类型上的CDI引发NPE

Question

I am trying to understand CDI. I have an interface:

public interface Person {
    public String getName();
}

And a class that implements Person :

public class Male implements Person {
    public Male() {}
    @Override
    public String getName() {
        return "ra";
    }
}

In the main class, I tried:

public class Main {
    @Inject
    private Person person;
    public static void main(String... args) {
        System.out.println(new Main().person.getName());
    }
}

but the above code throws NPE on person.getName() . I created the project using maven and the scope of javaee api is compile . What am I doing wrong?

Answer 1

You miss to start a CDI container it seems.

Note that any annotation on a class is just like a post-it. It is really only an additional info and not real code. This information has to be interpreted. This is what the CDI container does. I've written up an introduction article to CDI which explains a little bit of the basics: https://jaxenter.com/tutorial-introduction-to-cdi-contexts-and-dependency-injection-for-java-ee-jsr-299-104536.html

What do you miss:

1.) add an empty META-INF/beans.xml file. Or add a scope annotation like @ApplicationScoped to your Male.java class.

2.) add a CDI container. There are multiple options. If you want to use the Apache OpenWebBeans CDI container then you can find some infos here https://openwebbeans.apache.org/owbsetup_se.html You can also find a working sample over here

• https://github.com/apache/openwebbeans/tree/trunk/samples/standalone-sample/pom.xml
• https://github.com/apache/openwebbeans/blob/trunk/samples/standalone-sample/src/main/java/org/apache/webbeans/se/sample/Boot.java#L68