Springboot，使用Rest Controller从对象访问字段

Question

Having a bit of trouble with using Springboot to create an API, and understanding how Beans work.

I have three classes, and I have simplified them as much as I can;

ClassA

public class ClassA()
{
    private variable neededVar;

    public ClassA()
    {
        //initialise other variables;
    }

    public start()
    {
        /initialise neededVar;
    }

    @Bean
    public variable getNeededVar()
    {
        return neededVar;
    }
}

Application

@SpringBootApplication
public class Application
{
    private static ClassA myClass;

    public static void main( String[] args )
    {
        myClass = new ClassA();
        ClassA.start();

        SpringApplication.run( Application.class, args );
    }
}

Controller

@RestController
public class Controller
{
    @Autowired
    private variable neededVar;

    @RequestMapping( "/temp" )
    public string getVar()
    {
        return neededVar.toString();
    }
}

My problem is that in the controller, I am not getting the neededVar from the created ClassA object myClass, I'm actually not sure what I'm getting.

I also tried doing it this way, with similar results.

@SpringBootApplication
public class Application
{
    private static ClassA myClass;
    private static variable myNeededVar;

    @Bean
    public variable getNeededVar()
    {
        return myNeededVar;
    }

    public static void main( String[] args )
    {
        myClass = new ClassA();
        myNeededVar = myClass.getNeededVar();
        ClassA.start();

        SpringApplication.run( Application.class, args );
    }
}

If anyone could point me in the correct direction of getting the neededVar from the instantiated ClassA in the application into my rest controller (and subsequently all the others I will create), that would be very much appreciated.

Thanks!

Answer 1

• The spring doesn't exactly work in a way you are expecting. You should delegate object's lifecycle to spring. ie You do not need to manually create ClassA object and call its start method.
• @Bean annotation is used to notify spring that you want to create an object of a class that was not part of spring's autoscan mechanism.
• In order to delegate class object management to spring, class should be either annotated with @Component or it's other variants such as @Controller , @Service etc. or you should tell spring how to create object of that class using @Bean annotation.

To show how you can use @Component and @Bean , I'll mark Class A as component and inject Variable with @Bean .

 @Component
    public class ClassA()
    {
         /*
          * Notice this autowired annotation. It tells spring to insert Variable object.
          * What you were trying to do with getNeededVar() is done using Autowired annotation
          */
        @Autowired
        private Variable neededVar;

    }

Now tell spring how to create object to Variable as it's not marked as @Component

@SpringBootApplication
public class Application
{
    public static void main( String[] args ) {
        SpringApplication.run( Application.class, args );
    }

  // This is how you register non spring components to spring context.
  // so that you can autowire them wherever needed
   @Bean
   public Variable variable() { 
      return new Variable();
   }
}

Your rest controller code stays as is. As Variable is registred to spring via @Bean in SpringApplication class, you don't really need ClassA . You can delete it.

@RestController
public class Controller
{
    @Autowired
    private Variable neededVar;

    @RequestMapping( "/temp" )
    public string getVar()
    {
        return neededVar.toString();
    }
}