@Configuration注解和@Component注解的区别

首先看下@Configuration这个注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {

	@AliasFor(annotation = Component.class)
	String value() default "";
	
	boolean proxyBeanMethods() default true;
}

可以发现其实@Configuration注解本质上也是一个@Component注解.

@Configuration
@ComponentScan("com.aiqiong.admin")
public class AppConfig {

}

@Component
public class A {

}

    public static void main(String[] args) {
        AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
        A a = (A) annotationConfigApplicationContext.getBean("a");
        System.out.println("a = " + a);
    }

运行结果

可以看到A此时是一个普通的对象
把A类上的注解换成@Configuration

@Configuration
public class A {

}

运行结果:

可以看到加上Configuration注解之后,A变成了一个cglib的代理对象.

@Configuration注解生成cglib代理的作用

场景:

@Component
public class AppConfig {

    @Bean
    public B b(){
        B b = new B();
        b.setA(a());
        return b;
    }


    @Bean
    public A a(){
        return new A();
    }
    
}

public class A {

    public A(){
        System.out.println("A....");
    }
}

通过控制台的输出信息可以看到,其实A对象被创建了两次
换成@Configuration注解呢

@Configuration
public class AppConfig {

    @Bean
    public B b(){
        B b = new B();
        b.setA(a());
        return b;
    }

    @Bean
    public A a(){
        return new A();
    }
}

此时A只创建一次.

所以@Configuration注解就是为了避免用户显式地去调用@Bean方法，造成破坏了@Bean方法的bean的单例，而@Configuration注解能够做到这样避免用户显式调用@Bean方法的原理就是通过创建对应的配置类cglib代理对象去实现的.
下面我们重点看一下@Configuraton注解底层是怎么通过cglib代理去实现@Bean方法不被用户显式调用.

@Configuration动态代理的实现原理

ConfigurationClassPostProcessor实现了BeanDefinitionRegistryPostProcessor接口，而BeanDefinitionRegistryPostProcessor接口又是继承于BeanFactoryPostProcessor接口，众所周知BeanFactoryPostProcessor接口接口是一个BeanFactory的后置处理接口，也就是在BeanFactory初始化之后会调用该接口的postProcessBeanFactory方法，也就是上面的方法，其中里面会再调用enhanceConfigurationClasses方法，该方法就是生成cglib代理的关键方法

	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);
		}
		// 生成配置类cglib代理
		enhanceConfigurationClasses(beanFactory);
		beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
	}

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
		Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
		// 遍历当前spring容器中所有的bd名称，这个for循环的目的就是找到所有全配置类（加了@Configuration注解的配置类）
		for (String beanName : beanFactory.getBeanDefinitionNames()) {
			// 根据bd名称获取到bd
			BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
			// 获取到CONFIGURATION_CLASS_ATTRIBUTE对应的属性值
			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);
					}
				}
			}
			// 条件成立：说明这个bd对应的是一个全配置类
			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)) {
				
				configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
			}
		}
		// 条件成立：没有需要被增强的配置类
		if (configBeanDefs.isEmpty()) {
			// nothing to enhance -> return immediately
			return;
		}
		// 如果这个类是一个全配置类，那么就使用cglib进行代理增强，目的是防止@Bean方法的手动重复调用造成单例的破坏
		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();
			// 给这个全配置类创建一个cglib增强后的Class对象
			Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
			if (configClass != enhancedClass) {
				// 把cglib代理的Class对象设置到这个全配置类的bd中
				beanDef.setBeanClass(enhancedClass);
			}
		}
	}

上面代码其实分成了两大步骤:
第一步骤是用来找到扩展属性CONFIGURATION_CLASS_ATTRIBUTE对应的值等于CONFIGURATION_CLASS_FULL的BeanDefinition.
这个属性是在执行postProcessBeanFactory方法之前,spring会解析所有的配置类,然后解析成对应的BeanDefinition.
而BeanDefinition又分成了两种,一种是全配置类,一种是半配置类.

