【SpringBoot】自动装配原理（三）：ConfigurationClassBeanDefinitionReader 过滤条件注解

╰+攻爆jí腚メ 2022-11-03 15:30 202阅读 0赞

# 条件注解 #

条件注解是Spring4提供的一种bean加载特性，主要用于控制配置类和bean初始化条件。在springBoot，springCloud一系列框架底层源码中，条件注解的使用到处可见。

不少人在使用@ConditionalOnBean注解时会遇到不生效的情况，依赖的 bean 明明已经配置了，但就是不生效。到底@ConditionalOnBean和bean加载的顺序有没有关系呢？跟着源码，一探究竟。

*  问题演示：
    
        @Configuration
        public class Configuration1 { 
        
            @Bean
            @ConditionalOnBean(Bean2.class)
            public Bean1 bean1() { 
                return new Bean1();
            }
        }
        
        @Configuration
        public class Configuration2 { 
        
            @Bean
            public Bean2 bean2(){ 
                return new Bean2();
            }
        }

结果：

*  @ConditionalOnBean(Bean2.class) 返回false。
 *  命名定义了bean2，bean1却未加载。

# 源码分析 #

首先要明确一点，条件注解的解析一定发生在spring ioc的bean definition阶段，因为 spring bean初始化的前提条件就是有对应的bean definition，条件注解正是通过判断bean definition来控制bean能否实例化。

## ConfigurationClassPostProcessor ##

从 bean definition解析的入口开始：ConfigurationClassPostProcessor

@Override
        public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) { 
            int registryId = System.identityHashCode(registry);
            if (this.registriesPostProcessed.contains(registryId)) { 
                throw new IllegalStateException(
                        "postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
            }
            if (this.factoriesPostProcessed.contains(registryId)) { 
                throw new IllegalStateException(
                        "postProcessBeanFactory already called on this post-processor against " + registry);
            }
            this.registriesPostProcessed.add(registryId);
        
            // 解析bean definition入口
            processConfigBeanDefinitions(registry);
        }

跟进processConfigBeanDefinitions方法：

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) { 
    
                //省略不必要的代码...
    
                // 在这里 AutoConfigurationImportSelector 已经回调过了
                // 所有 @Conpoment 扫描到的类 和 @Import 导入的类，都已经注册了
                parser.parse(candidates);
                parser.validate();
        
                // 将所有扫到的配置类存入集合
                Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
                configClasses.removeAll(alreadyParsed);
        
                // Read the model and create bean definitions based on its content
                if (this.reader == null) { 
                    this.reader = new ConfigurationClassBeanDefinitionReader(
                            registry, this.sourceExtractor, this.resourceLoader, this.environment,
                            this.importBeanNameGenerator, parser.getImportRegistry());
                }
                // 开始解析配置类，也就是条件注解解析的入口
                this.reader.loadBeanDefinitions(configClasses);
                alreadyParsed.addAll(configClasses);
                //...
    
    }

## ConfigurationClassBeanDefinitionReader ##

### loadBeanDefinitions ###

public void loadBeanDefinitions(Set<ConfigurationClass> configurationModel) { 
            ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator trackedConditionEvaluator = new ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator();
            Iterator var3 = configurationModel.iterator();
    
            while(var3.hasNext()) { 
                ConfigurationClass configClass = (ConfigurationClass)var3.next();
                this.loadBeanDefinitionsForConfigurationClass(configClass, trackedConditionEvaluator);
            }
    
        }
    
    	private void loadBeanDefinitionsForConfigurationClass(ConfigurationClass configClass, ConfigurationClassBeanDefinitionReader.TrackedConditionEvaluator trackedConditionEvaluator) { 
    		// 如果要 skip，那么当前配置类即使在 ConpomentScanParser 中被保存到了 BeanDefitionRegistry 也要移除掉
    		// skip 的条件是有 @Conditional 注解，且不满足 Condition 的条件
            if (trackedConditionEvaluator.shouldSkip(configClass)) { 
                String beanName = configClass.getBeanName();
                if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) { 
         
                    this.registry.removeBeanDefinition(beanName);
                }
    			
    			// 对于 selectImport 返回的类名，并没有进行注册，而是保存在了 importRegistry
    			// 不牵扯移除 @Import 导入的类中用 @Bean 注入的类，因为 shouldSkip 不成立才解析 @Bean
                this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
            } else { 
                if (configClass.isImported()) { 
                    this.registerBeanDefinitionForImportedConfigurationClass(configClass);
                }
    
