SpringBoot(四) SpringBootBean扫描介入及自动装配

叁歲伎倆 2023-01-01 02:46 210阅读 0赞

这里开始分析SpringBoot如何让spring扫描并自动装配。

## BeanDefition ##

这里需要提到Spring一个重要知识点—BeanDefition。Spring实例化bean是一个复杂的过程，不仅仅金是new出实例，Bean有很多属性例如作用域、懒加载、别名等。Spring通过BeanDefition记录bean构造时的属性值、构造函数参数值以及具体实现提供的进一步信息。

## 核心-SpringBoot扫描bean ##

Spring 启动过程首先扫描获取所有BeanDefition，放入一个map中，然后执行初始化步骤，将所有单例bean初始化(实例化)并放入容器中。

而SpringBoot的自动装配，即是在扫描阶段，把需要自动装配的bean加入BeanDefition Map中。然后Spring自动初始化这些bean。

SpringBoot如何介入SpringBean扫描阶段，通过扫描获取到这些用户没有配置但会自动装配的BeanDefition 呢？

下面是Spring容器Context刷新的源码，仅看名字，貌似也看不出来哪个方法执行了bean扫描。每个方法点进去都是庞大的一堆代码，好在这段代码结构比较清晰，我们通过断点一步步查看，到底哪个方法扫描了需要自动装配的BeanDefition 。

@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.
    				postProcessBeanFactory(beanFactory);
    
    				// Invoke factory processors registered as beans in the context.
    				invokeBeanFactoryPostProcessors(beanFactory);
    
    				// Register bean processors that intercept bean creation.
    				registerBeanPostProcessors(beanFactory);
    
    				// Initialize message source for this context.
    				initMessageSource();
    
    				// Initialize event multicaster for this context.
    				initApplicationEventMulticaster();
    
    				// Initialize other special beans in specific context subclasses.
    				onRefresh();
    
    				// Check for listener beans and register them.
    				registerListeners();
    
    				// Instantiate all remaining (non-lazy-init) singletons.
    				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();
    			}
    		}
    	}

执行完postProcessBeanFactory方法后，beanDefinitionMap中存在6个BeanDefition，除了入口类，都是processor，显然不对，继续走。。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70]

执行完invokeBeanFactoryPostProcessors方法，beanDefinitionMap中的对象暴增到124了，其中还有我自己定义的一个bean myConfig。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 1]  
可猜测invokeBeanFactoryPostProcessors承载了SpringBoot接入bean扫描的工作，看方法描述主要作用是实例化并调用所有已注册的BeanFactoryPostProcessor bean。

invokeBeanFactoryPostProcessors方法比较长，大致的处理逻辑如下：  
分离出BeanDefinitionRegistryPostProcessor和BeanFactoryPostProcessor类型的处理器，优先对BeanDefinitionRegistryPostProcessor类型处理，然后处理BeanFactoryPostProcessor类型，且每次都按照优先级及处理，优先级为：  
实现接口PriorityOrdered > 实现接口Ordered > 剩下的

而符合BeanDefinitionRegistryPostProcessor且实现接口PriorityOrdered的处理器为：  
org.springframework.context.annotation.internalConfigurationAnnotationProcessor

这是容器创建时AnnotatedBeanDefinitionReader自动添加了其对应的BeanDefinition，对应的class为ConfigurationClassPostProcessor。  
ConfigurationClassPostProcessor是一个重量级的处理器，加载所有Configuration配置类

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
          PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware

按照上述逻辑，ConfigurationClassPostProcessor是最早被执行的，因为它既是BeanDefinitionRegistryPostProcessor类型，而且实现了PriorityOrdered接口，所以首先对其进行实例化。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 2]

然后开始执行BeanDefinition的发现与注册，发现步骤用于解析获取Configuration中定义了哪些获取bean的方式，注册步骤用于将所有定义的bean转化注册，会立即注册扫描到的Configuration类。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 3]

#### BeanDefinition发现 ####

1.  使用ConfigurationClassParser工具类转化配置类（如SpringBoot项目入口main方法的类）。
2.  解析配置类的元数据处理转化为ConfigurationClass，doProcessConfigurationClass，包括@PropertySource、@ComponentScan、@Import（延时处理）、@ImportResource、@Bean及接口方法等，基本的configuration已经处理完毕  
    。
3.  处理@Import涉及的所有配置的Configuration，SpringBoot项目时使用**EnableAutoConfiguration.AutoConfigurationImportSelector**将META-INF/spring.factories下的所有自动注入类，用于接下来转化为ConfigurationClass，这个注解是在入口类的SpringBootApplication里的EnableAutoConfiguration注解中设置的。

