Spring @Autowired not working

Question

I have some problems wth autowire annotation. My app looks like this:

Here is controller:

@Controller
public class MyController {
    @Autowired
    @Qua         


        

Answer 1

You should include this section of XML code in spring-config.xml :

<context:component-scan base-package="Fully.Qualified.Package.Name" />

but you should know the difference between <context:annotation-config> vs <context:component-scan> as most people are suggesting these two :

1) First big difference between both tags is that <context:annotation-config> is used to activate applied annotations in already registered beans in application context. Note that it simply does not matter whether bean was registered by which mechanism e.g. using <context:component-scan> or it was defined in application-context.xml file itself.

2) Second difference is driven from first difference itself. It does register the beans in context + it also scans the annotations inside beans and activate them. So <context:component-scan>; does what <context:annotation-config> does, but additionally it scan the packages and register the beans in application context.

Answer 2

Important points:

1. Sometimes, @Component may leads to a problem where it might say no default constructor found. The class which is defined as a @Component annotation, it must have a default constructor.
2. Suppose, we have applied @Autowired annotation at field which is a user defined class reference. Now, if we also apply @Component to that class then it will always be initialized with null. So, a field with @Autowired should not have @Component at its class definition.
3. By default @Autowired is byType.

Address bean is autowired at Student class. Let’s see what happens if we apply @Component at Address.java.

CollegeApp.java:

package com.myTest
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import com.bean.Address;
import com.bean.Student;
//Component scanning will for only those classes
//which is defined as @Component. But, all the class should not use
//@Component always even if the class is enabled with auto
//component scanning, specially the class which is Autowired
//Or which is a property of another class 
@Configuration
@ComponentScan(basePackages={"com.bean"})
public class CollegeApp {
    @Bean
    public Address getAddress(){
        return new Address("Elgin street");
}
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(CollegeApp.class);
        Student student=context.getBean(Student.class);
        System.out.println(student.toString());
        context.close();
    }
}

We want Elgin street to be autowired with Student address.

Address.java:

package com.bean;
import org.springframework.stereotype.Component;
@Component
public class Address {
    private String street;
    public Address()
    {
    }
    public Address(String theStreet)
    {
        street=theStreet;
    }
    public String toString()
    {
        return (" Address:"+street);
    }
}

Student.java:

package com.bean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class Student {
    private String name;
    private int age;
    private Address address;
    public Student()
    {
    }
    public Student(String theName,int theAge)
    {
        name=theName;age=theAge;
    }
    @Autowired
    public void setAddress(Address address) {
        this.address = address;
    }
    public String toString()
    {
        return ("Name:"+name+" Age:"+age+ " "+address);
    }
}

Output: - Name:null Age:0 Address:null //Address not Autowired here.

To resolve the issue, only change the Address.java as below:

Address.java:

package com.bean;
public class Address {
    private String street;
    public Address(String theStreet)
    {
        street=theStreet;
    }
    public String toString()
    {
        return (" Address:"+street);
    }
}

Output:- Name:null Age:0 Address:Elgin street

Answer 3

I found the solution. As Javi said (thanks a lot for you, Javi), I have to annotate DAO and Service layer classes with @Repository and @Service annotation. Now I've tried to write like this:

@Service("someService")
public class SomeServiceImpl implements SomeService{    
    @Autowired
    @Qualifier("myDAO")
    private MyDAO myDAO;

    ....
}

and

@Repository("myDAO")
    public class JDBCDAOImpl implements MyDAO {    
    @Autowired
    @Qualifier("dataSource")
    private DataSource dataSource;    
    ....
}

and all works fine!!!

But I still not found an answer for this quesion: if application will be more complex, and will have more complex structure, where @Repositore and @Service annotation are not preferred for some classes, how to inject correctly beans, which located in lower levels (in a fields of classes, or in a field of fields of classes) (with @Autowire annotation, of course)?

Answer 4

There can be two reasons for this.

1. When you have not annotated the injected object or say service with proper @Service/@Component/@Repository annotations.

2. Once you have made sure of point 1 ,next check for whether the class package of your annotated service class is included in the class-path for your spring boot application in the main class.You can configure this using the following annotation.

@SpringBootApplication(scanBasePackages = { "com.ie.efgh.somepackage","com.ie.abcd.someotherpackage" })

Doing this you tell spring to look into the packages for the classes during class loading.

Answer 5

I guess you need <context:annotation-config />.

Answer 6

You can use

<context:component-scan base-package="PATH OF THE BASE PACKAGE"/>  

entry in your configuration .xml file. This entry will scan/read all the stated type and annotations from the java classes.