为什么我们的项目里出现两个配置类继承WebMvcConfigurationSupport时,只有一个会生效。我在网上找了半天都是说结果的,没有人分析源码到底是为啥,博主准备讲解一下,希望可以帮到大家!
大家基本遇到过一种情况,就是我配置类中已经配置了,为什么就是没有生效呢?其中一种原因就是,自己写的配置类也继承了WebMvcConfigurationSupport,当项目出现两个配置类都继承该类时,只会讲第一个配置类生效,至于为什么,就是今天博主需要讲解的,我们必须了解一些springboot的bean的创建过程也就是其生命周期:
https://www.processon.com/view/link/5f704050f346fb166d0f3e3c
虽然画的比较简单,有许多细节都没有解析,但是对于当前我们的话题来讲已经基本可以了;
第一步:我们的配置类是从哪里开始创建解析的:大家可以看到图示bean的流程中doProcessConfigurationClass(configClass, sourceClass, filter);方法,我们看一下是如何调用它 的:
1 protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException { 2 if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) { 3 return; 4 } 5 6 ConfigurationClass existingClass = this.configurationClasses.get(configClass); 7 if (existingClass != null) { 8 if (configClass.isImported()) { 9 if (existingClass.isImported()) { 10 existingClass.mergeImportedBy(configClass); 11 } 12 // Otherwise ignore new imported config class; existing non-imported class overrides it. 13 return; 14 } 15 else { 16 // Explicit bean definition found, probably replacing an import. 17 // Let‘s remove the old one and go with the new one. 18 this.configurationClasses.remove(configClass); 19 this.knownSuperclasses.values().removeIf(configClass::equals); 20 } 21 } 22 23 // Recursively process the configuration class and its superclass hierarchy. 24 SourceClass sourceClass = asSourceClass(configClass, filter); 25 do { 26 //从这里开始解析我们的当前配置类 27 sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter); 28 } 29 while (sourceClass != null); 30 31 this.configurationClasses.put(configClass, configClass); 32 }
这里可以看到一个while循环,为什么要这么设计呢?我们再看看doProcessConfigurationClass(configClass, sourceClass, filter);方法的源码
1 protected final SourceClass doProcessConfigurationClass( 2 ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter) 3 throws IOException { 4 5 if (configClass.getMetadata().isAnnotated(Component.class.getName())) { 6 // Recursively process any member (nested) classes first 7 processMemberClasses(configClass, sourceClass, filter); 8 } 9 10 // Process any @PropertySource annotations 11 for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable( 12 sourceClass.getMetadata(), PropertySources.class, 13 org.springframework.context.annotation.PropertySource.class)) { 14 if (this.environment instanceof ConfigurableEnvironment) { 15 processPropertySource(propertySource); 16 } 17 else { 18 logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() + 19 "]. Reason: Environment must implement ConfigurableEnvironment"); 20 } 21 } 22 23 // Process any @ComponentScan annotations 24 Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable( 25 sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class); 26 if (!componentScans.isEmpty() && 27 !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) { 28 for (AnnotationAttributes componentScan : componentScans) { 29 // The config class is annotated with @ComponentScan -> perform the scan immediately 30 Set<BeanDefinitionHolder> scannedBeanDefinitions = 31 this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName()); 32 // Check the set of scanned definitions for any further config classes and parse recursively if needed 33 for (BeanDefinitionHolder holder : scannedBeanDefinitions) { 34 BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition(); 35 if (bdCand == null) { 36 bdCand = holder.getBeanDefinition(); 37 } 38 if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) { 