Reconstructing generic types at runtime with Guice

Question

I have a few types that are like this

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

How would one recreate Foo such that you could consume it in Guice? The bit I'm stuck on is how to cross reference the K from Bar to the 2nd parameterized type of Foo.

So for example,

WildcardType kType = Types.subtypeOf(Object.class);
WildcardType barType = 
   Types.subtypeOf(Types.newParameterizedType(Bar.class, pipeKey));
ParameterizedType fooType = 
   Types.newParameterizedType(Foo.class, pipelineableType, pipeKey);

Really this seems wrong as it's basically:

Foo<V extends Bar<? extends Object>, ? extends Object> 

Which is not the same thing as:

Foo<V extends Bar<K>, K>

As in the latter case I know that K is a consistent type.

Any ideas?

Cheers

Matt

Answer 1

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>.

Answer 2

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;
  }
}