@Configuration简介
用于标识一个类为配置类，与xml配置效果类似
用法简介
public class TestApplication {
public static void main(String args[]) {
AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(AppConfig.class);
}
}
@Configuration
public class AppConfig {
@Bean
public A a(){
return new A();
}
@Bean
public B b(){
return new B();
}
}
public class A {
public A(){
System.out.println("Call A constructor");
}
}
public class B {
public B(){
System.out.println("Call B constructor");
}
}上面的例子应该是@Configuration最普遍一种使用场景了，在@Configuration class下面配置@Bean method，用于想Spring Ioc容器注入bean.但其实我们把AppConfig的@Configuration注解去掉，对应的Bean也是可以被注入到容器中去的。
那么问题来了@Configuration到底有什么作用？
我们给AppConfig改写一下,如下：
@Configuration
@EnableAspectJAutoProxy
public class AppConfig {
@Bean
public A a(){
b();
return new A();
}
@Bean
public B b(){
return new B();
}再去执行TestApplication#main,那么执行结果会是什么样呢？
Call A constructor
Call B constructor嗯哼？按照Java的语法，B的构造函数应该是被调用了两次啊？为什么只有输出一句Call B constructor?
这其实就是@Configuration再发挥作用啦，不信你去掉@Configuration，再去运行一下，就会发现B的构造函数被执行了两次。
官方给出了这样一段解释对于被@Configuration注解的类
In common scenarios,@Beanmethods are to be declared within@Configurationclasses, ensuring that “full” mode is always used and that cross-method references therefore get redirected to the container’s lifecycle management.
在一般情况下，@Bean method 是被声明在@Configuration类中的，以确保 full mode总是被使用，并且跨方法的引用会被重定向到容器生命周期管理。
怎么理解呢？
原来Spring将被@Configuration注解的配置类定义为full configuration, 而将没有被@Configuration注解的配置类定义为lite configuration。full configuration能重定向从跨方法的引用，从而保证上述代码中的b bean是一个单例.
源码中是如何实现@Configuration语意
public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
/**
* 调用无参构造函数，实例化AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner
* 同时会调用父类GenericApplicationContext无参构造函数实例化一个关键的工厂DefaultListableBeanFactory
* 同时还会注册一些开天辟地的后置处理器到beanDefinitionMap，这些后置处理器有bean工厂后置处理器;有bean后置处理器
*/
this();
//将componentClasses注册到beanDefinitionMap集合中去
register(componentClasses);
refresh();
}跟踪refresh()
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
//供上下文(Context)子类继承，允许在这里后置处理bean factory
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//按顺序调用BeanFactoryPostProcessor,这里的按顺序仅实现了PriorityOrdered和Ordered的语意，未实现@Order注解的语意
//通过调用ConfigurationConfigPostProcessor#postProcessBeanDefinitionRegistry
//解析@Configuration配置类，将自定义的BeanFactoryPostProcessor、BeanPostProcessor注册到beanDefinitionMap
//接着实例化所有（包括开天辟地）的BeanFactoryPostProcessor，然后再调用BeanFactoryPostProcessor#postProcessBeanFactory
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//按顺序将BeanPostProcessor实例化成bean并注册到beanFactory的beanPostProcessors,
//这里的按顺序仅实现了PriorityOrdered和Ordered的语意，未实现@Order注解的语意
//因为BeanPostProcessor要在普通bean初始化（）前后被调用，所以需要提前完成实例化并注册到beanFactory的beanPostProcessors
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//注册国际化相关的Bean
initMessageSource();
// Initialize event multicaster for this context.
//为上下文注册应用事件广播器（用于ApplicationEvent的广播），如果有自定义则使用自定义的，如果没有则内部实例化一个
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
//注册所有（静态、动态）的listener,并广播earlyApplicationEvents
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//实例化用户自定义的普通单例Bean（非开天辟地的、非后置处理器）
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}跟踪invokeBeanFactoryPostProcessors(beanFactory)
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
}
}跟踪invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<>();
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
BeanDefinitionRegistryPostProcessor registryProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
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<>();
// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
//此处调用ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry，
//解析配置类，为配置中的bean定义生成对应beanDefinition,并注入到registry的beanDefinitionMap
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
boolean reiterate = true;
while (reiterate) {
reiterate = false;
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);
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
}
// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
//调用ConfigurationClassPostProcessor#postProcessBeanFactory增强配置类(通过cglib生成增强类,load到jvm内存，
//设置beanDefinition的beanClass为增强类)
//为什么要增强配置类？主要是为了让@Bean生成的bean是单例,
invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
}
else {
// Invoke factory processors registered with the context instance.
invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
}
// 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)) {
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// 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);
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// 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);
// Clear cached merged bean definitions since the post-processors might have
// modified the original metadata, e.g. replacing placeholders in values...
beanFactory.clearMetadataCache();
}跟踪invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
private static void invokeBeanFactoryPostProcessors(
Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
for (BeanFactoryPostProcessor postProcessor : postProcessors) {
postProcessor.postProcessBeanFactory(beanFactory);
}
}跟踪ConfigurationClassPostProcessor#postProcessBeanFactory
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
int factoryId = System.identityHashCode(beanFactory);
if (this.factoriesPostProcessed.contains(factoryId)) {
throw new IllegalStateException(
"postProcessBeanFactory already called on this post-processor against " + beanFactory);
}
this.factoriesPostProcessed.add(factoryId);
if (!this.registriesPostProcessed.contains(factoryId)) {
// BeanDefinitionRegistryPostProcessor hook apparently not supported...
// Simply call processConfigurationClasses lazily at this point then.
processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
}
//为@Configuration注解的类生成增强类（如果有必要），并替换bd中的beanClass属性，
enhanceConfigurationClasses(beanFactory);
beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}到了这一步谜底几乎已经揭晓了，@Configuration class是通过增强来实现它的语义的。通过增强把跨方法的引用调用重定向到Spring生命周期管理.我们近一步探索下这个enhanceConfigurationClasses方法
public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
for (String beanName : beanFactory.getBeanDefinitionNames()) {
BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
Object configClassAttr = beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE);
MethodMetadata methodMetadata = null;
if (beanDef instanceof AnnotatedBeanDefinition) {
methodMetadata = ((AnnotatedBeanDefinition) beanDef).getFactoryMethodMetadata();
}
if ((configClassAttr != null || methodMetadata != null) && beanDef instanceof AbstractBeanDefinition) {
// Configuration class (full or lite) or a configuration-derived @Bean method
// -> resolve bean class at this point...
AbstractBeanDefinition abd = (AbstractBeanDefinition) beanDef;
if (!abd.hasBeanClass()) {
try {
abd.resolveBeanClass(this.beanClassLoader);
}
catch (Throwable ex) {
throw new IllegalStateException(
"Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
}
}
}
//在ConfigurationClassUtils.checkConfigurationClassCandidate方法中会标记Configuration is full or lite
if (ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(configClassAttr)) {
if (!(beanDef instanceof AbstractBeanDefinition)) {
throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
}
else if (logger.isInfoEnabled() && beanFactory.containsSingleton(beanName)) {
logger.info("Cannot enhance @Configuration bean definition '" + beanName +
"' since its singleton instance has been created too early. The typical cause " +
"is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
"return type: Consider declaring such methods as 'static'.");
}
configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
}
}
if (configBeanDefs.isEmpty()) {
// nothing to enhance -> return immediately
return;
}
ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
for (Map.Entry<String, AbstractBeanDefinition> entry : configBeanDefs.entrySet()) {
AbstractBeanDefinition beanDef = entry.getValue();
// If a @Configuration class gets proxied, always proxy the target class
beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);
// Set enhanced subclass of the user-specified bean class
Class<?> configClass = beanDef.getBeanClass();
//为@Configuration注解的类生成增强类
Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
if (configClass != enhancedClass) {
if (logger.isTraceEnabled()) {
logger.trace(String.format("Replacing bean definition '%s' existing class '%s' with " +
"enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
}
beanDef.setBeanClass(enhancedClass);
}
}
}看到有那么一句话Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
很明显了，使用cglib技术为config class生成一个enhancedClass，再通过beanDef.setBeanClass(enhancedClass);修改beanDefinition的BeanClass属性，在bean实例化阶段，会利用反射技术将beanClass属性对应的类实例化出来，所以最终实例化出来的@Configuration bean是一个代理类的实例。这里稍微提一下为什么要使用cglib，而不是jdk动态代理，主要是因为jdk动态代理是基于接口的，而这里AppConfig并没有实现任何接口，所以必须用cglib技术。
总结
被@Configuration 注解的类，是 full configuration class，该类会被增强(通过cglib)，从而实现跨方法引用调用被重定向到Spring 生命周期管理，最终保证@Bean method生成的bean是一个单例。