39 parse(bdCand.getBeanClassName(), holder.getBeanName()); 40 } 41 } 42 } 43 } 44 45 // Process any @Import annotations 46 processImports(configClass, sourceClass, getImports(sourceClass), filter, true); 47 48 // Process any @ImportResource annotations 49 AnnotationAttributes importResource = 50 AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class); 51 if (importResource != null) { 52 String[] resources = importResource.getStringArray("locations"); 53 Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader"); 54 for (String resource : resources) { 55 String resolvedResource = this.environment.resolveRequiredPlaceholders(resource); 56 configClass.addImportedResource(resolvedResource, readerClass); 57 } 58 } 59 //这里也很重要,这里开始会解析当前配置类里的bean,然后解析父类里面的bean,就是这里才会把WebMvcConfigurationSupport的所有bean 60 //都解析出来并添加到configClass里面,不管解析当前类还是父类,configClass都是自己当前的配置类,所以WebMvcConfigurationSupport 61 // Process individual @Bean methods 62 Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass); 63 for (MethodMetadata methodMetadata : beanMethods) { 64 configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass)); 65 } 66 67 // Process default methods on interfaces 68 processInterfaces(configClass, sourceClass); 69 70 //最主要的就是这里,解析当前类的父类 71 // Process superclass, if any 72 if (sourceClass.getMetadata().hasSuperClass()) { 73 String superclass = sourceClass.getMetadata().getSuperClassName(); 74 if (superclass != null && !superclass.startsWith("java") && 75 !this.knownSuperclasses.containsKey(superclass)) { 76 //如果我们第一个继承了WebMvcConfigurationSupport的配置类,已经被扫描到,就会添加一个map缓存, 77 //下一个也继承了WebMvcConfigurationSupport的配置类,将不在解析,直接返回null。结束循环,这也是外面一层为什么要添加while循环 78 this.knownSuperclasses.put(superclass, configClass); 79 // Superclass found, return its annotation metadata and recurse 80 return sourceClass.getSuperClass(); 81 } 82 } 83 84 // No superclass -> processing is complete 85 return null;
所以就现在来讲,基本已经决定了,解析第一个配置类的时候,第二个配置类重写的任何方法基本没什么用了,因为父类所有的bean已经在第一个配置类中解析扫描到了,就剩下如何去创建bean了。我们再继续往下看会更明白;
第二步:现在当所有bean已经扫描到,并且bean定义已经完成,该开始实例化了,看一下createBeanInstance的创建过程,最后生成的时候会找到 factoryBean也就是我们自己的配置类
1 private Object instantiate(String beanName, RootBeanDefinition mbd, 2 @Nullable Object factoryBean, Method factoryMethod, Object[] args) { 3 4 try { 5 if (System.getSecurityManager() != null) { 6 return AccessController.doPrivileged((PrivilegedAction<Object>) () -> 7 this.beanFactory.getInstantiationStrategy().instantiate( 8 mbd, beanName, this.beanFactory, factoryBean, factoryMethod, args), 9 this.beanFactory.getAccessControlContext()); 10 } 11 else { 12 return this.beanFactory.getInstantiationStrategy().instantiate( 13 mbd, beanName, this.beanFactory, factoryBean, factoryMethod, args); 14 } 15 } 16 catch (Throwable ex) { 17 throw new BeanCreationException(mbd.getResourceDescription(), beanName, 18 "Bean instantiation via factory method failed", ex); 19 } 20 }
其中factoryBean就是我们的当前第一个被解析到的配置类bean,截图为证,我自己写了两个配置类,第一个被加载的是MyASD,瞎写的名,好区分,第二个配置类是WebConfiguration,我们只看WebMvcConfigurationSupport里面的其中一个bean的创建过程,就是requestMappingHandlerAdapter,为啥要看这个,正好跟上节json自定义衔接。
https://www.cnblogs.com/guoxiaoyu/p/13667961.html
到这里,我们可以看到在生成requestMappingHandlerAdapter时,调用extendMessageConverters方法时,一定会调用第一个配置类中的重写方法,因为所有的WebMvcConfigurationSupport里面 bean都被第一个配置类解析完了,所有的factoryBean都是当前第一个配置类,就算第二个配置完没有报错,也不会生效了。
我直接把这个问题用源码的方式讲解清楚,方便大家明白为什么配置两个WebMvcConfigurationSupport类,只有一个生效。