getting null values from bean

Question

Hi I am trying to understand autowiring in beans , I am able to autowire but still getting null values here is code snippets

Edited entire code with current output

 @Component
public class A {
    private String value;
    private B b;

    public void display() {
        System.out.println(value);
        System.out.println(b.m());
    }

    public String getValue() {
        return value;
    }


    public void setValue(String value) {
        this.value = value;
    }

    public B getB() {
        return b;
    }

    @Autowired
    public void setB(B b) {
        this.b = b;
    }

}

B class

@Component
public class B {

    private String b;

    public void setB(String b) {
        this.b = b;
    }

    public String m() {
        return b;
    }

}

configuration.java

@Service
@ComponentScan(basePackageClasses = A.class)
public class AppConfig {

    @Autowired
    public A setBean() {
        A a = new A();
        a.setValue("inside A");
        return a;
    }

    @Autowired
    public B setB() {
        B b = new B();
        b.setB("inside B");
        return b;
    }
}

Main

 @ContextConfiguration(classes = AppConfig.class)
public class Application {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext configApplicationContext = new AnnotationConfigApplicationContext(
                AppConfig.class);
        A app = (A) configApplicationContext.getBean("a");
        app.display();
    }

}

Now the question is, I am supposed to get all values but instead of that I am getting null value from each bean , So why I am getting this value as null and how can I sort this?

Edit

the output is

log4j:WARN No appenders could be found for logger (org.springframework.core.env.StandardEnvironment). log4j:WARN Please initialize the log4j system properly. log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info. null null

Edit After debugging it i found that , in config class it is passing values as specified but when it comes to display values in display() it gives null

Answer 1

Possible causes of your receiving null values:

1. @Component should be written with a capital letter and the proper annotation from the Spring package must be imported;

2. Class 'ApCfg' should look like this:

   @Configuration // this annotation is needed by Spring to know it is a config class
   @ComponentScan(basePackages = { "java" }) // you have to add the package NAMES here, not a regex
   public class ApCfg {
   
   @Bean // here, you're telling Spring to register a bean in its container, so you have to mark it as so, by using the @Bean annotation
   public A getA() {
       A a = new A(); // constructors are methods too, so they need to be called, using paranthesis
       a.setAValue("asdf"); // setters usually return void, so you shouldn't be able to return this
       return a; // this instance here is what will be registered in the container
    }
   }
   

If you've annotated class A with @Component, then you shouldn't be able to register it twice, so you should choose a way - either use the @Component annotation, or define it manually in the Config class in a method annotated by @Bean.

Also, I tend to agree that your choice of package name is rather peculiar. Are you certain that the package definition at the top of your classes is:

package java;

?

If you don't have any statement starting with the word package, then all you're classes are in the default package and, in that case, I strongly recommend grouping them in real packages instead.

Answer 2

It seems you've not quite grasped the difference between @Autowired, @Bean, and @Component.

• @Autowired means that a field or method should be set to or called with autowired (injected) beans.
• @Bean is used in a class marked with @Configuration to tell Spring about a bean that can be autowired
• @Component is used on a class to tell Spring that this class should be instantiated as a bean that can be autowired.

In your code you've marked both A and B as @Components which means that they will be instantiated as beans using their default constructors, using default values.

You've then tried to instantiate these as beans (again) in AppConfig but used @Autowired to mark the definitions which means that Spring won't pick them up.

You need to choose one way if telling Spring about your beans, so you can either remove @Component from A and B and use @Bean instead of @Autowired on the methods that create them in AppConfig, or remove the bean definitions completely from AppConfig and initialise the members inside A and B upon construction.

First option:

// No @Component here
public class A {
    ...
}

// No @Component here
public class B {
    ...
}

@Configuration
// You probably don't need @ComponentScan anymore since
// no classes are annotated with @Component
@ComponentScan(basePackageClasses = A.class)
public class AppConfig {

    @Bean
    public A setBean() {
        A a = new A();
        a.setValue("inside A");
        return a;
    }

    @Bean
    public B setB() {
        B b = new B();
        b.setB("inside B");
        return b;
    }
}

Second option:

@Component
public class A {
    private String value = "inside A";
    ...
}

@Component
public class B {
    private String b = "inside B";
    ...
}

@Configuration
@ComponentScan(basePackageClasses = A.class)
public class AppConfig {
    // No bean definitions here, but required for @ComponentScan
}