Question

I have a maven project with the following structure:

spring-di-test
 +--spring-di-test-core
 |     com.example.core.DITestMain
 +--spring-di-test-store
       com.example.store.DITest
       com.example.store.RandomStringService

where spring-di-test is the root project and the two below are modules.

My classes look like this:

DITestMain located in spring-di-test-core

public class DITestMain {
    public static void main(String[] args) {
        new DITest().run();
    }
}

applicationContext.xml located in spring-di-test-core’s resources folder

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
                           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
                           http://www.springframework.org/schema/context 
                           http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:spring-configured/>
    <context:component-scan base-package="com.example.*" annotation-config="true"/>

</beans>

DITest located in spring-di-test-store

@Configurable(preConstruction = true)
@Controller
public class DITest {   
    @Autowired(required=true)
    private RandomStringService randomStringService;

    public void run() {
        System.out.println(randomStringService.getRandomString());
    }
}

RandomStringService located in spring-di-test-store

@Service("randomStringService")
public class RandomStringService {

    private final Random random;

    public RandomStringService() {
        random = new Random();
    }

    public String getRandomString() {
        StringBuilder sb = new StringBuilder();
        int length = random.nextInt(20);
        for (int i = 0; i < length + 1; i++) {
            sb.append(new Character((char) ('a' + random.nextInt(20))));
        }
        return sb.toString();
    }
}

applicationContext.xml located in spring-di-test-store

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
                           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
                           http://www.springframework.org/schema/context 
                           http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:spring-configured/>
    <context:component-scan base-package="com.example.*" annotation-config="true"/>

</beans>

When I run DITestMain, I get a NullPointerException for randomStringService. What went wrong?

Answer 1

1: You’re not creating the Spring context in your main method

2: Have you done all the required configuration to enable load-time weaving (so that your @Configurable bean will work)?

Take a look at http://static.springsource.org/spring/docs/3.0.x/reference/aop.html#aop-using-aspectj and read the entire chapter closely.