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
@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。
如果您觉得本文不错，欢迎关注我的微信公众号，您的关注是我坚持的动力！
