How to @Inject Property File Properties With CDI

Problem

You use CDI for your Java application and want to use a Property file for some needed configuration, but don't know how to do this...

Solution

Create @Producer(s) and an annotation to direct it.

Annotation

package nl.ivonet.config;

import javax.enterprise.util.Nonbinding;
import javax.inject.Qualifier;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

/**
 * Represents an property key to be injected
 */
@Qualifier
@Retention(RUNTIME)
@Target({TYPE, METHOD, FIELD, PARAMETER})
public @interface Property {
    @Nonbinding String value() default "";
    @Nonbinding boolean required() default true;
}

Producer

The examples provided here enable parsing of Strings, Booleans and Integers but you can, of course, add producers as needed. Note that the getKey method first looks if the @Property annotation has a value and if so it will take that as the key for the property file, but if you don't provide a value it will take the annotated field name as the key.

package nl.ivonet.config;

import javax.annotation.PostConstruct;
import javax.enterprise.inject.Produces;
import javax.enterprise.inject.spi.InjectionPoint;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class PropertyProducer {
    private Properties properties;

    @Property
    @Produces
    public String produceString(final InjectionPoint ip) {
        return this.properties.getProperty(getKey(ip));
    }

    @Property
    @Produces
    public int produceInt(final InjectionPoint ip) {
        return Integer.valueOf(this.properties.getProperty(getKey(ip)));
    }

    @Property
    @Produces
    public boolean produceBoolean(final InjectionPoint ip) {
        return Boolean.valueOf(this.properties.getProperty(getKey(ip)));
    }

    private String getKey(final InjectionPoint ip) {
        return (ip.getAnnotated()
                  .isAnnotationPresent(Property.class) && 
                !ip.getAnnotated()
                   .getAnnotation(Property.class)
                    value().isEmpty()) ? ip.getAnnotated()
                                           .getAnnotation(Property.class)
                                           .value() 
                                       : ip.getMember()
                                           .getName();
    }

    @PostConstruct
    public void init() {
        this.properties = new Properties();
        final InputStream stream = PropertyProducer.class
                                      .getResourceAsStream("/application.properties");
        if (stream == null) {
            throw new RuntimeException("No properties!!!");
        }
        try {
            this.properties.load(stream);
        } catch (final IOException e) {
            throw new RuntimeException("Configuration could not be loaded!");
        }
    }
}

Usage

  @Inject
  @Property
  private String rootFolder;

or

  @Inject
  @Property("root.folder.key.here")
  private String rootFolder;

application.properties

rootFolder=/helloWorldFolder
root.folder.key.here=/another/folder/name

Extra Consideration

Note that when using this method for injecting Strings and stuff it might get difficult to write Unit tests as Mocking frameworks cannot Mock final classes (e.g. String). If you get to this problem you might want to look at this article.

Have fun,

Ivo.