Spring的@Configuration使用与原理
@Configuration的使用
@Configuration是一个被@Component注解修饰的注解，先来看一个现象：
package com.morris.spring.config;
import com.morris.spring.entity.Message;
import com.morris.spring.entity.Order;
import com.morris.spring.entity.User;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;
//@Component
@Configuration
public class UserConfig {
@Bean
public User user() {
System.out.println("create user");
return new User();
}
@Bean
public Message message() {
return new Message(user());
}
}现象：
- UserConfig类上面使用@Configuration，user()只会被调用一次。
- UserConfig类上面使用@Component，user()会被调用两次，也就是会创建两个user对象，破坏了Spring中Bean的单例。
为什么会出现上面这种现象呢？初步猜测使用@Configuration会使用动态代理，下面从源码来分析原理。
@Configuration的源码分析
标识@Configuration为full
在BeanDefinition会有一个属性来标识是否有@Configuration注解：
org.springframework.context.annotation.ConfigurationClassUtils#checkConfigurationClassCandidate
public static boolean checkConfigurationClassCandidate(
BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {
... ...
Map<String, Object> config = metadata.getAnnotationAttributes(Configuration.class.getName());
if (config != null && !Boolean.FALSE.equals(config.get("proxyBeanMethods"))) {
// 这里对@Configuration做了特殊处理，往BD中设置了属性full
beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);
}
else if (config != null || isConfigurationCandidate(metadata)) {
// 带有@Component、@ComponentScan、@Import、@ImportResource、@Bean注解的BD，设置属性lite
beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
}
else {
return false;
}
// It's a full or lite configuration candidate... Let's determine the order value, if any.
Integer order = getOrder(metadata);
if (order != null) {
beanDef.setAttribute(ORDER_ATTRIBUTE, order);
}
return true;
}@Bean注解的收集
org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass
...
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
confiGClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
...将@Bean的方法封装为MethodMetadata对象放入到ConfigClass中。
解析BeanMethod注入BeanDefinition
org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader#loadBeanDefinitionsForBeanMethod
private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
ConfigurationClass configClass = beanMethod.getConfigurationClass();
MethodMetadata metadata = beanMethod.getMetadata();
String methodName = metadata.getMethodName();
... ...
if (metadata.isStatic()) {
// static @Bean method
// 静态 @Bean method
if (configClass.getMetadata() instanceof StandardAnnotationMetadata) {
beanDef.setBeanClass(((StandardAnnotationMetadata) configClass.getMetadata()).getIntrospectedClass());
}
else {
beanDef.setBeanClassName(configClass.getMetadata().getClassName());
}
beanDef.setUniqueFactoryMethodName(methodName);
}
else {
// 实例 @Bean method
// instance @Bean method
beanDef.setFactoryBeanName(configClass.getBeanName());
beanDef.setUniqueFactoryMethodName(methodName);
}
... ...
this.reGIStry.registerBeanDefinition(beanName, beanDefToRegister);
}从上面的源码可以发现@Bean注解注入的BeanDefinition都会有个FactoryMethodName，也就是后面实例化时将会使用factory-method流程进行实例化。
替换BeanClass为CGlib代理类
org.springframework.context.annotation.ConfigurationClassPostProcessor#postProcessBeanFactory
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);
}
// 对@Configuration注解的类做增强，CGlib代理
enhanceConfigurationClasses(beanFactory);
beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}
public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
// 收集所有带有@Configuration的BD，也就是有个attribute为full的属性
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);
}
}
}
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();
// 遍历所有带有@Configuration的BD
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();
// 生成CGlib代理类
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);
}
}
}org.springframework.context.annotation.ConfigurationClassEnhancer#enhance
public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
if (EnhancedConfiguration.class.isAssignableFrom(configClass)) {
if (logger.isDebugEnabled()) {
logger.debug(String.format("Ignoring request to enhance %s as it has " +
"already been enhanced. This usually indicates that more than one " +
"ConfigurationClassPostProcessor has been registered (e.g. via " +
"<context:annotation-config>). This is harmless, but you may " +
"want check your configuration and remove one CCPP if possible",
configClass.getName()));
}
return configClass;
}
// 创建代理类
Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));
if (logger.isTraceEnabled()) {
logger.trace(String.format("Successfully enhanced %s; enhanced class name is: %s",
configClass.getName(), enhancedClass.getName()));
}
return enhancedClass;
}
private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(configSuperClass);
enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});
enhancer.setUseFactory(false);
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));
// 这里会注入几个Callback
enhancer.setCallbackFilter(CALLBACK_FILTER);
enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());
return enhancer;
}
private Class<?> createClass(Enhancer enhancer) {
Class<?> subclass = enhancer.createClass();
// Registering callbacks statically (as opposed to thread-local)
// is critical for usage in an OSGi environment (SPR-5932)...