• 全配置类:加了@Configuration注解并且proxyBeanMethods属性等于true
• 半配置类:加了@Component,@ComponentScan,@Import,@ImportResource这几个注解,或者@Configuration注解并且proxyBeanMethods属性等于false

对于全配置类CONFIGURATION_CLASS_ATTRIBUTE值等于CONFIGURATION_CLASS_FULL
对于半配置类CONFIGURATION_CLASS_ATTRIBUTE值等于CONFIGURATION_CLASS_LITE

第二步是把上面找出来的BeanDefinition创建cglib代理.使用的就是ConfigurationClassEnhancer这个工具类.

创建代理对象详解

概述

	public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {
		if (EnhancedConfiguration.class.isAssignableFrom(configClass)) {
			
			return configClass;
		}
		Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));
		
		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));
		enhancer.setCallbackFilter(CALLBACK_FILTER);
		enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());
		return enhancer;
	}

上面都比较简单,就是常规的创建代理对象需要的参数

	private Class<?> createClass(Enhancer enhancer) {
		Class<?> subclass = enhancer.createClass();
		Enhancer.registerStaticCallbacks(subclass, CALLBACKS);
		return subclass;
	}

最后调用enhancer的createClass方法，就可以得到一个增强后的Class对象，返回出去之后把这个增强的Class对象设置到BeanDefinition中就可以了
创建代理对象参数详解
(1) CallbackFilter

enhancer.setCallbackFilter(CALLBACK_FILTER);

作用:当执行目标方法的时候会被accept方法拦截,在accept方法中会根据传入的目标方法去返回一个Callback增强.
通过这个CallbackFilter就可以实现调用不同的方法使用不同的增强Callback.spring这里传入的是ConditionalCallbackFilter,这个CallbackFilter在创建的时候需要传入一个Callback数组.
这个数组会被accept方法过滤,返回一个具体的Callback返回.

private static class ConditionalCallbackFilter implements CallbackFilter {

		private final Callback[] callbacks;

		private final Class<?>[] callbackTypes;

		public ConditionalCallbackFilter(Callback[] callbacks) {
			this.callbacks = callbacks;
			this.callbackTypes = new Class<?>[callbacks.length];
			for (int i = 0; i < callbacks.length; i++) {
				this.callbackTypes[i] = callbacks[i].getClass();
			}
		}
		// 
		@Override
		public int accept(Method method) {
			for (int i = 0; i < this.callbacks.length; i++) {
				Callback callback = this.callbacks[i];
				if (!(callback instanceof ConditionalCallback) || ((ConditionalCallback) callback).isMatch(method)) {
					return i;
				}
			}
			throw new IllegalStateException("No callback available for method " + method.getName());
		}

		public Class<?>[] getCallbackTypes() {
			return this.callbackTypes;
		}
	}

上面提到一个Callback数组,那么这个数组中的元素是啥呢?

	private static final Callback[] CALLBACKS = new Callback[] {
			new BeanMethodInterceptor(),
			new BeanFactoryAwareMethodInterceptor(),
			// 不进行增强
			NoOp.INSTANCE
	};

根据对应isMatch方法来看一下另外两个都是什么时候回被调用

1、BeanMethodInterceptor

		public boolean isMatch(Method candidateMethod) {
		     // 不是Object对象、不是setBeanFactory方法,并且方法上要有@Bean注解
			return (candidateMethod.getDeclaringClass() != Object.class &&
					!BeanFactoryAwareMethodInterceptor.isSetBeanFactory(candidateMethod) &&
					BeanAnnotationHelper.isBeanAnnotated(candidateMethod));
		}

2、BeanFactoryAwareMethodInterceptor

		public boolean isMatch(Method candidateMethod) {
			return isSetBeanFactory(candidateMethod);
		}
		
		public static boolean isSetBeanFactory(Method candidateMethod) {
			// 是setBeanFactory方法
			return (candidateMethod.getName().equals("setBeanFactory") &&
					candidateMethod.getParameterCount() == 1 &&
					BeanFactory.class == candidateMethod.getParameterTypes()[0] &&
					BeanFactoryAware.class.isAssignableFrom(candidateMethod.getDeclaringClass()));
		}

