microprofile-config custom ConfigSource using JPA

Question

I am currently trying to setup a custom ConfigSource reading config values from our DB2. As the ConfigSources are loaded via ServiceLoader it looks like there is no way to access the database via JPA as the ServiceLoader is scanning for custom ConfigSources very early.

Any ideas?

Answer 1

You can anotate your ConfigSource as a singleton session bean and mark it for eager initialization during the application startup sequence. Also you need to define a static member variable holding your config values.

With this setup you can lazy load your properties values from an injected JPA source or also from any other CDI or EJB.

See the following example Code

@Startup
@Singleton
public class MyConfigSource implements ConfigSource {

    public static final String NAME = "MyConfigSource";
    public static Map<String, String> properties = null; // note to use static here!

    @PersistenceContext(unitName = ".....")
    private EntityManager manager;

    @PostConstruct
    void init() {
        // load your data from teh JPA source or EJB
        ....
    }

    @Override
    public int getOrdinal() {
        return 890;
    }

    @Override
    public String getValue(String key) {
        if (properties != null) {
            return properties.get(key);
        } else {
            return null;
        }
    }

    @Override
    public String getName() {
        return NAME;
    }

    @Override
    public Map<String, String> getProperties() {
        return properties;
    }

}

ConfigSources are POJO’s because if a CDI bean expected config to be injected into it at startup based on a ConfigSource that had dependencies on CDI’s you could get into startup looping issues.

For this reason the example CongigSoruce is constructed twice - once at the beginning from the Config-API and later from the CDI implemenation on @PostConstruct. With the static variable 'properties' we overload the values from the already constructed ConfigSource. Of course you can also separate the code in two classes if you like.