Spring注解扫描原理浅析
Spring注解扫描原理浅析
一、概述
本篇文章将会带着大家通过阅读源码的方式去揭秘Spring的注解扫描原理，如果你想让源码可编辑，那么可以把源码下载下来然后进行调试，关于编译Spring源码，我推荐这篇文章——(spring源码系列（六）——番外篇如何编译spring的源码。
很多人一提到源码都会比较排除，可能是因为注释都是英文，又或者类名或方法名太长记不住等原因。本篇文章会手把手带你看源码，中间过程不重要，很多代码不需要过多关注，只需要对重点代码片段和方法进行关注即可。废话不多说，我们这就开始吧！
二、测试代码
public static void main(String[] args) {
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
// 这里可以注册一些配置类
// applicationContext.register(MyConfig.class);
applicationContext.refresh();
}
三、源码探究
1、ConfigurationClassPostProcessor 的注册
点进AnnotationConfigApplicationContext的空构造函数，可以看到下面的代码
public AnnotationConfigApplicationContext() {
// new了一个Reader
this.reader = new AnnotatedBeanDefinitionReader(this);
// new了个Scanner
this.scanner = new ClassPathBeanDefinitionScanner(this);
}
进入new AnnotatedBeanDefinitionReader(this)
public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
this(registry, getOrCreateEnvironment(registry));
}
进入this(registry, getOrCreateEnvironment(registry))
public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
Assert.notNull(environment, "Environment must not be null");
this.registry = registry;
this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
进入AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry)方法
public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
registerAnnotationConfigProcessors(registry, null);
}
继续往里走，就回到达我们的目的地
public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
BeanDefinitionRegistry registry, @Nullable Object source) {
// 省略...
if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
// 创建ConfigurationClassPostProcessor.class对应的BeanDefinition
RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
def.setSource(source);
// 注册BeanDefinition
beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
}
// 省略...
}
正如你所见，在这个方法中，Spring往容器中事先注册了许多的Bean，其中ConfigurationClassPostProcessor这个Bean是和我们注解的扫描息息相关的
2、ConfigurationClassPostProcessor 的功能
public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
// 省略...
}
进入ConfigurationClassPostProcessor这个类可以很清晰的看到，它实现了一个BeanDefinitionRegistryPostProcessor接口，接着我们进入到这个接口中去
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;
}
通过代码可以看到它是接口BeanFactoryPostProcessor的子类
public interface BeanFactoryPostProcessor {
void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;
}
其这个接口就只有一个方法。请记住现在这个方法，一会儿后面会解释为什么这个类那么重要
3、refresh() —— 启动方法
现在，进入到之前测试代码中的refresh()方法中，这个方法是用于启动容器的，现在我们来看一下它的总体流程
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 省略...
try {
// 调用一系列的BeanFactoryPostProcessor对Bean工厂进行后置处理
// 实例化并调用所有已注册的 BeanFactoryPostProcessor bean
// 会先执行所有 BeanDefinitionRegistryPostProcessors 类中的方法，这个是BeanFactoryPostProcessor的子类
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// 省略...
}
// 省略...
}
这个方法中，我们只需要关注invokeBeanFactoryPostProcessors这一个方法，接下来我们进入其中一探究竟
/**
* Instantiate and invoke all registered BeanFactoryPostProcessor beans,
* respecting explicit order if given.
* Must be called before singleton instantiation.
*/
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
// 省略...
}
通过翻译注释，我们可以知道，这个方法会注册所有实现了BeanFactoryPostProcessor接口的bean，接着继续深入。下面的方法可能会比较长，但是就一个
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// 保存了已经执行过的BeanDefinitionRegistryPostProcessor
// 目的是防止重复执行
// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<>();
// 如果实现了BeanDefinitionRegistry
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
// 存放所有找出来的BeanFactoryPostProcessor（父类）
List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
// 找出所有BeanDefinitionRegistryPostProcessor（子类）
List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
// 遍历用户通过API往容器添加的BeanFactoryPostProcessor对象
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
BeanDefinitionRegistryPostProcessor registryProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
// 如果是子类就直接执行postProcessBeanDefinitionRegistryf方法
registryProcessor.postProcessBeanDefinitionRegistry(registry);
registryProcessors.add(registryProcessor);
}
else {
regularPostProcessors.add(postProcessor);
}
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
// Separate between BeanDefinitionRegistryPostProcessors that implement
// PriorityOrdered, Ordered, and the rest.
