Spring 中的 Java 类是否自动成为@Bean？

Question

If I create some classes, can I automatically @Autowire those classes somewhere else without using the @Bean , @Component , @Service ? Or if I want to @Autowire a class somewhere else, do I need to use the annotations described before?

I want to @Autowire a class to another that uses the @Configuration and @ComponentScan annotation

For example:

public class SomeClass{
    //do Something
}

or

@Bean
public class SomeClass{
     //do Something
}

to achieve this..

@Configuration
@ComponentScan("some.package")
public class SomeOtherClass{

    @Autowire
    private SomeClass someClass;

}

Answer 1

If i create some class, can i automatically @Autowire that class somewhere else without using the @Bean, @Component, @Service

No

You can read more about creating Spring beans .

Answer 2

The short answer is No , as you need to inform Spring as to which classes are meant to be "components" and which ones aren't. If this wasn't the case, it would become all to easy to initialise as components classes that are not meant for that purpose, causing all sorts of problems.

If you don't want to put a @Component annotation on your classes, however (say because you're building a library that is supposed to be framework-agnostic and you don't want to carry around dependencies to Spring), the users of your class will indeed have other options to initialise your classes and turn them into @Component instances.

One option is to use XML configuration:

https://docs.spring.io/spring/docs/4.2.x/spring-framework-reference/html/xsd-configuration.html

Another option is to create instances of your beans within classes annotated with @Configuration :

https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/annotation/Configuration.html

In the latter case, what you do is create a method (in which you will create a new instance of your class) and annotate it with `@Bean'.

Both approaches are valid and are used across all sorts of applications, my preference leaning towards the @Configuration annotation variant.

If you are 100% certain that the package in question will contain ONLY classes meant to be initialised as beans, you can write a method to scan the classes in the package, create instances for each class and register them as beans. I must say however that I don't see a valid use-case for such approach that, in fact, has probably more risks than benefits.

Answer 3

Bean is Java object living inside the Spring Container (Application Context).

Class is not a bean. Class will become the candidate of becoming bean, if you declare that either in XML or an annotation way ( @Component , @Service , @Controller or etc.). When Spring Container starts, it will scan for all the candidate-bean classes, and if everything is OK, candidates will become beans, meaning that they will be Java objects living inside a Spring Container . By default, they will be singletons.

Answer 4

Think that way: You have a Math exam and are not ready for the exam. So you decided to prepare some cheatSheet to remember some stuff during the exam.

    @Component   
    public class CheetSheet{
   //Some code to prepare Cheatsheet
    }

Let's say we have 2 Spring Classes

     @Component  
     public class Exam{
     // some code here
    }

You have Cheetsheet and Exam Spring Beans but what about preparing the piece of the sheet? So you may need helper class that it will read things from file(resources) and will give you the exact things you have problem(or you need)

 //Helper class

 public class HelpToPrepare{

 public String ReadFromFile(someList){
  // I am reading from a file method
  return listOfData;
 }

//I am sorting data and collecting the important ones that I have problem
 public String sortAndSave(){
  readFromFile();
  //some code for sort
  //some code for save
   return saveList;
}

So, the last class will not be part of Spring but it will help to sort out things or keep things that you will use them when you need.

Hope it will help