                Iterator var3 = configClass.getBeanMethods().iterator();
    
                while(var3.hasNext()) { 
                    BeanMethod beanMethod = (BeanMethod)var3.next();
                    // 解析 @Bean 注解（即遍历 method）
                    // 如果 method 上有 @Condtional 还是会调用 shouldSkip 判断）
                    this.loadBeanDefinitionsForBeanMethod(beanMethod);
                }
    
                this.loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
                this.loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
            }
        }

loadBeanDefinitionsForBeanMethod

private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) { 
            ConfigurationClass configClass = beanMethod.getConfigurationClass();
            MethodMetadata metadata = beanMethod.getMetadata();
            String methodName = metadata.getMethodName();
            
            if (this.conditionEvaluator.shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN)) { 
                configClass.skippedBeanMethods.add(methodName);
            } else if (!configClass.skippedBeanMethods.contains(methodName)) { 
            	....
            	this.registry.registerBeanDefinition(beanName, beanDefToRegister);
            }
        }

### shouldSkip ###

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) { 
    
    
            //判断是否有条件注解，否则直接返回
            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);
            }
        
            //获取当前定义bean的方法上，所有的条件注解
            List<Condition> conditions = new ArrayList<>();
            for (String[] conditionClasses : getConditionClasses(metadata)) { 
                for (String conditionClass : conditionClasses) { 
                    Condition condition = getCondition(conditionClass, this.context.getClassLoader());
                    conditions.add(condition);
                }
            }
        
            //根据Order来进行排序
            AnnotationAwareOrderComparator.sort(conditions);
        
            //遍历条件注解，开始执行条件注解的流程
            for (Condition condition : conditions) { 
                ConfigurationPhase requiredPhase = null;
                if (condition instanceof ConfigurationCondition) { 
                    requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
                }
                //这里执行条件注解的 condition.matches 方法来进行匹配，返回布尔值
                if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) { 
                    return true;
                }
            }
        
            return false;
        }

## Condition ##

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### SpringBootCondition ###

@Override
    	public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { 
    		String classOrMethodName = getClassOrMethodName(metadata);
    		try { 
    			// 这里进行查询
    			ConditionOutcome outcome = getMatchOutcome(context, metadata);
    			logOutcome(classOrMethodName, outcome);
    			recordEvaluation(context, classOrMethodName, outcome);
    			return outcome.isMatch();
    		}
    		catch (NoClassDefFoundError ex) { 
    			throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to "
    					+ ex.getMessage() + " not " + "found. Make sure your own configuration does not rely on "
    					+ "that class. This can also happen if you are "
    					+ "@ComponentScanning a springframework package (e.g. if you "
    					+ "put a @ComponentScan in the default package by mistake)", ex);
    		}
    		catch (RuntimeException ex) { 
    			throw new IllegalStateException("Error processing condition on " + getName(metadata), ex);
    		}
    	}

### OnBeanCondition ###

@Override
    	public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) { 
    		ConditionMessage matchMessage = ConditionMessage.empty();
    		
    		// @ConditionalOnBean
    		if (metadata.isAnnotated(ConditionalOnBean.class.getName())) { 
    			BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ConditionalOnBean.class);
    			MatchResult matchResult = getMatchingBeans(context, spec);
    			if (!matchResult.isAllMatched()) { 
    				String reason = createOnBeanNoMatchReason(matchResult);
    				return ConditionOutcome
    						.noMatch(ConditionMessage.forCondition(ConditionalOnBean.class, spec).because(reason));
    			}
    			matchMessage = matchMessage.andCondition(ConditionalOnBean.class, spec).found("bean", "beans")
    					.items(Style.QUOTE, matchResult.getNamesOfAllMatches());
    		}
    		
    		// @ConditionalOnSingleCandidate
    		if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) { 
    			BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata,
    					ConditionalOnSingleCandidate.class);
    			MatchResult matchResult = getMatchingBeans(context, spec);
    			if (!matchResult.isAllMatched()) { 
    				return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class, spec)
    						.didNotFind("any beans").atAll());
    			}
    			else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matchResult.getNamesOfAllMatches(),
    					spec.getStrategy() == SearchStrategy.ALL)) { 
    				return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class, spec)
    						.didNotFind("a primary bean from beans")
    						.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
    			}
    			matchMessage = matchMessage.andCondition(ConditionalOnSingleCandidate.class, spec)
    					.found("a primary bean from beans").items(Style.QUOTE, matchResult.getNamesOfAllMatches());
    		}
    		
    		// @ConditionalOnMissingBean
    		if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) { 
    			BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ConditionalOnMissingBean.class);
    			MatchResult matchResult = getMatchingBeans(context, spec);
    			if (matchResult.isAnyMatched()) { 
    				String reason = createOnMissingBeanNoMatchReason(matchResult);
    				return ConditionOutcome
    						.noMatch(ConditionMessage.forCondition(ConditionalOnMissingBean.class, spec).because(reason));
    			}
    			matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class, spec).didNotFind("any beans")
    					.atAll();
    		}
    		return ConditionOutcome.match(matchMessage);
    	}

