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Tencent two sides: @bean and @component are used on the same class. What happens?

2022-06-30 12:29:00 【Java technology stack】

source ：cnblogs.com/youzhibing/p/15354706.html

Doubt background

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.

Spring Boot I won't introduce the foundation , Recommend this practical tutorial ：

https://github.com/javastacks/spring-boot-best-practice

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 ）. If you want to know more , You can read ：Spring The cycle of dependence , Detailed analysis of source code → Do you really need level 3 cache ？

https://www.cnblogs.com/youzhibing/p/14337244.html

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, You can see ：

https://www.cnblogs.com/youzhibing/p/10559337.html

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 the definition 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 [email protected] Embellished UserManager Definition .Bean The type is also defined by ScannedGenericBeanDefinition replaced ConfigurationClassBeanDefinition.

Subsequently passed BeanDefinition When creating an instance , The nature created is @[email protected] 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 .

Recommend a Spring Boot Basic tutorials and practical examples ：

https://github.com/javastacks/spring-boot-best-practice

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-project\spring-boot-bean-component\target\classes\com\lee\qsl\manager\UserManager.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 .

total junction

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 @[email protected] 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, Wrong front , I went to 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 .

Spring4.1.2 introduction 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 .