據，獲取元數據中註解，方法等信息
#3 doProcessConfigurationClass方法很關鍵，處理@Component，@PropertySources，@ComponentScans，@Import，@ImportResource，帶@Bean的方法，接口及父類。
#4 若是ConfigurationClass存在父類，doProcessConfigurationClass返回父類，這裏遞歸處理父類數據
#5 將該ConfigurationClass加入configurationClasses，以便ConfigurationClassPostProcessor#processConfigBeanDefinitions方法#5步驟使用
該方法是處理ConfigurationClass的入口，doProcessConfigurationClass中引入了新的ConfigurationClass，也會調用該方法處理。post

ConfigurationClassParser#doProcessConfigurationClassui

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;
}

這裏只關注該方法AutoConfigure功能相關的代碼
#1 若是Class存在@Component註解，會查詢Class的內部類，若是內部類也是ConfigurationClass，會調用processConfigurationClass方法處理內部類（注意，@Configuration註解上標識了@Component註解）
#2 處理@Import註解
#3 處理@ImportResource，添加引入資源信息到ConfigurationClass#importedResources，ConfigurationClassPostProcessor#processConfigBeanDefinitions方法#5步驟會處理
#4 選擇Class中存在@Bean標註的方法，加入到ConfigurationClass#beanMethods中
#5 選擇接口中存在@Bean標註的方法，一樣加入到ConfigurationClass#beanMethods中
#6 若是存在父類，返回父類到ConfigurationClassParser#processConfigurationClass中，遞歸處理父類。

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引入的類是ImportSelector實現類
#2 DeferredImportSelector接口須要延遲處理，加入到deferredImportSelectorHandler中
ConfigurationClassParser#parse方法最後會調用deferredImportSelectorHandler#process方法處理DeferredImportSelector接口
#3 調用ImportSelector#selectImports方法
#4 使用ImportSelector#selectImports返回Class Name數組，加載對應的SourceClass
#5 使用processImports方法繼續處理這些SourceClass
#6 @Import引入的類是ImportBeanDefinitionRegistrar實現類
#7 將該類加入到ConfigurationClass#importBeanDefinitionRegistrars中，ConfigurationClassPostProcessor#processConfigBeanDefinitions方法#5步驟會處理
#8 @Import引入的類是其餘類，轉發爲ConfigurationClass，使用processConfigurationClass方法處理

這裏對應了@Import註解的三種用法，引入ImportSelector，ImportBeanDefinitionRegistrar或者具體的ConfigurationClass。

@Import最後都要processConfigurationClass處理它引入的ConfigurationClass

回到ConfigurationClassPostProcessor#processConfigBeanDefinitions方法#5步驟，
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 註冊ConfigurationClass自身的BeanDefinition
#2 註冊@Bean註解標識方法引入的bean
#3 從@ImportResource引入的資源中讀取BeanDefinition
#4 處理@Import引入的ImportBeanDefinitionRegistrar

@EnableAutoConfiguration註解引入的AutoConfigurationImportSelector，實現的是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 從spring.factories文件中獲取@EnableAutoConfiguration對應的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的實現類（OnBeanCondition，OnClassCondition，OnWebApplicationCondition）過濾部分ConfigurationClass，這裏處理@ConditionalOnBean，@ConditionalOnClass，@ConditionalOnMissingClass等註解。
OnClassCondition能夠判斷當前Java環境中存在或者不存在某一個class，SpringBoot AutoConfigure功能能夠實現當咱們引入某個框架jar後，自動配置完成該框架的配置，正是經過該條件判斷類實現。

來看一個例子，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是一個ConfigurationClass，他使用@Bean標識方法引入其餘bean。
（RedisAutoConfiguration在spring-boot-autoconfigure這個jar下的spring.factories文件中已經被配置爲@EnableAutoConfiguration的ConfigurationClass）
@ConditionalOnClass代表classpath只有存在RedisOperations這個類，RedisAutoConfiguration的配置才生效
（引入spring-data-redis的jar後有這個類了，RedisAutoConfiguration也就生效了）
@Import引入的 LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class用於與redis創建鏈接，並生成RedisConnectionFactory。
一樣，引入 Lettuce相關jar後，LettuceConnectionConfiguration生效，引入Jedis相關jar後，JedisConnectionConfiguration生效。

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