List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();
// 通过翻译注释可以知道，此处会调用实现了PriorityOrdered接口的BeanDefinitionRegistryPostProcessors,通过前面的代码可以知道，
// Spring内置的ConfigurationClassPostProcessor是实现了的，并且只有它实现了，所以此处的size = 1
// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
// 实现联动PriorityOrdered接口最优先执行
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// 实例化，并放入单例池中
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
// 对实现了PriorityOrdered的BeanDefinitionRegistryPostProcessors进行排序
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
// 先调用实现了PriorityOrdered接口的BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法
// 在此处调用了接口中的postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// 这里需要再次去容器中获取是因为在上一步过后，容器中可能注册了新的BeanDefinitionRegistryPostProcessors
// 现在调用的是实现了Ordered接口的容器中可能注册了新的BeanDefinitionRegistryPostProcessors
// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
// Ordered仅次于POrdered执行
if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
// 不难看出执行顺序是PriorityOrdered -> Order -> 普通的，即使前面又注册了新的实现了PriorityOrdered的，在排序后，PriorityOrdered一定会优先执行
// 每次执行都会优先执行实现了POrder接口的
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
// 再调用所实现了Ordered接口的BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
boolean reiterate = true;
// 因为可能会出现Registry注册Registry这样的套娃情况，所以用了while-true
while (reiterate) {
reiterate = false;
// 如果容器中还有BeanDefinitionRegistryPostProcessor就再循环一边
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
reiterate = true;
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
// 最后调用再调用两个几口都没实现的BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
}
// 调用BeanDefinitionRegistryPostProcessor的postProcessBeanFactory方法，因为这个Registry接口继承了BeanFactoryPostProcessor
// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
// 先调用子类的
invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
// 再调用父类的（API提供的）
invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
}
else {
// 调用BeanFactoryPostProcessor接口的postProcessBeanFactory(beanFactory)方法
// Invoke factory processors registered with the context instance.
invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
}
// 执行扫描出来的（父类）
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
List<String> orderedPostProcessorNames = new ArrayList<>();
List<String> nonOrderedPostProcessorNames = new ArrayList<>();
for (String ppName : postProcessorNames) {
if (processedBeans.contains(ppName)) {
// skip - already processed in first phase above
}
else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
// getBean会进行实例化
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
}
// 后面只保存了Bean的名字，并没有先实例化，可能是因为BD会被上面实现了PriorityOrdered的BD修改
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// 最先调用实现了PriorityOrdered的BeanFactoryPostProcessors中的postProcessBeanFactory方法
// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
for (String postProcessorName : orderedPostProcessorNames) {
orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
sortPostProcessors(orderedPostProcessors, beanFactory);
// 再调用实现了Ordered接口的BeanFactoryPostProcessor中的postProcessBeanFactory方法
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// 最后才调用普通的BeanFactoryPostProcessor的方法
// Finally, invoke all other BeanFactoryPostProcessors.
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
for (String postProcessorName : nonOrderedPostProcessorNames) {
nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
// 因为上面的BeanFactoryPostProcessor都执行完之后可能会对一些BeanDefinition进行了修改操作，因此讲MergedBD都标记为已已过期的
// Clear cached merged bean definitions since the post-processors might have
// modified the original metadata, e.g. replacing placeholders in values...
beanFactory.clearMetadataCache();
}
**小结：**总结一下这个方法，首先会根据PriorityOrdered -> Ordered-> 没实现这两个接口 这样的顺序去调用所有BeanDefinitionRegistryPostProcessor的postProcessBeanDefinitionRegistry方法，最后会调用所有BeanFactoryPostProcessor的postProcessBeanFactory方法。换句话说，上面提到的ConfigurationClassPostProcessor因为实现了这两个接口，所以这两个方法都会被调用。接下来我们去看看这个接口里面都是一些什么内容吧！
4、ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法
public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
@Override
public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
// 省略...
processConfigBeanDefinitions(registry);
}
}
进入processConfigBeanDefinitions方法
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
// 省略...
do {
// 通过pase方法解析配置类，其实也就是在这个方法中去做的Spring的扫描
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
candidates.clear();
if (registry.getBeanDefinitionCount() > candidateNames.length) {
String[] newCandidateNames = registry.getBeanDefinitionNames();
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) {
// 找出所有没有被解析过的配置类
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));
}
}
}
candidateNames = newCandidateNames;
}
}
while (!candidates.isEmpty());
}
接着进入parse方法
public void parse(Set<BeanDefinitionHolder> configCandidates) {
// 省略...
