Dependency Injection in a Java 7 standalone application

Question

I would like to use dependency injection in a large Java 7 standalone application, but I am not really sure where to start.

I have written a small test application:

public class Main {

    @Inject
    MyInterface myInterface;

    public static void main( String[] args ) {

        Main m = new Main();
        System.out.println(m.myInterface.getMessage());

    }

}

with an interface:

public interface MyInterface {

    String getMessage();

}

and an interface implementation:

@Singleton
public class MyInterfaceImpl implements MyInterface {

    public String getMessage() {
        return "Hello World!";
    }

}

The pom.xml contains one dependency:

<dependencies>
    <dependency>
        <groupId>javax</groupId>
        <artifactId>javaee-api</artifactId>
        <version>7.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

This application compiles, but of course, it crashes with a NPE when trying to print the message. The injection has not happened.

So, my question are:

1. Can dependency injection be achieved in a Java 7 standalone application?
2. What other dependencies do I have to include to make it work?
3. Does anyone have a simple operational example to share (I could not find any)?

Answer 1

There is not only one way to use dependency injection with Java.

(1) You could for instance use the standard CDI, where the reference implementation is Weld. There is documentation about using Weld in a Java SE environment, what is probably what you mean by standalone application.

You could alternatively also use Spring Framework, which also supports the common CDI annotation (e.g. @Inject). In this case, you will typically create a ClasspathXmlApplicationContext at the program startup and let Spring manages (create/destroy) all the beans you need.

(2) You current dependencies only imports the API of Java EE. Thus I'm not surprised if you get a NullPointerException at the execution. You need to add an implementation (like Weld) or use Spring.

(3) See links above.

Also take a look at Differences between Java EE 6 CDI Implementations to get reference about other Java CDI available implementations.