/** * Return the auto-configuration class names that should be considered. By default * this method will load candidates using {@link SpringFactoriesLoader} with * {@link #getSpringFactoriesLoaderFactoryClass()}. * @param metadata the source metadata * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation * attributes} * @return a list of candidate configurations */
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { 
       List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
             getBeanClassLoader());
       Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
             + "are using a custom packaging, make sure that file is correct.");
       return configurations;
    }

#### BeanDefinition条件判断与跳过 ####

上一步中获取需要注入的类后，执行ConfigurationClassParser.parse方法，将所有扫描到的configurationClass进一步处理，判断是否需要转化，如需要则递归获取configuration类。

protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException { 
    		if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) { 
    			return;
    		}
    
    		ConfigurationClass existingClass = this.configurationClasses.get(configClass);
    		if (existingClass != null) { 
    			if (configClass.isImported()) { 
    				if (existingClass.isImported()) { 
    					existingClass.mergeImportedBy(configClass);
    				}
    				// Otherwise ignore new imported config class; existing non-imported class overrides it.
    				return;
    			}
    			else { 
    				// Explicit bean definition found, probably replacing an import.
    				// Let's remove the old one and go with the new one.
    				this.configurationClasses.remove(configClass);
    				this.knownSuperclasses.values().removeIf(configClass::equals);
    			}
    		}
    
    		// Recursively process the configuration class and its superclass hierarchy.
    		SourceClass sourceClass = asSourceClass(configClass, filter);
    		do { 
    			sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
    		}
    		while (sourceClass != null);
    
    		this.configurationClasses.put(configClass, configClass);
    	}

**这就是springboot最核心的东西了，上面代码中的判断是否跳过，基于Spring条件注解，满足注解条件则会加载类，最终定义的bean会被加入到BeanDefinitionMap中，最终会自动装配，否则会被忽略。**

#### 条件判断 ####

条件判断的支持来源于autoconfiguration包，判断是否是web项目，判断是否存在配置，判断是否存在类等。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 4]  
如下示例，如果配置文件中配置了“myconfig”则类会被加载并实例化到容器中，否则不会。

/** * <p></p> * * @author nec * @since 2020-10-09 15:18 */
    @Component
    @ConditionalOnProperty({ "myconfig"})
    public class MyConfig { 
    
        @Bean
        public TestUtil getTestUtil() { 
            System.out.println("============>start init getTestUtil!!!");
            return new TestUtil();
        }
        
    }

如果判断条件是否满足呢，根据类头上的条件注解，就像上述类，注解了@ConditionalOnProperty(\{“myconfig”\})，那么判断这个类是否可以加载就在于condition.matches能否匹配到Property中存在"myconfig"这个条件，存在就返回true，这个类就会执行转化操作最终获取beanDefination，否则跳过处理下一个configuration。

/** * Determine if an item should be skipped based on {@code @Conditional} annotations. * @param metadata the meta data * @param phase the phase of the call * @return if the item should be skipped */
    	public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) { 
    		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) { 
    			return false;
    		}
    
    		if (phase == null) { 
    			if (metadata instanceof AnnotationMetadata &&
    					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) { 
    				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
    			}
    			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
    		}
    
    		List<Condition> conditions = new ArrayList<>();
    		for (String[] conditionClasses : getConditionClasses(metadata)) { 
    			for (String conditionClass : conditionClasses) { 
    				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
    				conditions.add(condition);
    			}
    		}
    
    		AnnotationAwareOrderComparator.sort(conditions);
    
    		for (Condition condition : conditions) { 
    			ConfigurationPhase requiredPhase = null;
    			if (condition instanceof ConfigurationCondition) { 
    				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
    			}
    			if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) { 
    				return true;
    			}
    		}
    
    		return false;
    	}

再进一步，怎么匹配的？如果是ConditionalOnProperty条件，则由对应的OnPropertyCondition处理，会根据条件值，从environment中找是否存在对应的配置。找到返回匹配，否则不匹配！  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 5]

