NullPointerException when using @Inject annotation

Question

I got this error even thought I @Inject Message object in MyClass. I just try to learn it now how can I Inject an object.

Please help me, Thanks!

Here are my example.

Main:

public class Main {

    public static void main(String [] arg){
        new MyClass().print();
    }

}

MyClass

import javax.inject.Inject;

public class MyClass {

    @Inject private Message message;

    public void print() {
        System.out.println(message.getMessage());
    }

}

Message

public class Message {

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

}

What do you add to tell the DI framework that the parent class MyClass is under its control? — duffymo, Jan 24 '18 at 21:06
I just using javax from javaee-api. Do I need some Framework like Spring? — Balázs Fodor-Pap, Jan 24 '18 at 21:08
@duffymo I don't think this is quite a duplicate of the question you've marked. The confusion is with DI, not with the presence of an NPE. — Gian, Jan 24 '18 at 21:08
@BalázsFodor-Pap Yes, you'll need a framework. `javax.inject` just provides a standardized set of interfaces and annotations for specific frameworks to use. It is not a DI implementation itself. — Gian, Jan 24 '18 at 21:10
All right, thank you all. And which framework should i learn first? — Balázs Fodor-Pap, Jan 24 '18 at 21:14
I suggested two popular ones in my answer. The skills you'll learn from one are largely transferable to others, so I wouldn't stress too much about the specifics. — Gian, Jan 24 '18 at 21:15
After the good answers i using [Guice](https://github.com/google/guice/wiki/Motivation) and i can injecting the Message object . Thanks again! — Balázs Fodor-Pap, Jan 24 '18 at 21:55

score 2 · Accepted Answer · answered Jan 24 '18 at 21:06

You've not got anything that I can see which would interpret or make use of the @Inject annotation. You'll need some kind of DI framework present that will actually do the dependency injection for you. Perhaps consider looking at the minimal examples for something like Guice or Spring?

If you analyze your current program, you're just declaring a field (which is defaulting to null), and never setting it to anything, hence the NPE.

NullPointerException when using @Inject annotation

1 Answers1