// 解析通过注解得到的Bean
parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
// 省略...
}
进入到解析AnnotatedBeanDefinition的parse方法中，然后再进入processConfigurationClass方法
protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
return;
}
// 用ConfigurationClass保存配置类种的信息
ConfigurationClass existingClass = this.configurationClasses.get(configClass);
if (existingClass != null) {
if (configClass.isImported()) {
if (existingClass.isImported()) {
existingClass.mergeImportedBy(configClass);
}
// Otherwise ignore new imported config class; existing non-imported class overrides it.
return;
}
else {
// Explicit bean definition found, probably replacing an import.
// Let's remove the old one and go with the new one.
this.configurationClasses.remove(configClass);
this.knownSuperclasses.values().removeIf(configClass::equals);
}
}
// 在此处递归地处理配置类及其超类层次结构
// Recursively process the configuration class and its superclass hierarchy.
SourceClass sourceClass = asSourceClass(configClass, filter);
do {
// doProcessConfigurationClass中会解析配置类上的注解
sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
}
while (sourceClass != null);
this.configurationClasses.put(configClass, configClass);
}
接着进入到进入到doProcessConfigurationClass中就会看到核心代码了
protected final SourceClass doProcessConfigurationClass(
ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
throws IOException {
// 如果加了@Component注解就会首先递归地处理内部类
if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
// Recursively process any member (nested) classes first
processMemberClasses(configClass, sourceClass, filter);
}
// 解析@PropertySource注解
// Process any @PropertySource annotations
for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), PropertySources.class,
org.springframework.context.annotation.PropertySource.class)) {
if (this.environment instanceof ConfigurableEnvironment) {
// 处理注解中的内容，并抽象成PropertySource对象
processPropertySource(propertySource);
}
else {
logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
"]. Reason: Environment must implement ConfigurableEnvironment");
}
}
// 开始解析@ComponentScans和@ComponentScan注解
// Process any @ComponentScan annotations
Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
if (!componentScans.isEmpty() &&
!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
for (AnnotationAttributes componentScan : componentScans) {
// The config class is annotated with @ComponentScan -> perform the scan immediately
Set<BeanDefinitionHolder> scannedBeanDefinitions =
this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
// Check the set of scanned definitions for any further config classes and parse recursively if needed
for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
if (bdCand == null) {
bdCand = holder.getBeanDefinition();
}
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
parse(bdCand.getBeanClassName(), holder.getBeanName());
}
}
}
}
// 解析@Import注解
// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), filter, true);
// 解析@ImportResource注解
// Process any @ImportResource annotations
AnnotationAttributes importResource =
AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
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);
}
}
// 解析加了@Bean的方法
// Process individual @Bean methods
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
// 处理接口
// Process default methods on interfaces
processInterfaces(configClass, sourceClass);
// 处理父类
// Process superclass, if any
if (sourceClass.getMetadata().hasSuperClass()) {
String superclass = sourceClass.getMetadata().getSuperClassName();
if (superclass != null && !superclass.startsWith("java") &&
!this.knownSuperclasses.containsKey(superclass)) {
this.knownSuperclasses.put(superclass, configClass);
// Superclass found, return its annotation metadata and recurse
return sourceClass.getSuperClass();
}
}
// No superclass -> processing is complete
return null;
}
从上面的代码中不难看出，注解解析的顺序是 @Component —> @PropertySource —> @ComponentScans和@ComponentScan —> @Import —> @ImportResource —> @Bean —> 处理接口上的注解 —> 处理父类的注解，对于一些可能引入新的Bean的注解，内部还会调用上面提到过的processConfigBeanDefinitions方法，然后又会进入到这里来，很明显，这是一个递归的过程，因为配置类可能会引入新的配置类。
四、小结
Spring容器通过refresh()方法启动，在启动过程中，会最先执行实现了BeanDefinitionRegistryPostProcessor接口和PriorityOrdered接口的类的postProcessBeanDefinitionRegistry方法，而满足上面这个条件的就只有ConfigurationClassPostProcessor这一个类。
他会去解析所有的配置类，并且解析配置类的方法是递归调用的。因源码过于庞大，所有本篇文章只是带大家简单看了一下Spring注解扫描的原理，对某个注解的解析原理大家可以根据自己想法去方法内部查看。
如果以上内容有问题，欢迎评论指出