Spring Component versus Bean

Question

I understand that  Component annotation was introduced in spring 2 5 in order to get rid of xml bean definition by using classpath scanning    Bean was introduced in spring 3 0 and can be used with  Configuration in order to fully get rid of xml file and use java config instead   Would it have been possible to re-use the  Component annotation instead of introducing  Bean annotation  My understanding is that the final goal is to create beans in both cases

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Component Preferable for component scanning and automatic wiring    When should you use  Bean    Sometimes automatic configuration is not an option  When  Let s imagine that you want to wire components from 3rd-party libraries  you don t have the source code so you can t annotate its classes with  Component   so automatic configuration is not possible    The  Bean annotation returns an object that spring should register as bean in application context  The body of the method bears the logic responsible for creating the instance

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You can use  Bean to make an existing third-party class available to your Spring framework application context    Bean public ViewResolver viewResolver          InternalResourceViewResolver viewResolver   new InternalResourceViewResolver         viewResolver setPrefix   WEB-INF view         viewResolver setSuffix   jsp         return viewResolver      By using the  Bean annotation  you can wrap a third-party class  it may not have  Component and it may not use Spring   as a Spring bean  And then once it is wrapped using  Bean  it is as a singleton object and available in your Spring framework application context  You can now easily share reuse this bean in your app using dependency injection and  Autowired    So think of the  Bean annotation is a wrapper adapter for third-party classes  You want to make the third-party classes available to your Spring framework application context   By using  Bean in the code above  I m explicitly declare a single bean because inside of the method  I m explicitly creating the object using the new keyword  I m also manually calling setter methods of the given class  So I can change the value of the prefix field  So this manual work is referred to as explicit creation  If I use the  Component for the same class  the bean registered in the Spring container will have default value for the prefix field   On the other hand  when we annotate a class with  Component  no need for us to manually use the new keyword  It is handled automatically by Spring

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Component and  Bean do two quite different things  and shouldn t be confused    Component  and  Service and  Repository  are used to auto-detect and auto-configure beans using classpath scanning  There s an implicit one-to-one mapping between the annotated class and the bean  i e  one bean per class   Control of wiring is quite limited with this approach  since it s purely declarative    Bean is used to explicitly declare a single bean  rather than letting Spring do it automatically as above   It decouples the declaration of the bean from the class definition  and lets you create and configure beans exactly how you choose   To answer your question        would it have been possible to re-use the  Component annotation instead of introducing  Bean annotation     Sure  probably  but they chose not to  since the two are quite different  Spring s already confusing enough without muddying the waters further

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When you use the  Component tag  it s the same as having a POJO  Plain Old Java Object  with a vanilla bean declaration method  annotated with  Bean   For example  the following method 1 and 2 will give the same result   Method 1   Component public class SomeClass        private int number       public SomeClass Integer theNumber           this number   theNumber intValue               public int getNumber            return this number            with a bean for  theNumber     Bean Integer theNumber        return new Integer 3456       Method 2    Note  no  Component tag public class SomeClass        private int number       public SomeClass Integer theNumber           this number   theNumber intValue               public int getNumber            return this number            with the beans for both    Bean Integer theNumber        return new Integer 3456       Bean SomeClass someClass Integer theNumber       return new SomeClass theNumber       Method 2 allows you to keep bean declarations together  it s a bit more flexible etc  You may even want to add another non-vanilla SomeClass bean like the following    Bean SomeClass strawberryClass        return new SomeClass new Integer 1

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You have two ways to generate beans     One is to create a class with an annotation  Component    The other is to create a method and annotate it with  Bean  For those classes containing method with  Bean should be annotated with  Configuration   Once you run your spring project  the class with a  ComponentScan annotation would scan every class with  Component on it  and restore the instance of this class to the Ioc Container  Another thing the  ComponentScan would do is running the methods with  Bean on it and restore the return object to the Ioc Container as a bean   So when you need to decide which kind of beans you want to create depending upon current states  you need to use  Bean  You can write the logic and return the object you want   Another thing worth to mention is the name of the method with  Bean is the default name of bean

