Reconstructing generic types at runtime with Guice via Types and TypeLiterals

Question

I have a few types that are like this

// a value that is aware of its key type (K)
Bar
// something that deals with such values and keys
Foo

        

Answer 1

From the JavaDoc for Binder:

Guice cannot currently bind or inject a generic type, such as Set<E> all type parameters must be fully specified.

You can create bindings for Foo when K and V are bound. If you need to make bindings for Foo for more than one type of key, you can make a method that makes it easier to do these bindings. One way to do that is to create a method like this in your module:

<K, V extends Bar<K>> AnnotatedBindingBuilder<Foo<V, K>> bind(Class<K> keyType,
    Class<V> barType) {
  ParameterizedType bType = Types.newParameterizedType(Bar.class, keyType);
  ParameterizedType fType = Types.newParameterizedType(Foo.class, barType,
      keyType);

  @SuppressWarnings("unchecked")
  TypeLiteral<Foo<V, K>> typeLiteral =
      (TypeLiteral<Foo<V, K>>) TypeLiteral.get(fType);

  return bind(typeLiteral);
}

Then if you have these classes:

class StringValue implements Bar<String> {
  ...
}

class StringValueProcessor implements Foo<StringValue, String> {
  ...
}

You can create a binding like this:

bind(String.class, StringValue.class).to(StringValueProcessor.class);

...so that Guice could inject into a class like this:

static class Target {
  private final Foo<StringValue, String> foo;

  @Inject
  public Target(Foo<StringValue, String> foo) {
    this.foo = foo;
  }
}

Answer 2

Guice's factory cannot build TypeVariable instances. You'll need to implement this interface directly as you need it.

Note that Guice doesn't allow bindings for types that aren't fully-qualified. For example, you can bind a Map<String, Integer> but you can't bind a Map<K, V>.