Tencent two sides: @bean and @component are used in the same class, what will happen?

Doubt description

lately , In the process of development , Found a way to write before , It's like this

In my understanding ,@Configuration Add @Bean Will create a userName Not for null Of UserManager object , and @Component It will also create a userName by null Of UserManager object

So we inject... Into other objects UserManager Object time , Which object is injected ？

Because the project has been online for a long time , Therefore, this writing method does not compile and report errors , There was no problem with the operation

I'll find my colleagues later to find out , Actually, I want

take effect , In fact, it did take effect

So here comes the question ：Spring How many... Are there in the container UserManager Object of type ？

Spring Boot edition

Used in the project Spring Boot The version is ：2.0.3.RELEASE

Object's scope Is the default value , That is to say singleton

The results verify that

There are many ways to verify , Sure debug Follow the source code , have a look Spring How many... Are there in the container UserManager object , You can also go straight from UserManager Construction method , See which constructor methods are called , wait

Let's start with the construction method , have a look UserManager How many instances did you instantiate

Only parameterized constructors are called , The nonparametric construction method remains intact （ Not called at all ）

since UserManager The constructor is called only once , So the previous question ： Which object is injected

The answer is clear , There's no choice , Can only be @Configuration Add @Bean Created userName Not for null Of UserManager object

Here comes the question ： Why not @Component Created userName by null Of UserManager object ？

The source code parsing

@Configuration And @Component It's very close

therefore @Configuration It can be component scan

among ConfigurationClassPostProcessor And @Configuration Is closely linked , The class inheritance structure is as follows ：

It has achieved BeanFactoryPostProcessor Interface and PriorityOrdered Interface , About BeanFactoryPostProcessor,

So we're from AbstractApplicationContext Of refresh Method called invokeBeanFactoryPostProcessors(beanFactory) Start , Follow the source code

Now it's done com.lee.qsl Under bag component scan , com.lee.qsl Package and sub package UserConfig 、 UserController and UserManager All scanned

Be careful , At the moment @Bean The processing of has not yet started , UserManager It's through @Component And the scanned ; here Spring In the container beanDefinitionMap Medium UserManager That's true

The next step is important , It is closely related to the answer we want

Loop recursive processing UserConfig 、 UserController and UserManager , Package them all into ConfigurationClass , Recursive scanning BeanDefinition

After the cycle , Let's see configClasses

UserConfig bean In definition information beanMethods There's an element in [BeanMethod:name=userManager,declaringClass=com.lee.qsl.config.UserConfig]

Then we went down , Let's take a closer look at the link where the answer appears

Did you find anything ？@Component Embellished UserManager The definition is directly overridden as @Configuration + @Bean Embellished UserManager Definition

Bean The type is also defined by ScannedGenericBeanDefinition replaced ConfigurationClassBeanDefinition

Subsequently passed BeanDefinition When creating an instance , The nature created is @Configuration + @Bean Embellished UserManager , That is, it will reflect the call UserManager The parametric construction method of

Since then , The answer is clear

Spring In fact, it gives a hint

2021-10-03 20:37:33.697 INFO 13600 --- [main] o.s.b.f.s.DefaultListableBeanFactory : Overriding bean definition for bean 'userManager' with a different definition: replacing [Generic bean: class [com.lee.qsl.manager.UserManager]; scope=singleton; abstract=false; lazyInit=false; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodName=null; destroyMethodName=null; defined in file [D:\qsl-project\spring-boot-bean-component\target\classes\com\lee\qsl\manager\UserManager.class]] with [Root bean: class [null]; scope=; abstract=false; lazyInit=false; autowireMode=3; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=userConfig; factoryMethodName=userManager; initMethodName=null; destroyMethodName=(inferred); defined in class path resource[com/lee/qsl/config/UserConfig.class]]

Only the log level is info , It's too inconspicuous

Spring Upgrade optimization

Probably Spring The team realized info The level is too inconspicuous , Or realize that the direct coverage is not reasonable

So in Spring 5.1.2.RELEASE （Spring Boot It is 2.1.0.RELEASE ） Optimization is made

Let's see in detail

Start direct error reporting ,Spring Tips are also given

The bean 'userManager', defined in class path resource [com/lee/qsl/config/UserConfig.class], could not be registered. A bean with that name has already been defined in file [D:qsl-projectspring-boot-bean-componenttargetclassescomleeqslmanagerUserManager.class] and overriding is disabled.

Let's follow the source code , Mainly look at and Spring 5.0.7.RELEASE The difference between

New configuration item allowBeanDefinitionOverriding To control whether BeanDefinition Cover , Not allowed by default

We can configure... In the configuration file ：spring.main.allow-bean-definition-overriding=true , allow BeanDefinition Cover

This treatment is better , Leave the choice to the developer , Instead of dealing with it secretly , It has achieved the effect that developers want

summary

Spring 5.0.7.RELEASE （ Spring Boot 2.0.3.RELEASE ） Support @Configuration + @Bean And @Component Act on the same class at the same time

When starting, it will give info Level log prompt , At the same time @Configuration + @Bean Embellished BeanDefinition overwrite @Component Embellished BeanDefinition

Maybe Spring The team realized that the above treatment was not appropriate , So in Spring 5.1.2.RELEASE Optimization is made

Added configuration item ：allowBeanDefinitionOverriding , Give the initiative to the developer , It's up to the developer to decide whether to allow coverage

Add

About allowBeanDefinitionOverriding , What I said earlier is wrong , Later, I went to look through the source code , Supplemented as follows

Spring 1.2 introduction DefaultListableBeanFactory When it's time private boolean allowBeanDefinitionOverriding = true;, The default is to allow BeanDefinition Cover

Spring 4.1.2 The introduction of isAllowBeanDefinitionOverriding() Method

Spring From beginning to end, the default is to allow BeanDefinition Covered , The change is Spring Boot , Spring Boot 2.1.0 Not covered before Spring Of allowBeanDefinitionOverriding The default value is , Still allowed BeanDefinition Covered

Spring Boot 2.1.0 in SpringApplication Defines private properties ：allowBeanDefinitionOverriding

The specified value is not displayed , So the default is false , After the Spring Boot During startup , Will overwrite with this value Spring Medium allowBeanDefinitionOverriding The default value of

About allowBeanDefinitionOverriding , I think you should have made it clear .