那么最后我们返回的是哪一个增强器呢?
(2) EnhancedConfiguration

enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});

public interface EnhancedConfiguration extends BeanFactoryAware {
}

所以自然地cglib代理对象中自然就会有这个BeanFactoryAware接口的setBeanFactory方法了.
所以可以回答上面一个问题了,创建代理的时候使用的增强器是BeanFactoryAwareMethodInterceptor
那么setBeanFactory方法是在什么时候被调用的呢?
这就要回顾之前讲到的Bean的生命周期了.如果一个bean实现了BeanFactoryAware接口的话,在实例化的过程中会回调BeanFactoryAware接口的setBeanFactory方法,然后在spring回调setBeanFactory方法的时候,BeanFactoryAwareMethodInterceptor这个Callback就能拦截到,执行intercept方法中的增强逻辑.
在intercept方法中做的其实就是把回调回来的BeanFactory对象赋值给属性名称为beanFactory的属性.
beanFactory这个属性是哪里的?在哪里设置的呢?

(3) beanFactory

	enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));

setStrategy属性可以设置生成代理类的class文件字节码策略，这里具体设置的策略是BeanFactoryAwareGeneratorStrategy.

	private static class BeanFactoryAwareGeneratorStrategy extends
			ClassLoaderAwareGeneratorStrategy {

		public BeanFactoryAwareGeneratorStrategy(@Nullable ClassLoader classLoader) {
			super(classLoader);
		}

		@Override
		protected ClassGenerator transform(ClassGenerator cg) throws Exception {
			ClassEmitterTransformer transformer = new ClassEmitterTransformer() {
				@Override
				public void end_class() {
					// 给生成的代理类添加一个属性
					// 属性名为 beanFactory
					// 访问域是public
					// 属性类型是BeanFactory
					declare_field(Constants.ACC_PUBLIC, BEAN_FACTORY_FIELD, Type.getType(BeanFactory.class), null);
					super.end_class();
				}
			};
			return new TransformingClassGenerator(cg, transformer);
		}

	}

	private static final String BEAN_FACTORY_FIELD = "$$beanFactory";

总结下BeanFactoryAwareMethodInterceptor这个Callback的作用了，它的作用就是在spring回调BeanFactoryAware接口的seetBeanFactory方法的时候，把回调的BeanFactory对象赋值给
beanFactory属性，而这个属性的由来就是cglib在创建代理类的时候添加进来的.

下面讲讲另外一种增强器
BeanMethodInterceptor

		public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,
					MethodProxy cglibMethodProxy) throws Throwable {
			// 获取到$$beanFactory属性的值，也就是BeanFactory对象
			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;
				}
			}

			
			if (factoryContainsBean(beanFactory, BeanFactory.FACTORY_BEAN_PREFIX + beanName) &&
					factoryContainsBean(beanFactory, beanName)) {
				Object factoryBean = beanFactory.getBean(BeanFactory.FACTORY_BEAN_PREFIX + beanName);
				if (factoryBean instanceof ScopedProxyFactoryBean) {
					
				}
				else {
					// It is a candidate FactoryBean - go ahead with enhancement
					return enhanceFactoryBean(factoryBean, beanMethod.getReturnType(), beanFactory, beanName);
				}
			}

			// 条件成立：说明此时spring正在执行反射执行这个@Bean方法
			if (isCurrentlyInvokedFactoryMethod(beanMethod)) {
				// 调用父类的原始@Bean方法返回bean实例，返回的bean实例会注册到spring容器中
				return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
			}

			// 代码来到这里说明此时的@Bean方法并不是由spring调用的，而是用户自身的代码显示调用的
   			// 这种情况也就是在@Bean方法B中调用@Bean方法A，
   			// 在调用@Bean方法B的时候上面的条件就不会成立，所以会执行到这里
   			// 从BeanFactory中获取bean
			return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
		}

		private boolean isCurrentlyInvokedFactoryMethod(Method method) {
			Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();
			return (currentlyInvoked != null && method.getName().equals(currentlyInvoked.getName()) &&
					Arrays.equals(method.getParameterTypes(), currentlyInvoked.getParameterTypes()));
		}

