helidon/docs-internal/generic-type.md

Helidon Generic Type Support Proposal

Provide a utility class to express generic type literals and provide API guidance for the Helidon APIs (config, webserver etc).

Problematic

Currently (v0.10.5) entity mapping is done with as methods that accept a Class<T> argument:

request.content().as(Dog.class).thenAccept((Dog dog) -> { /* ... */ });
config.get("doggy").as(Dog.class).ifPresent((Dog dog) -> { /* ... */ });

Generic type mapping is in most cases needed for collections, List and Map.

This can be solved by having more variants of the as methods:

config.get("doggies").asList(Dog.class).ifPresent((List<Dog> dogs) -> { /* ... */ });

This model enables the most common use-cases of generic type mapping, however it limits the generic type mapping to just List<T> and Map<T> ; there is simply no way of mapping a custom parameterized type (e.g. Pet<Dog>).

In Java there is no way to express a generic type as a literal. I.e List<Dog>.class is not valid. This is a known problem that has been solved by many popular Java frameworks.

E.g.

• javax.ws.rs.GenericType
• javax.enterprise.util.TypeLiteral

Proposal

Make a copy of javax.ws.rs.GenericType under io.helidon.common.GenericType.

Overload the existing as(Class<T> type) methods with as(GenericType<T> type).

Retain or add the shorthand asList(Class<T> type) to satisfy the most common use-cases. Remove every other shorthand if any.

Examples

Get the request content as List<JsonObject>:

request.content().asList(JsonObject.class)
    .thenAccept((List<JsonObject> jsons) -> { /* ... */ });

Get the request content as Pet<Dog>:

request.content().as(new GenericType<Pet<Dog>>(){})
    .thenAccept((Pet<Dog> pet) ->  { /* ... */ });

Notes

java.lang.reflect.Type

Changing as(Class<T> type) for as(Type type) could potentially work as a single signature for both use-cases.

as(Dog.class)
as(new GenericType<Pet<Dog>>(){}.getType())

However since Type is not parameterized we would loose the ability to provide a fluent API. The example below does not compile:

// does not compile
request.content().as(new GenericType<Pet<Dog>>(){}.getType())
    .thenAccept((Pet<Dog> pet) ->  { /* ... */ });