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What type literal must I use to have CDI's Instance::select method work properly?

Suppose I have an interface like this:

public interface Converter<T> { /*...*/ }

And suppose in a CDI environment I have successfully done this:

@Inject
@Any
private Instance<Converter<?>> converters;

(By "successfully" I mean that I can do the following and see several converters in the output, so beans are being discovered and supplied properly:

for (final Object o : converters) {
  System.out.println("*** converter: " + o);
}

…so bean discovery is not the issue.)

And now suppose that given Integer.class, I'd like to do this:

final TypeLiteral<Converter<Integer>> typeLiteral = new TypeLiteral<Converter<Integer>>(){};
final Instance<Converter<Integer>> subInstance = converters.select(typeLiteral);
final Converter<Integer> converter = subInstance.get();

This works fine.

Now, in my actual code, Integer.class is passed in, as a value fulfilling a parameter declared as Class<T>, so what I really have is this:

final TypeLiteral<Converter<T>> typeLiteral = new TypeLiteral<Converter<T>>(){};
final Instance<Converter<T>> subInstance = converters.select(typeLiteral);
final Converter<T> converter = subInstance.get(); // this does not work

The get() call fails with a stack trace that starts with something that looks like the following:

org.jboss.weld.exceptions.UnsatisfiedResolutionException: WELD-001334: Unsatisfied dependencies for type Converter<T> with qualifiers @Any 
    at org.jboss.weld.bean.builtin.InstanceImpl.get(InstanceImpl.java:105)

What must I do to make this selection succeed?

One thing I notice is that the stack is reporting that a Converter<T> cannot be found. This looks suspicious: I would have expected it to talk in terms of Converter<Integer> instead, since the T "slot" is being "filled" with Integer.class at runtime, although, to be fair, I did indeed supply a new TypeLiteral<Converter<T>>(){}, not new TypeLiteral<Converter<Integer>>(){}.

Anyway, all this tells me that TypeLiteral<T> is using T as the type to look for, not the actual value "filling" the T "slot", and indeed, there is no converter declared as implements Converter<T>, only a converter declared as implements Converter<Integer>, and so I'm worried that what I'd like to do here is fundamentally impossible.

Solution

  • Creating a TypeLiteral to capture generic parameters only works if those parameters are known at compile time, so new TypeLiteral<Converter<Integer>>(){}.

    If the type parameter is not known at compile time then a TypeLiteral cannot capture the parameter information because that information has been removed due to type erasure. So creating a new TypeLiteral<Converter<T>>(){} actually just creates a new TypeLiteral<Converter<object>>(){}.

    This means your select(typeLiteral) will not work as expected since it will receive the type literal for Converter<object>.