Enhancer.registerStaticCallbacks(subclass, CALLBACKS);
return subclass;
}
private static final Callback[] CALLBACKS = new Callback[] {
new BeanMethodInterceptor(),
// 重点
new BeanFactoryAwareMethodInterceptor(),
NoOp.INSTANCE
};
private static final ConditionalCallbackFilter CALLBACK_FILTER = new ConditionalCallbackFilter(CALLBACKS);上面一堆代码就是为了给@Configuration注解的类生成CGlib代理类，所以关键就是这个类长什么样，是怎么做增强的，重点就在上面的BeanMethodInterceptor中。
@Bean方法的调用
org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#intercept
public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,
MethodProxy cglibMethodProxy) throws Throwable {
ConfigurableBeanFactory beanFactory = getBeanFactory(enhancedConfigInstance);
String beanName = BeanAnnotationHelper.determineBeanNameFor(beanMethod);
// Determine whether this bean is a scoped-proxy
if (BeanAnnotationHelper.isScopedProxy(beanMethod)) {
String scopedBeanName = ScopedProxyCreator.getTargetBeanName(beanName);
if (beanFactory.isCurrentlyInCreation(scopedBeanName)) {
beanName = scopedBeanName;
}
}
// To handle the case of an inter-bean method reference, we must explicitly check the
// container for already cached instances.
// First, check to see if the requested bean is a FactoryBean. If so, create a subclass
// proxy that intercepts calls to getObject() and returns any cached bean instance.
// This ensures that the semantics of calling a FactoryBean from within @Bean methods
// is the same as that of referring to a FactoryBean within XML. See SPR-6602.
if (factoryContainsBean(beanFactory, BeanFactory.FACTORY_BEAN_PREFIX + beanName) &&
factoryContainsBean(beanFactory, beanName)) {
Object factoryBean = beanFactory.getBean(BeanFactory.FACTORY_BEAN_PREFIX + beanName);
if (factoryBean instanceof ScopedProxyFactoryBean) {
// Scoped proxy factory beans are a special case and should not be further proxied
}
else {
// It is a candidate FactoryBean - Go ahead with enhancement
return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);
}
}
if (isCurrentlyInvokedFactoryMethod(beanMethod)) {
// The factory is calling the bean method in order to instantiate and register the bean
// (i.e. via a getBean() call) -> invoke the super implementation of the method to actually
// create the bean instance.
if (logger.isInfoEnabled() &&
BeanFactoryPostProcessor.class.isAssignableFrom(beanMethod.getReturnType())) {
logger.info(String.format("@Bean method %s.%s is non-static and returns an object " +
"assignable to Spring's BeanFactoryPostProcessor interface. This will " +
"result in a failure to process annotations such as @Autowired, " +
"@Resource and @PostConstruct within the method's declaring " +
"@Configuration class. Add the 'static' modifier to this method to avoid " +
"these container lifecycle issues; see @Bean javadoc for complete details.",
beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName()));
}
// 第一次调用走这
return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
}
// 第二次调用走这
return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
}isCurrentlyInvokedFactoryMethod何时返回true？
private boolean isCurrentlyInvokedFactoryMethod(Method method) {
// SimpleInstantiationStrategy这个值哪里设置进去的
Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();
return (currentlyInvoked != null && method.getName().equals(currentlyInvoked.getName()) &&
Arrays.equals(method.getParameterTypes(), currentlyInvoked.getParameterTypes()));
}第一次调用发生在Spring容器启动时实例化user对象，@Bean注解注入的对象会通过factory-method进行实例化，此处是UserConfig.user()，调用此方法前会往ThreadLocal中放入当前的方法：
org.springframework.beans.factory.support.SimpleInstantiationStrategy#instantiate
...
try {
// 放入ThreadLocal，@Bean走cglib代理会使用这个ThreadLocal
currentlyInvokedFactoryMethod.set(factoryMethod);
Object result = factoryMethod.invoke(factoryBean, args);
if (result == null) {
result = new NullBean();
}
return result;
}
...然后再反射调用factoryMethod，此时调用的是代理对象，进入BeanMethodInterceptor#intercept，然后判断当前ThreadLocal中有FactoryMethod，就会调用父类也就是目标类的目标方法，此处是UserConfig.user()创建对象。
第二次进入BeanMethodInterceptor#intercept时，是实例化Message，此时虽然ThreadLocal中有FactoryMethod，但是这个Method是message()，而不是user()，所以会调用下面的方法从BeanFactory中获得对象，而不是调用FactoryMethod创建对象。
org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#resolveBeanReference
private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,
ConfigurableBeanFactory beanFactory, String beanName) {
// The user (i.e. not the factory) is requesting this bean through a call to
// the bean method, direct or indirect. The bean may have already been marked
// as 'in creation' in certain autowiring scenariOS; if so, temporarily set
// the in-creation status to false in order to avoid an exception.
boolean alreadyInCreation = beanFactory.isCurrentlyInCreation(beanName);
try {
if (alreadyInCreation) {
beanFactory.setCurrentlyInCreation(beanName, false);
}
boolean useArgs = !ObjectUtils.isEmpty(beanMethodArgs);
if (useArgs && beanFactory.isSingleton(beanName)) {
// Stubbed null arguments just for reference purposes,
// expecting them to be autowired for regular singleton references?