其中比较重要的是Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod()
这里就要提到spring实例化bean的时候了.看下面这里实例化bean的代码

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
      @Nullable Object factoryBean, final Method factoryMethod, Object... args) {
 
   try {
      if (System.getSecurityManager() != null) {
         AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            ReflectionUtils.makeAccessible(factoryMethod);
            return null;
         });
      }
      else {
         ReflectionUtils.makeAccessible(factoryMethod);
      }
 
      Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
      try {
         // 把要执行的工厂方法放到currentlyInvokedFactoryMethod中
         currentlyInvokedFactoryMethod.set(factoryMethod);
         // 通过反射执行工厂实例的工厂方法
         Object result = factoryMethod.invoke(factoryBean, args);
         if (result == null) {
            result = new NullBean();
         }
         return result;
      }
      finally {
         if (priorInvokedFactoryMethod != null) {
            currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
         }
         else {
            currentlyInvokedFactoryMethod.remove();
         }
      }
   }  
}

可以看到spring实例化bean的时候会通过反射去调用这个bean所在的配置类的@Bean方法,在调用前会把这个@Bean方法放到currentlyInvokedFactoryMethod中.这一步是spring实例化bean的时候触发的,如果手动调这个@Bean方法,根本不会调用上面的方法currentlyInvokedFactoryMethod中也就不会有这个@Bean方法.
所以isCurrentlyInvokedFactoryMethod方法里的

Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();

如果currentlyInvoked为空,说明是手动调用的,并且返回false,不为空是通过spring反射调用的,并且返回true.
如果返回的是false,则直接就执行resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);

		private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,
				ConfigurableBeanFactory beanFactory, String beanName) {

			// 判断当前bean是否正在创建
			boolean alreadyInCreation = beanFactory.isCurrentlyInCreation(beanName);
			try {
				if (alreadyInCreation) {
					beanFactory.setCurrentlyInCreation(beanName, false);
				}
				boolean useArgs = !ObjectUtils.isEmpty(beanMethodArgs);
				if (useArgs && beanFactory.isSingleton(beanName)) {
				
					for (Object arg : beanMethodArgs) {
						if (arg == null) {
							useArgs = false;
							break;
						}
					}
				}
				// 从spring容器中获取这个bean
				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)) {
						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);
					// 注册两个bean之间的依赖关系
					beanFactory.registerDependentBean(beanName, outerBeanName);
				}
				return beanInstance;
			}
			finally {
				if (alreadyInCreation) {
					beanFactory.setCurrentlyInCreation(beanName, true);
				}
			}
		}

总结
Configuration注解能够使得加了它的对应的配置类在spring容器中是一个cglib代理bean，让配置类实例变成cglib代理实例的作用就是防止@Bean方法B又调用@Bean方法A的这种场景破坏了bean的单例性。
解决这种场景的原理就是在spring实例化@Bean方法B对应的bean的时候，会通过反射调用配置类的@Bean方法B，在反射调用之前@Bean方法B放到currentlyInvokedFactoryMethod中，然后由于配置类已经是一个cglib代理对象了，
所以会被BeanMethodInterceptor这个Callback进行拦截增强，此时判断到currentlyInvokedFactoryMethod中存在方法B，那么就直接执行这个@Bean方法B了，而@Bean方法B中又调用了@Bean方法A，
所以此时在执行@Bean方法B的时候，@Bean方法A也会被BeanMethodInterceptor这个Callback所拦截增强，但是，此时currentlyInvokedFactoryMethod中的方法是@Bean方法B，所以isCurrentlyInvokedFactoryMethod方法判断返回的是false，也就是走resolveBeanReference方法从spring容器中去获取@Bean方法A对应的bean.

该文章是转载大佬的为何加上@Configuration注解的类会生成cglib代理？