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Spring: @Component versus @Bean

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.

Is there anywhere @Bean can be used apart from Configuration class?
@Willa Yes, there is. That's called Lite mode. And it is not recommended. See here: docs.spring.io/spring/docs/current/spring-framework-reference/…
I'd summarize it saying a method with @bean returns a customizable instance of spring bean, while @component defines a class that may be later instanciated by spring IoC engine when needed.
@Willa yes, @Bean can be used in inside a class annotiated with @Component. "You can use the @Bean annotation in a @Configuration-annotated or in a @Component-annotated class." docs.spring.io/spring-framework/docs/current/reference/html/…

S
Sebas

@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.


I think this makes the most sense. If I understand correctly @Component goes on classes themselves while @Bean goes on class methods (which yield instances of class objects).
I was in a limbo, until I read what if you don't have the source codes ? And then, kablaam! Of course, @Component if you own the source code, but if ever you want to componentize other classes of which you don't have the source code, then @Bean. I'm sure there are other differences, but this, ladies and gentlemen, nails it.
Simple and straight, thanks mate.
I still didn't get this - if you don't have the source code, how does spring knows the about the class that we have @Beaned?
@ajaysinghnegi: You don't own the code in the libraries that your app depends on, and yet you (and Spring) know about the classes in those libraries.
s
skaffman

@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.


So I can only use @Component when autowired is needed? It seems @Bean can not affect @Autowired
use '@component' for service based classes, '@Bean' as factory more tailor made objects, e.g jdbc datasource
@Jaskey you can use @Autowired with @Bean if you have annotated your bean class with @Configuration
Sorry but I can't understand a word of your explanation. You clearly understand this so please would you write a clear explanation or point to the appropriate documentation?
Now that I understand the concept (from reading other people's answers), your explanation makes sense. Which tells me all the more that your explanation is no good to anyone who doesn't already understand the concepts.
o
outdev

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.


How you you call that example class?
@PowerFlower This method should be in a configuration class, annotated with @Configuration
@Lookup public SomeService getSomeService(){ return null; } Cuz it is prototype scoped
U
Unmitigated

@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.


4. Actually @Bean could be declared in non-configuration class. It's known as lite mode
Regarding point 5. I think we put a bean inside the spring container. So, every class is outside the spring container. I guess, point 5 should be rewarded
It is the one of the most answering question of spring boot.
Point 5. What is the definition of "outside of Spring container"?
@FuadEfendi If the class is in a library, so you cannot annotate the source code. But you can instantiate it and make a Bean out of it.
C
Community

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.


What is a type?
@JacFrall: Simply put, a type is a class. Any instance of that class is of that class' type. So if you have a class "Dog" and create a Dog rex = new Dog(), then the object named "rex" is of type "Dog", because it's an instance of the "Dog" class.
Not just that. Those beans handled by spring differently. Just got into trouble. @ConditionalOnBean(SomeBean) doesn't work, if SomeBean is made of @Bean annotation. But it works if SomeBean is @Component. This even doesn't work despite @Bean=> SomeBean method is called and bean is created and added to the context. Still, @ConditionaOnBean unable to see this bean (or may be order of beans created using @Bean vs (@Component @Service @Respository @Service @Configuration) differs and @Bean annotated methods are called at the end of application context after scanning is done).
P
Paulo Merson

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("getUser");

@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.


I think it is not necessary to get the User object from ApplicationContext when you use @Bean approach. You can still use @Autowire to get the bean as you would do in case of @Component. @Bean just adds the Bean to the Spring Container just as would @Component does. The difference is as follows. 1. Using @Bean, you can add Third Party Classes to Spring Container. 2. Using @Bean, you can get the desired implementation of an interface at run-time(Using factory design pattern)
@Andy You can still use @ Autowire to get the bean as you would do in case of @ Component how ? When you use @ Autowired and User user, it will take the default implementation right. How to code it to take specific implementation.
There are a fews ways you can achieve this. Let's say you hav Employee interface & 2 implementations EmployeeImpla & EmployeeImplb. Let's say you have a class called SalaryImpl which is annotated as @Component and you want to use EmployeeImplA implementation in that class. OPTION 1 you can do something like Employee employeeImplA OPTION 2 @Qulaifier("employeeImplA") Employee employee
e
elvis

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.


Would be nice if this answer was updated with an example of how that bean is used as well
How would you wrap an @Bean over a third party class if the source code does not allow modification?
usage ``` @AutoWired ViewResolver viewResolver ```
S
SomeGuy

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));
}

D
Dai Niu

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.


A
AzarEJ

Difference between Bean and Component:

https://i.stack.imgur.com/e8RlG.png


for 3rd party libs, I think @ Bean is possible as @ Bean does not require class definition to be present inside Spring container, on the contrary @ Component is not possible in this case.
a
alok

@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-- }

D
Daniel Andres Pelaez Lopez

@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/


P
Partho

Spring supports multiple types annotations such as @Component, @Service, @Repository. All theses can be found under the org.springframework.stereotype package.

@Bean can be found under the org.springframework.context.annotation package.

When classes in our application are annotated with any of the above mentioned annotation then during project startup spring scan(using @ComponentScan) each class and inject the instance of the classes 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.

@Component

If we mark a class with @Component or one of the other Stereotype annotations these classes will be auto-detected using classpath scanning. As long as these classes are in under our base package or Spring is aware of another package to scan, a new bean will be created for each of these classes.

package com.beanvscomponent.controller;

import org.springframework.stereotype.Controller;

@Controller
public class HomeController {

    public String home(){
       return "Hello, World!";
   }

 }

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. It is also important to note that the stereotype annotations are class level annotations.

@Bean

@Bean is used to explicitly declare a single bean, rather than letting Spring do it automatically like we did with @Controller. It decouples the declaration of the bean from the class definition and lets you create and configure beans exactly how you choose. With @Bean you aren't placing this annotation at the class level. If you tried to do that you would get an invalid type error. The @Bean documentation defines it as:

Indicates that a method produces a bean to be managed by the Spring container.

Typically, @Bean methods are declared within @Configuration classes.We have a user class that we needed to instantiate and then create a bean using that instance. This is where I said earlier that we have a little more control over how the bean is defined.

package com.beanvscomponent;

public class User {

private String first;
private String last;

public User(String first, String last) {
    this.first = first;
    this.last = last;
   }
}

As i mentioned earlier @Bean methods should be declared within @Configuration classes.

package com.beanvscomponent;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class ApplicationConfig {

@Bean
public User superUser() {
    return new User("Partho","Bappy");
   }

}

The name of the method is actually going to be the name of our bean. If we pull up the /beans endpoint in the actuator we can see the bean defined.

{
"beans": "superUser",
"aliases": [],
"scope": "singleton",
"type": "com.beanvscomponent.User",
"resource": "class path resource 
    [com/beanvscomponent/ApplicationConfig.class]",
"dependencies": []
}

@Component vs @Bean

https://i.stack.imgur.com/zJpux.png

I hope that cleared up some things on when to use @Component and when to use @Bean. It can be a little confusing but as you start to write more applications it will become pretty natural.


Please make a small project on it. I dont understand how you will call the specific implementation of the User class
ס
סטנלי גרונן

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();
      }
    }

A
AzarEJ

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.