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Additional Points from above answers  Let   s say we got a module which is shared in multiple apps and it contains a few services  Not all are needed for each app   If use  Component on those service classes and the component scan in the application       we might end up detecting more beans than necessary   In this case  you either had to adjust the filtering of the component scan or provide the configuration that even the unused beans can run  Otherwise  the application context won   t start   In this case  it is better to work with  Bean annotation and only instantiate those beans       which are required individually in each app   So  essentially  use  Bean for adding third-party classes to the context  And  Component if it is just inside your single application

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I see a lot of answers and almost everywhere it s mentioned  Component is for autowiring where component is scanned  and  Bean is exactly declaring that bean to be used differently  Let me show how it s different    Bean  First it s a method level annotation  Second you generally use it to configure beans in Java code  if you are not using xml configuration  and then call it from a class using the ApplicationContext getBean method  Example   Configuration class MyConfiguration       Bean     public User getUser             return new User             class User                    Getting Bean  User user   applicationContext getBean  quot getUser quot       Component  It is the general way to annotate a bean and not a specialized bean  It is a class level annotation and is used to avoid all that configuration stuff through java or xml configuration  We get something like this   Component class User         to get Bean  Autowired User user   That s it  It was just introduced to avoid all the configuration steps to instantiate and use that bean

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Difference between Bean and Component

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Let s consider I want specific implementation depending on some dynamic state   Bean is perfect for that case    Bean  Scope  prototype   public SomeService someService         switch  state        case 1          return new Impl1        case 2          return new Impl2        case 3          return new Impl3        default          return new Impl              However there is no way to do that with  Component

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Bean was created to avoid coupling Spring and your business rules in compile time  It means you can reuse your business rules in other frameworks like PlayFramework or JEE    Moreover  you have total control on how create beans  where it is not enough the default Spring instantation   I wrote a post talking about it   https   coderstower com 2019 04 23 factory-methods-decoupling-ioc-container-abstraction

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component and its specializations  Controller   service   repository  allow for auto-detection using classpath scanning  If we see component class like  Controller   service   repository will be scan automatically by the spring framework using the component scan   Bean on the other hand can only be used to explicitly declare a single bean in a configuration class    Bean used to explicitly declare a single bean  rather than letting spring do it automatically  Its make septate declaration of bean from the class definition  In short  Controller   service   repository are for auto-detection and  Bean to create seprate bean from class        -  Controller     public class LoginController        --code--        -  Configuration     public class AppConfig        Bean     public SessionFactory sessionFactory         --code--

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Both approaches aim to register target type in Spring container   The difference is that  Bean is applicable to methods  whereas  Component is applicable to types   Therefore when you use  Bean annotation you control instance creation logic in method s body  see example above   With  Component annotation you cannot

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1  About  Component  Component functs similarily to  Configuration  They both indicate that the annotated class has one or more beans need to be registered to Spring-IOC-Container  The class annotated by  Component  we call it Component of Spring  It is a concept that contains several beans  Component class needs to be auto-scanned by Spring for registering those beans of the component class   2  About  Bean  Bean is used to annotate the method of component-class as mentioned above   It indicate the instance retured by the annotated method needs to be registered to Spring-IOC-Container   3  Conclusion The difference between them two is relatively obivious  they are used in different circumstances   The general usage is           Configuration is implemented by  Component      Configuration     public ComponentClass           Bean       public FirstBean FirstBeanMethod             return new FirstBean                    Bean       public SecondBean SecondBeanMethod             return new SecondBean

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Component auto detects and configures the beans using classpath scanning whereas  Bean explicitly declares a single bean  rather than letting Spring do it automatically   Component does not decouple the declaration of the bean from the class definition where as  Bean decouples the declaration of the bean from the class definition   Component is a class level annotation whereas  Bean is a method level annotation and name of the method serves as the bean name   Component need not to be used with the  Configuration annotation where as  Bean annotation has to be used within the class which is annotated with  Configuration   We cannot create a bean of a class using  Component  if the class is outside spring container whereas we can create a bean of a class using  Bean even if the class is present outside the spring container   Component has different specializations like  Controller   Repository and  Service whereas  Bean has no specializations

[java] Spring: @Component versus @Bean

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