// A safe assumption since @Bean singleton arguments cannot be optional...
for (Object arg : beanMethodArgs) {
if (arg == null) {
useArgs = false;
break;
}
}
}
// 第二次调用直接从BeanFactory中取
Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :
beanFactory.getBean(beanName));
if (!ClassUtils.isAssignableValue(beanMethod.getReturnType(), beanInstance)) {
// Detect package-protected NullBean instance through equals(null) check
if (beanInstance.equals(null)) {
if (logger.isDebugEnabled()) {
logger.debug(String.format("@Bean method %s.%s called as bean reference " +
"for type [%s] returned null bean; resolving to null value.",
beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),
beanMethod.getReturnType().getName()));
}
beanInstance = null;
}
else {
String msg = String.format("@Bean method %s.%s called as bean reference " +
"for type [%s] but overridden by non-compatible bean instance of type [%s].",
beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),
beanMethod.getReturnType().getName(), beanInstance.getClass().getName());
try {
BeanDefinition beanDefinition = beanFactory.getMergedBeanDefinition(beanName);
msg += " Overriding bean of same name declared in: " + beanDefinition.getResourceDescription();
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore - simply no detailed message then.
}
throw new IllegalStateException(msg);
}
}
Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();
if (currentlyInvoked != null) {
String outerBeanName = BeanAnnotationHelper.determineBeanNameFor(currentlyInvoked);
beanFactory.registerDependentBean(beanName, outerBeanName);
}
return beanInstance;
}
finally {
if (alreadyInCreation) {
beanFactory.setCurrentlyInCreation(beanName, true);
}
}
}如果将user()方法改为static，那么又会创建两个user对象，因为spring并没有为@Configuration修饰的类创建代理，只是为@Configuration修饰的类的对象创建了代理。
FactoryBean的代理
如果@Bean注入的是一个FactoryBean，Spring会为FactoryBean生成代理，保证getObject()方法只会被调用一次。
package com.morris.spring.config;
import com.morris.spring.entity.Message;
import com.morris.spring.entity.UserFactoryBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class UserFactoryBeanConfig {
@Bean
public UserFactoryBean userFactoryBean() {
System.out.println("userFactoryBean");
return new UserFactoryBean();
}
@Bean
public Message message() throws Exception {
return new Message(userFactoryBean().getObject());
}
}关键源码如下： org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#intercept
...
if (factoryContainsBean(beanFactory, BeanFactory.FACTORY_BEAN_PREFIX + beanName) &&
factoryContainsBean(beanFactory, beanName)) {
Object factoryBean = beanFactory.getBean(BeanFactory.FACTORY_BEAN_PREFIX + beanName);
if (factoryBean instanceof ScopedProxyFactoryBean) {
// Scoped proxy factory beans are a special case and should not be further proxied
}
else {
// It is a candidate FactoryBean - go ahead with enhancement
// 为FactoryBean创建代理
return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);
}
}
...org.springframework.context.annotation.ConfigurationClassEnhancer.BeanMethodInterceptor#enhanceFactoryBean
private Object enhanceFactoryBean(final Object factoryBean, Class<?> exposedType,
final ConfigurableBeanFactory beanFactory, final String beanName) {
try {
Class<?> clazz = factoryBean.getClass();
boolean finalClass = Modifier.isFinal(clazz.getModifiers());
boolean finalMethod = Modifier.isFinal(clazz.getMethod("getObject").getModifiers());
if (finalClass || finalMethod) {
if (exposedType.isInterface()) {
if (logger.isTraceEnabled()) {
logger.trace("Creating interface proxy for FactoryBean '" + beanName + "' of type [" +
clazz.getName() + "] for use within another @Bean method because its " +
(finalClass ? "implementation class" : "getObject() method") +
" is final: Otherwise a getObject() call would not be routed to the factory.");
}
// 创建jdk动态代理
return createInterfaceProxyForFactoryBean(factoryBean, exposedType, beanFactory, beanName);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Unable to proxy FactoryBean '" + beanName + "' of type [" +
clazz.getName() + "] for use within another @Bean method because its " +
(finalClass ? "implementation class" : "getObject() method") +
" is final: A getObject() call will NOT be routed to the factory. " +
"Consider declaring the return type as a FactoryBean interface.");
}
return factoryBean;
}
}
}
catch (NoSuchMethodException ex) {
// No getObject() method -> shouldn't happen, but as long as nobody is trying to call it...
}
// 使用cglib创建代理
return createCglibProxyForFactoryBean(factoryBean, beanFactory, beanName);
}到此这篇关于Spring的@Configuration使用与原理的文章就介绍到这了,更多相关Spring的@Configuration内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持！
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