#### 示例-自动开启aop ####

最后来看一个完整的自动装配示例，aop如何自动启用。

Spring项目如果需要开启AOP，需要手动设置@EnableAspectJAutoProxy。

而SpringBoot AopAutoConfiguration默认是支持自动启用aop的，因为matchIfMissing =true表示即使未配置spring.aop.auto，也会匹配，如果要关闭自动配置，则需要设置spring.aop.auto=false

当存在Advice.class时，AspectJAutoProxyingConfiguration 会自动启用，然后根据spring.aop.proxy-target-class是否配置来决定使用jdk动态代理还是cglib动态代理，可以看到

JdkDynamicAutoProxyConfiguration 的匹配matchIfMissing = false，表示条件匹配不到不生效  
CglibAutoProxyConfiguration 的匹配matchIfMissing = true，表示条件匹配不到也会生效

所以springboot默认启用的是cglib动态代理！

看下源码，注释表示自动开启aop相当于手动启用@EnableAspectJAutoProxy。  
Equivalent to enabling \{@link EnableAspectJAutoProxy @EnableAspectJAutoProxy\} in your configuration.

/** * {@link org.springframework.boot.autoconfigure.EnableAutoConfiguration * Auto-configuration} for Spring's AOP support. Equivalent to enabling * {@link EnableAspectJAutoProxy @EnableAspectJAutoProxy} in your configuration. * <p> * The configuration will not be activated if {@literal spring.aop.auto=false}. The * {@literal proxyTargetClass} attribute will be {@literal true}, by default, but can be * overridden by specifying {@literal spring.aop.proxy-target-class=false}. * * @author Dave Syer * @author Josh Long * @since 1.0.0 * @see EnableAspectJAutoProxy */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
    public class AopAutoConfiguration { 
    
    	@Configuration(proxyBeanMethods = false)
    	@ConditionalOnClass(Advice.class)
    	static class AspectJAutoProxyingConfiguration { 
    
    		@Configuration(proxyBeanMethods = false)
    		@EnableAspectJAutoProxy(proxyTargetClass = false)
    		@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false",
    				matchIfMissing = false)
    		static class JdkDynamicAutoProxyConfiguration { 
    
    		}
    
    		@Configuration(proxyBeanMethods = false)
    		@EnableAspectJAutoProxy(proxyTargetClass = true)
    		@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
    				matchIfMissing = true)
    		static class CglibAutoProxyConfiguration { 
    
    		}
    
    	}
    
    	@Configuration(proxyBeanMethods = false)
    	@ConditionalOnMissingClass("org.aspectj.weaver.Advice")
    	@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
    			matchIfMissing = true)
    	static class ClassProxyingConfiguration { 
    
    		ClassProxyingConfiguration(BeanFactory beanFactory) { 
    			if (beanFactory instanceof BeanDefinitionRegistry) { 
    				BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
    				AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
    				AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
    			}
    		}
    
    	}
    
    }

**下面来验证下**  
首先引入aop的starter

<dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-aop</artifactId>
    </dependency>

编写aop切面代码  
aop配置类

@Aspect
    @Component
    public class AopTest { 
    
        @Before("execution(* cn.nec.test.util.TestUtil.sayHi())")
        public void around() { 
            System.out.println("aop before");
        }
    }

被切的类

@Component
    public class TestUtil { 
    
        public void sayHi() { 
            System.out.println("Hi!");
        }
    
    }

测试类

@RestController
    @RequestMapping("/test")
    public class TestController { 
    
        @Autowired
        public TestUtil testUtil;
    
        @GetMapping("/test")
        public String test(){ 
            testUtil.sayHi();
            return "success";
        }
    }

1.  配置文件不做任何配置直接启动，访问接口http://127.0.0.1:8080/test/test，结果如下  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 6]
2.  手动关闭aop自动配置，再次测试，结果如下  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 7]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 8]

1.  关闭aop自动配置后，手动开启aop，结果如下  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 9]  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTIwOTgwMjE_size_16_color_FFFFFF_t_70 10]
2.  总结：springboot aop自动装配相当于帮我们自动设置了@EnableAspectJAutoProxy，而如果不引入aop-starter，则需要手动引入aop依赖jar的maven配置，并且设置@EnableAspectJAutoProxy

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