Does CDI work for regular Java application?

Question

I have simple maven project with three classes. In my META-INF I have my empty beans.xml. Everytime I run my Main.java I get a NullPointerException.

Exception in thread "main" java.lang.NullPointerException
at hom.World.helloWorld(World.java:12)
at hom.Main.main(Main.java:6)

Is what I'm trying to accomplish is way out of scope of what CDI is suppose to do?

Hello.java

package hom;
import javax.enterprise.inject.Default;

@Default
public class Hello {
   public String hello() {
      return "Hello ";
   }
}

World.java

package hom;
import javax.enterprise.inject.Default;
import javax.inject.Inject;

@Default
public class World {
   @Inject
   Hello hello;

   public String helloWorld() {
      return hello.hello() + "World!";
   }
}

Main.java

package hom;
public class Main {
   public static void main(String[] args) {
      World helloWorld = new World();
      System.out.println(helloWord.helloWorld());
   }
}

Answer 1

It is possible to use CDI in a standalone application.

Weld weld = new Weld();
WeldContainer container = weld.initialize();
World helloWorld = container.instance().select(World.class).get();
System.out.println(helloWorld.helloWorld());
weld.shutdown();

You will also need the weld-se dependency.

<dependency>
    <groupId>org.jboss.weld.se</groupId>
    <artifactId>weld-se-core</artifactId>
    <version>2.2.4.Final</version>
</dependency>

See also Weld documentation on the subject.

Answer 2

The following example works for CDI 2.

<dependency>
    <groupId>org.jboss.weld.se</groupId>
    <artifactId>weld-se-core</artifactId>
    <version>3.1.2.Final</version>
</dependency>

Dependency is the same.

package com.zetcode;

import com.zetcode.provider.HelloMessageProvider;
import com.zetcode.renderer.ConsoleMessageRenderer;
import com.zetcode.renderer.MessageRenderer;

import javax.enterprise.inject.se.SeContainer;
import javax.enterprise.inject.se.SeContainerInitializer;

public class CdiSimple {

    public static void main(String[] args) {

        SeContainerInitializer initializer = SeContainerInitializer.newInstance();

        try (SeContainer container = initializer.disableDiscovery()
                .addBeanClasses(HelloMessageProvider.class, ConsoleMessageRenderer.class).initialize()) {

            MessageRenderer messageRenderer = container.select(ConsoleMessageRenderer.class).get();
            messageRenderer.render();
        }
    }
}

We use SeContainerInitializer and SeContainer to set up CDI on Java SE.

package com.zetcode.provider;

public interface MessageProvider {

    String getMessage();
}

This is MessageProvider.

package com.zetcode.provider;

public class HelloMessageProvider implements MessageProvider {

    public String getMessage() {

        return "Hello there";
    }
}

This is HelloMessageProvider.

package com.zetcode.renderer;

public interface MessageRenderer {

    void render();
}

This is MessageRenderer.

package com.zetcode.renderer;

import com.zetcode.provider.MessageProvider;

import javax.inject.Inject;

public class ConsoleMessageRenderer implements MessageRenderer {

    @Inject
    private MessageProvider messageProvider;

    public void render() {

        var message = messageProvider.getMessage();

        System.out.println(message);
    }
}

And finally, this is ConsoleMessageRenderer.