• 場景：yml配置變量值是null 解析得到的是" "
• tip： 這篇最好結(jié)合 MBP中yml屬性注入及邏輯刪除過程一起看下

• 下面就來分析下原因：

1. yml解析過程中進行了默認處理
   分析這部分代碼一定要搞清楚Iterator邏輯 因為內(nèi)部增強for默認使用Iterator遍歷的
   最好知悉Iterator及Iterable 兩者區(qū)別
2. 設(shè)置yml屬性值時進行了默認處理（@Value是通過setter操作的 還是是直接反射字段賦值的）-反射字段賦值的

結(jié)果是: 1
org.springframework.boot.env.OriginTrackedYamlLoader.OriginTrackingConstructor#getValue：

        private Object getValue(Object value) {
            return (value != null) ? value : "";
        }

null：

null

“null”：

"null"

• 1. yml|yaml解析及默認處理的部分流程如下：

1. org.springframework.boot.SpringApplication#run(java.lang.String...)
2. org.springframework.boot.SpringApplication#prepareEnvironment
3. org.springframework.boot.SpringApplicationRunListeners#environmentPrepared
4. org.springframework.boot.context.event.EventPublishingRunListener#environmentPrepared
5. org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent)
6. org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)
7. org.springframework.context.event.SimpleApplicationEventMulticaster#invokeListener
8. org.springframework.context.event.SimpleApplicationEventMulticaster#doInvokeListener
9. org.springframework.boot.context.config.ConfigFileApplicationListener#onApplicationEvent
10. org.springframework.boot.context.config.ConfigFileApplicationListener#onApplicationEnvironmentPreparedEvent
11. org.springframework.boot.context.config.ConfigFileApplicationListener#postProcessEnvironment
12. org.springframework.boot.context.config.ConfigFileApplicationListener#addPropertySources
13. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load()
14. org.springframework.boot.context.config.FilteredPropertySource#apply
15. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load()
16. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load(org.springframework.boot.context.config.ConfigFileApplicationListener.Profile, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentFilterFactory, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentConsumer)
17. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load(java.lang.String, java.lang.String, org.springframework.boot.context.config.ConfigFileApplicationListener.Profile, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentFilterFactory, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentConsumer)
18. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#loadForFileExtension
19. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#load(org.springframework.boot.env.PropertySourceLoader, java.lang.String, org.springframework.boot.context.config.ConfigFileApplicationListener.Profile, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentFilter, org.springframework.boot.context.config.ConfigFileApplicationListener.DocumentConsumer)
20. org.springframework.boot.context.config.ConfigFileApplicationListener.Loader#loadDocuments
21. org.springframework.boot.env.YamlPropertySourceLoader#load
22. org.springframework.boot.env.OriginTrackedYamlLoader#load
23. org.springframework.beans.factory.config.YamlProcessor#process(org.springframework.beans.factory.config.YamlProcessor.MatchCallback)
24. org.springframework.beans.factory.config.YamlProcessor#process(org.springframework.beans.factory.config.YamlProcessor.MatchCallback, org.yaml.snakeyaml.Yaml, org.springframework.core.io.Resource)
25. java.util.Iterator#next
26. org.yaml.snakeyaml.constructor.BaseConstructor#getData
27. org.yaml.snakeyaml.constructor.BaseConstructor#constructDocument
28. org.springframework.boot.env.OriginTrackedYamlLoader.OriginTrackingConstructor#constructObject
29. org.yaml.snakeyaml.constructor.BaseConstructor#constructObject
30. org.yaml.snakeyaml.constructor.BaseConstructor#constructObjectNoCheck
31. org.yaml.snakeyaml.constructor.SafeConstructor.ConstructYamlMap#construct
32. org.yaml.snakeyaml.constructor.BaseConstructor#constructMapping
33. org.yaml.snakeyaml.constructor.SafeConstructor#constructMapping2ndStep
34. org.yaml.snakeyaml.constructor.BaseConstructor#constructMapping2ndStep
35. org.springframework.boot.env.OriginTrackedYamlLoader.OriginTrackingConstructor#constructObject
36. org.yaml.snakeyaml.constructor.BaseConstructor#constructObject
    ....

在org.springframework.boot.env.OriginTrackedYamlLoader.OriginTrackingConstructor#getValue 打個斷點 觀察具體的調(diào)用棧信息

具體的調(diào)用棧信息

• 2. @Value注解賦值的過程部分流程如下：

     
     
     @Value注入的核心代碼

結(jié)論：@Value是通過反射字段賦值的 并沒有進行null的處理操作
org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject

@Override
        protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
            Field field = (Field) this.member;
            Object value;
            if (this.cached) {
                value = resolvedCachedArgument(beanName, this.cachedFieldValue);
            }
            else {
                DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
                desc.setContainingClass(bean.getClass());
                Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
                Assert.state(beanFactory != null, "No BeanFactory available");
                TypeConverter typeConverter = beanFactory.getTypeConverter();
                try {
                    value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
                }
                catch (BeansException ex) {
                    throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
                }
                synchronized (this) {
                    if (!this.cached) {
                        if (value != null || this.required) {
                            this.cachedFieldValue = desc;
                            registerDependentBeans(beanName, autowiredBeanNames);
                            if (autowiredBeanNames.size() == 1) {
                                String autowiredBeanName = autowiredBeanNames.iterator().next();
                                if (beanFactory.containsBean(autowiredBeanName) &&
                                        beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                    this.cachedFieldValue = new ShortcutDependencyDescriptor(
                                            desc, autowiredBeanName, field.getType());
                                }
                            }
                        }
                        else {
                            this.cachedFieldValue = null;
                        }
                        this.cached = true;
                    }
                }
            }
            if (value != null) {
                ReflectionUtils.makeAccessible(field);
                field.set(bean, value);
            }
        }
    }