死磕源码系列【org.springframework.beans.factory.Aware】

Dear 丶 2022-12-08 05:49 119阅读 0赞

> 使用spring及springboot做开发及看源码的时候经常会看到很多Aware类，如EnvironmentAware、ApplicationContextAware等等，那么这些类又是些什么类呢？接下来我们就来分析；

##### 1.首先看下Aware接口源码 #####

/** * A marker superinterface indicating that a bean is eligible to be notified by the * Spring container of a particular framework object through a callback-style method. * The actual method signature is determined by individual subinterfaces but should * typically consist of just one void-returning method that accepts a single argument. * * <p>Note that merely implementing {@link Aware} provides no default functionality. * Rather, processing must be done explicitly, for example in a * {@link org.springframework.beans.factory.config.BeanPostProcessor}. * Refer to {@link org.springframework.context.support.ApplicationContextAwareProcessor} * for an example of processing specific {@code *Aware} interface callbacks. * * @author Chris Beams * @author Juergen Hoeller * @since 3.1 */
    public interface Aware { 
    
    }

> Aware接口其实就是一个空接口，没有任何的方法定义，解释下上面的注解：一个标记父接口，指示了一个bean有资格被spring容器通过一个回调风格的方法调用框架的特定对象。实际的方法签名（即方法）由各个子接口确定的，通常应该仅包含一个接受单个参数、返回值为void的方法。
> 
> 请注意，仅仅实现Aware接口不会提供默认功能。相反，处理必须明确的进行，例如在一个org.springframework.beans.factory.config.BeanPostProcessor中，请参考org.springframework.context.support.ApplicationContextAwareProcessor类对部分Aware接口类的处理实例；

##### 2.示例 #####

参考示例文档中的org.springframework.context.support.ApplicationContextAwareProcessor类的实现：

class ApplicationContextAwareProcessor implements BeanPostProcessor { 
    
    	private final ConfigurableApplicationContext applicationContext;
    
    	private final StringValueResolver embeddedValueResolver;
    
    
    	/** * Create a new ApplicationContextAwareProcessor for the given context. */
    	public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) { 
    		this.applicationContext = applicationContext;
    		this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
    	}
    
    
    	@Override
    	@Nullable
    	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { 
    		if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
    				bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
    				bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){ 
    			return bean;
    		}
    
    		AccessControlContext acc = null;
    
    		if (System.getSecurityManager() != null) { 
    			acc = this.applicationContext.getBeanFactory().getAccessControlContext();
    		}
    
    		if (acc != null) { 
    			AccessController.doPrivileged((PrivilegedAction<Object>) () -> { 
    				invokeAwareInterfaces(bean);
    				return null;
    			}, acc);
    		}
    		else { 
    			invokeAwareInterfaces(bean);
    		}
    
    		return bean;
    	}
    
    	private void invokeAwareInterfaces(Object bean) { 
    		if (bean instanceof EnvironmentAware) { 
    			((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
    		}
    		if (bean instanceof EmbeddedValueResolverAware) { 
    			((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
    		}
    		if (bean instanceof ResourceLoaderAware) { 
    			((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
    		}
    		if (bean instanceof ApplicationEventPublisherAware) { 
    			((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
    		}
    		if (bean instanceof MessageSourceAware) { 
    			((MessageSourceAware) bean).setMessageSource(this.applicationContext);
    		}
    		if (bean instanceof ApplicationContextAware) { 
    			((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
    		}
    	}
    
    }

> BeanPostProcessor接口为IOC容器类提供了钩子方法，在Bean实例化之前和实例化之后分别调用before和after方法调用，具体使用方法请参考：https://blog.csdn.net/yaomingyang/article/details/108628357，当回调方法postProcessBeforeInitialization被调用时，会调用invokeAwareInterfaces方法，那么就会调用Aware接口中的方法，可能会传入一个参数，就是上述文档中提到的框架对象，如：applicationContext。

GitHub地址：[https://github.com/mingyang66/spring-parent][https_github.com_mingyang66_spring-parent]

[https_github.com_mingyang66_spring-parent]: https://github.com/mingyang66/spring-parent