Simple Spring Java @Configuration to @Autowire without @Bean

Question

I want to make use of spring @Autowired in a java rest project. For the last days, I am trying to set up a simple spring java project with java configuration without explicit bean configuration to check that functionality out. But I can't get it to work. I may be missing something fundamental.

None of the approaches I found in the web and on this site solved my problem so far. I couldn't find a sample for exactly what I'm trying to achieve too. This is mainly due to the amount of different spring versions and approaches spread over the web.

Here is an as easy as I could come up with Java Spring rest sample. I added a few comments with how I interpret spring annotations, as I may err here too:

App base class

package restoverflow;

import javax.ws.rs.ApplicationPath;
import javax.ws.rs.core.Application;

@ApplicationPath("/")
public class App extends Application {

}

Config class

package restoverflow;

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

@Configuration  //this is a configuration class and also found by spring scan
@ComponentScan  //this package and its subpackages are being checked for components and its subtypes
public class AppConfig {

}

Some Pojo

package restoverflow;

public class Pojo {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

A service

package restoverflow;

import org.springframework.stereotype.Service;

@Service //this is a subtype of component and found by the componentscan
public class PojoService {
    public Pojo getPojo(){
        Pojo pojo = new Pojo();
        pojo.setName("pojoName");
        return pojo;
    }
}

And finally a resource where the autowiring of the service should be done

package restoverflow;

import javax.ws.rs.GET;

import org.springframework.beans.factory.annotation.Autowire;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.stereotype.Service;

import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

@Path("/resource")
@Controller //this is a subtype of component and found by the componentscan
public class Resource {

    @Autowired //this tells to automatically instantiate PojoService with a default contructor instance of PojoService
    private PojoService pojoService;

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public Pojo getPojo() {
        return pojoService.getPojo();
    }
}

Pom:

...
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>5.1.7.RELEASE</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-core</artifactId>
    <version>5.1.7.RELEASE</version>
</dependency>
...

I want the pojoService to be instantiated. But I get a NullPointerException.

Answer 1

It looks like you are using Field level injection.

Please go through below link to understand all types of injections: https://www.vojtechruzicka.com/field-dependency-injection-considered-harmful/

Can't see any clear reason why pojoService is coming null. Please check whether pojoService bean is being initialized properly. It might be due to pojoService bean has not been initialized and you are getting null in your controller.

Answer 2

A nullpointer instead of a NoSuchBeanDefinitionException is more of an indication that the Spring context is not loaded at all, rather than loaded improperly.

If you're using Spring boot, modify your main class to initialize Spring:

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

Or else (as the pom.xml snippet did not mention Spring boot), initialize Spring manually by initializing a ClassPathXmlApplicationContext and adding <context:component-scan base-package="restoverflow" /> in your applicationContext.xml .