There are quite a few configuration libraries available in Java, such as this one available from Apache Commons, and they usually follow a very similar pattern: they parse a variety of configuration files and in the end, give you a Property or Map like structure where you can query your values:

Double double = config.getDouble("number");
Integer integer = config.getInteger("number");

I have always been unsatisfied with this approach for a couple of reasons:

  • A lot of boiler plate to retrieve these parameters.
  • Having to share the whole configuration object even if I only need one parameter from it.
  • It’s very easy to misspell a property and received incorrect values.

A while ago, I was reading the Guice documentation and I came across a paragraph that made me realize that maybe, we could do better. Here is the relevant excerpt:

Guice supports binding annotations that have attribute values. In the rare case that you need such an annotation:

Create the annotation @interface.
Create a class that implements the annotation interface. Follow the guidelines for equals() and hashCode() specified in the Annotation Javadoc. Pass an instance of this to the annotatedWith() binding clause.

I thought that using this technique might be exactly what I needed to create a smarter configuration framework, even though I had different plans than using this trick with the annotatedWith method, as suggested by this paragraph. The relevance of this snippet will become clear later, so let’s start with the goals.

Objectives

I want to:

  • Be able to inject individual configuration values anyhere in my code base and I want this to be type safe. No @Named or other string-based lookup.
  • Have a canonical list of all the properties available to the application, with their full type, default value, documentation and leaving the door open for improvements (e.g. is this option mandatory or optional, detecting when some properties are not used anywhere, deprecation, aliasing, etc…).

I don’t care much about the front end: how these properties get gathered is not relevant to this framework, they can come from XML, JSON, the network, a database, and they can have arbitrarily complex resolution and overriding rules, let’s save this for a future post. The input of this framework is a Map of properties and I take it from there.

By the time we’re done, we will be able to do something like this:

    # Some property file
    host=foo.com
    port=1234

Using these configuration values in your code:

public class A {
    @Inject
    @Prop(Property.HOST)
    private String host;

    @Inject
    @Prop(Property.PORT)
    private Integer port;

    // ...
}

Implementation

The definition of the Prop annotation is trivial:

@Retention(RUNTIME)
@Target({ ElementType.FIELD, ElementType.PARAMETER })
@BindingAnnotation
public @interface Prop {
  Property value();
}

Property is an enum that captures all the information necessary for all your properties. In our case:

public enum Property {
    HOST("host",
        "The host name",
        new TypeLiteral<String>() {},
        "foo.com"),

    PORT("port",
        "The port",
        new TypeLiteral<Integer>() {},
        1234);
}

This enum contains the string name of the property, a description, its default value and its type. Note that this type is a TypeLiteral, so we can even offer properties that have generic types that would otherwise be erased, a trick that comes in handy to inject caches or other generic collections. Obviously, you can have additional parameters as you see fit (e.g. “boolean deprecated“).

The next step is to tie all the properties that we parsed as input — we’ll use a Map called "allProps" — into our module so that Guice knows how to inject them.

In order to do this, we iterate all these properties and bind them to their own provider. Because we are using typed names, note the use of Key.get from the Guice API, which lets us specifically target each property with a specific annotation:

    for (Property prop : Property.values()) {
        Object value = PropertyConverters.getValue(prop.getType(), prop, allProps.asMap());
        binder.bind(Key.get(prop.getType(), new PropImpl(prop)))
                .toProvider(new PropertyProvider(prop, value));
    }

There are three classes in this piece of code that I haven’t explained yet. The first one is PropertyConverters, which simply reads the string version of the property and converts it to a Java type. The second one is PropertyProvider, a trivial Guice provider:

public class PropertyProvider implements Provider {
    private final T value;
    private final Property property;

    public PropertyProvider(Property property, T value) {
        this.property = property;
        this.value = value;
    }

    @Override
    public T get() {
        return value;
    }
}

PropImpl is more tricky and also the one thing that has always prevented me from implementing such a framework, until I came across this obscure tidbit of the Guice documentation quoted above. In order to understand the necessity of its existence, we need to understand how Guice’s Key.get() works. Guice uses this class to translate a type into a unique key that it can use to inject the correct value. The important part here is to notice that not only does this method work with both Class and TypeLiteral (which we are using), but it can also be given a specific annotation. This annotation can be @Named, which I’m not a big fan of because it’s a string, so susceptible to typos, or a real annotation, which is what we want. However, annotations are special beasts in Java and you can’t get an instance of them just like that.

This is where the trick mentioned at the top of this article comes into play: Java actually allows you to implement an annotation with a regular class. The implementation turns out to be fairly trivial, the difficulty was realizing that this was possible at all.

Now that we have all this in place, let’s back track and dissect how the magic happens:

    @Inject
    @Prop(Property.HOST)
    private String host;

When Guice encounters this injection point, it looks into its binders and it finds multiple bindings for Strings. However, because they have all been bound with a Key, the key is actually a pair: (String, a Prop). In this case, it will look up the pair String, Property.HOST and it will find a provider there. This provider was instantiated with the value found in the property file, so it knows what value to return.

Generalizing

Once I had the basic logic in place, I wondered if I could turn this mini framework into a library so that others could use it. The only missing piece would be to allow the specification of a more general Prop annotation. In the example above, this annotation has a value of type Property, which is specific to my application:

@Retention(RUNTIME)
@Target({ ElementType.FIELD, ElementType.PARAMETER })
@BindingAnnotation
public @interface Prop {
    Property value();
}

In order to make this more general, I need to make this attribute return an enum instead of my own:

@Retention(RUNTIME)
@Target({ ElementType.FIELD, ElementType.PARAMETER })
@BindingAnnotation
public @interface Prop {
    Enum value();
}

Unfortunately, this is not legal Java, because according to the JLS section 8.9, Enum and its generic variants are not enum types, something that Josh Bloch confirmed, to my consternation.

Therefore, this cannot be turned into a library, so if you are interested in using it in your project, you will have to copy the source and make a few modifications to adjust it to your needs, starting by having Prop#value have the type of the enum that captures your configuration.

You can find a small proof of concept here, which I hope you’ll find useful.

Note: this is a copy of the article I posted on our work blog.