[FIXED] Java spring noob beans and configuration

Issue

I'm trying to run the fallowing classes.

package com.example.demo;

import org.apache.catalina.core.ApplicationContext;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {

        var factory = new AnnotationConfigApplicationContext(AppConfig.class);
        test tst = factory.getBean(test.class);
        tst.getDummy().test();
        SpringApplication.run(DemoApplication.class, args);
    }

}

App config class:

package com.example.demo;

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

@Configuration
public class AppConfig {
    @Bean
    public test gettest(){
        return new test();
    }
//
    @Bean
    public test2 gettest2(){
        return new test2();
    }
}

First test class:

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;

@Component
public class test{
    @Autowired
    test2 dummy;

    public void setDummy(test2 dummy) {
        this.dummy = dummy;
    }

    public test2 getDummy() {
        return dummy;
    }

    void test(){
        System.out.println("test");
    }
}

Second test class:

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.stereotype.Component;

@Component
public class test2{

    public test2(){

    }
    void test(){
        System.out.println("test2");
    }

}

I get this error at run time

Field dummy in com.example.demo.test required a single bean, but 2 were found:
    - test2: defined in file [/Users/biliuta/IdeaProjects/demo/build/classes/java/main/com/example/demo/test2.class]
    - gettest2: defined by method 'gettest2' in class path resource [com/example/demo/AppConfig.class]

If i comment out the public test2 gettest2() bean from AppConfig I'll get this error:

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'gettest': Unsatisfied dependency expressed through field 'dummy'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.demo.test2' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

How come in the first case test2 bean is found and when I comment it out from AppConfig it is unable to find it anymore (in the first error message you can clearly see that test2 is found)? Is @Configure annotation mandatory to add beans to the context?

Solution

how @Autowired works is it first try to find the required bean by type and a fallback mechanism is by bean name(ex: there are multiple beans for the same type)

but when you disable the @Bean annotation from AppConfig there should have been at least one bean of type Test2 as you have annotated Test2 with @Component annotation but it can't find it. To solve this you need to add @ComponentScan(basePackageClasses = { Test2.class }) on your AppConfig class as follows

@Configuration
@ComponentScan(basePackageClasses = { Test2.class })
public class AppConfig {
@Bean
public Test gettest(){
    return new Test();
}

//@Bean
public Test2 gettest2(){
    return new Test2();
}
}

but if you enable the bean annotation as follows

@Configuration
@ComponentScan(basePackageClasses = { Test2.class })
public class AppConfig {
@Bean
public Test gettest(){
    return new Test();
}

@Bean
public Test2 gettest2(){
    return new Test2();
}
}

you need to change the injection of Test2 on Test as by using

 @Autowired
 Test2 test2;

or by using

 @Autowired
 Test2 gettest2;

Answered By - Tanmoy Majumdar
Answer Checked By - Timothy Miller (JavaFixing Admin)