@Configuration簡介
用於標識一個類為配置類，與xml配置效果類似
用法簡介
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");
}
}上面的例子應該是@Configuration最普遍一種使用場景了，在@Configuration class下面配置@Bean method，用於想Spring Ioc容器注入bean.但其實我們把AppConfig的@Configuration註解去掉，對應的Bean也是可以被注入到容器中去的。
那麼問題來了@Configuration到底有什麼作用？
我們給AppConfig改寫一下,如下：
@Configuration
@EnableAspectJAutoProxy
public class AppConfig {
@Bean
public A a(){
b();
return new A();
}
@Bean
public B b(){
return new B();
}再去執行TestApplication#main,那麼執行結果會是什麼樣呢？
Call A constructor
Call B constructor嗯哼？按照Java的語法，B的構造函數應該是被調用了兩次啊？為什麼只有輸出一句Call B constructor?
這其實就是@Configuration再發揮作用啦，不信你去掉@Configuration，再去運行一下，就會發現B的構造函數被執行了兩次。
官方給出了這樣一段解釋對於被@Configuration註解的類
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.
在一般情況下，@Bean method 是被聲明在@Configuration類中的，以確保 full mode總是被使用，並且跨方法的引用會被重定向到容器生命周期管理。
怎麼理解呢？
原來Spring將被@Configuration註解的配置類定義為full configuration, 而將沒有被@Configuration註解的配置類定義為lite configuration。full configuration能重定向從跨方法的引用，從而保證上述代碼中的b bean是一個單例.
源碼中是如何實現@Configuration語意
public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
/**
* 調用無參構造函數，實例化AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner
* 同時會調用父類GenericApplicationContext無參構造函數實例化一個關鍵的工廠DefaultListableBeanFactory
* 同時還會註冊一些開天闢地的後置處理器到beanDefinitionMap，這些後置處理器有bean工廠後置處理器;有bean後置處理器
*/
this();
//將componentClasses註冊到beanDefinitionMap集合中去
register(componentClasses);
refresh();
}跟蹤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)子類繼承，允許在這裏後置處理bean factory
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//按順序調用BeanFactoryPostProcessor,這裏的按順序僅實現了PriorityOrdered和Ordered的語意，未實現@Order註解的語意
//通過調用ConfigurationConfigPostProcessor#postProcessBeanDefinitionRegistry
//解析@Configuration配置類，將自定義的BeanFactoryPostProcessor、BeanPostProcessor註冊到beanDefinitionMap
//接着實例化所有（包括開天闢地）的BeanFactoryPostProcessor，然後再調用BeanFactoryPostProcessor#postProcessBeanFactory
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//按順序將BeanPostProcessor實例化成bean並註冊到beanFactory的beanPostProcessors,
//這裏的按順序僅實現了PriorityOrdered和Ordered的語意，未實現@Order註解的語意
//因為BeanPostProcessor要在普通bean初始化（）前後被調用，所以需要提前完成實例化並註冊到beanFactory的beanPostProcessors
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//註冊國際化相關的Bean
initMessageSource();
// Initialize event multicaster for this context.
//為上下文註冊應用事件廣播器（用於ApplicationEvent的廣播），如果有自定義則使用自定義的，如果沒有則內部實例化一個
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
//註冊所有（靜態、動態）的listener,並廣播earlyApplicationEvents
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//實例化用戶自定義的普通單例Bean（非開天闢地的、非後置處理器）
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();
}
}
}跟蹤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()));
}
}跟蹤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);
//此處調用ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry，
//解析配置類，為配置中的bean定義生成對應beanDefinition,並注入到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.
//調用ConfigurationClassPostProcessor#postProcessBeanFactory增強配置類(通過cglib生成增強類,load到jvm內存，
//設置beanDefinition的beanClass為增強類)
//為什麼要增強配置類？主要是為了讓@Bean生成的bean是單例,
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();
}跟蹤invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
private static void invokeBeanFactoryPostProcessors(
Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {
for (BeanFactoryPostProcessor postProcessor : postProcessors) {
postProcessor.postProcessBeanFactory(beanFactory);
}
}跟蹤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);
}
//為@Configuration註解的類生成增強類（如果有必要），並替換bd中的beanClass屬性，
enhanceConfigurationClasses(beanFactory);
beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}到了這一步謎底幾乎已經揭曉了，@Configuration class是通過增強來實現它的語義的。通過增強把跨方法的引用調用重定向到Spring生命周期管理.我們近一步探索下這個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方法中會標記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();
//為@Configuration註解的類生成增強類
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);
}
}
}看到有那麼一句話Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
很明顯了，使用cglib技術為config class生成一個enhancedClass，再通過beanDef.setBeanClass(enhancedClass);修改beanDefinition的BeanClass屬性，在bean實例化階段，會利用反射技術將beanClass屬性對應的類實例化出來，所以最終實例化出來的@Configuration bean是一個代理類的實例。這裏稍微提一下為什麼要使用cglib，而不是jdk動態代理，主要是因為jdk動態代理是基於接口的，而這裏AppConfig並沒有實現任何接口，所以必須用cglib技術。
總結
被@Configuration 註解的類，是 full configuration class，該類會被增強(通過cglib)，從而實現跨方法引用調用被重定向到Spring 生命周期管理，最終保證@Bean method生成的bean是一個單例。
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