#### BeanSearchSpec ####

条件注解依赖的bean被封装成了BeanSearchSpec，从名字可以看出是要寻找的对象，这是一个静态内部类，构造方法如下：

BeanSearchSpec(ConditionContext context, AnnotatedTypeMetadata metadata,
    				Class<?> annotationType) { 
    			this.annotationType = annotationType;
    			//读取 metadata中的设置的value
    			MultiValueMap<String, Object> attributes = metadata
    					.getAllAnnotationAttributes(annotationType.getName(), true);
    			//设置各参数，根据这些参数进行寻找目标类
    			collect(attributes, "name", this.names);
    			collect(attributes, "value", this.types);
    			collect(attributes, "type", this.types);
    			collect(attributes, "annotation", this.annotations);
    			collect(attributes, "ignored", this.ignoredTypes);
    			collect(attributes, "ignoredType", this.ignoredTypes);
    			this.strategy = (SearchStrategy) metadata
    					.getAnnotationAttributes(annotationType.getName()).get("search");
    			BeanTypeDeductionException deductionException = null;
    			try { 
    				if (this.types.isEmpty() && this.names.isEmpty()) { 
    					addDeducedBeanType(context, metadata, this.types);
    				}
    			}
    			catch (BeanTypeDeductionException ex) { 
    				deductionException = ex;
    			}
    			validate(deductionException);
    		}

#### getMatchingBeans() ####

MatchResult matchResult = getMatchingBeans(context, spec);

private MatchResult getMatchingBeans(ConditionContext context, BeanSearchSpec beans) { 
    		ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    		if (beans.getStrategy() == SearchStrategy.ANCESTORS) { 
    			BeanFactory parent = beanFactory.getParentBeanFactory();
    			Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
    					"Unable to use SearchStrategy.PARENTS");
    			beanFactory = (ConfigurableListableBeanFactory) parent;
    		}
    		MatchResult matchResult = new MatchResult();
    		boolean considerHierarchy = beans.getStrategy() != SearchStrategy.CURRENT;
    		List<String> beansIgnoredByType = getNamesOfBeansIgnoredByType(
    				beans.getIgnoredTypes(), beanFactory, context, considerHierarchy);
    				
    		//因为实例代码中设置的是类型，所以这里会遍历类型，根据type获取目标bean是否存在
    		for (String type : beans.getTypes()) { 
    			Collection<String> typeMatches = getBeanNamesForType(beanFactory, type,
    					context.getClassLoader(), considerHierarchy);
    			typeMatches.removeAll(beansIgnoredByType);
    			if (typeMatches.isEmpty()) { 
    				matchResult.recordUnmatchedType(type);
    			}
    			else { 
    				matchResult.recordMatchedType(type, typeMatches);
    			}
    		}
    		
    		//根据注解寻找
    		for (String annotation : beans.getAnnotations()) { 
    			List<String> annotationMatches = Arrays
    					.asList(getBeanNamesForAnnotation(beanFactory, annotation,
    							context.getClassLoader(), considerHierarchy));
    			annotationMatches.removeAll(beansIgnoredByType);
    			if (annotationMatches.isEmpty()) { 
    				matchResult.recordUnmatchedAnnotation(annotation);
    			}
    			else { 
    				matchResult.recordMatchedAnnotation(annotation, annotationMatches);
    			}
    		}
    		
    		//根据设置的name进行寻找
    		for (String beanName : beans.getNames()) { 
    			if (!beansIgnoredByType.contains(beanName)
    					&& containsBean(beanFactory, beanName, considerHierarchy)) { 
    				matchResult.recordMatchedName(beanName);
    			}
    			else { 
    				matchResult.recordUnmatchedName(beanName);
    			}
    		}
    		return matchResult;
    	}

getBeanNamesForType()方法

*  最终会委托给BeanTypeRegistry类的getNamesForType方法来获取对应的指定类型的bean name

Set<String> getNamesForType(Class<?> type) { 
    		//同步spring容器中的bean
    		updateTypesIfNecessary();
    		//返回指定类型的bean
    		return this.beanTypes.entrySet().stream()
    				.filter((entry) -> entry.getValue() != null
    						&& type.isAssignableFrom(entry.getValue()))
    				.map(Map.Entry::getKey)
    				.collect(Collectors.toCollection(LinkedHashSet::new));
    	}

## 小结 ##

我们来分析一下上面示例bean1为何没有注册？

*  因为在 ConfigurationClassBeanDefinitionReader 的loadBeanDefinitionsForBeanMethod 时，也会判断是否有 @Conditional，有的话也会调用 shouldSkip，而此时注册部分 @Bean 可能还并没有注册。

解决（有两种方式）

*  项目中条件注解依赖的类，大多会交给spring容器管理，所以如果要在配置中Bean通过@ConditionalOnBean依赖配置中的Bean时，完全可以用@ConditionalOnClass(Bean2.class)来代替。
 *  如果一定要区分两个**配置类的先后顺序**，可以将这两个类交与EnableAutoConfiguration管理和触发。也就是定义在META-INF\\spring.factories中声明是配置类，然后通过@AutoConfigureBefore、AutoConfigureAfter、AutoConfigureOrder控制先后顺序。因为这三个注解只对自动配置类生效。

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