Fix JSON sources to convert to Lists and other types

[Michael1993]

Closes #654

Proposed commit message:

Fix JSON sources to convert to Java types (#654 / #664)

The initial implementation of the JSON sources for ParameterizedTest
was missing the functionality to map from JSON to more complex Java
types like List. This commit fixes that by using Jackson's
ObjectMapper and JavaType.

Closes: #654 
PR: #664

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[filiphr]

Sorry for intruding, but since I was working on the Json support I wanted to chime in.

I believe that this approach is a bit strict and it has a dependency on Jackson.

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Jackson has support for converting to a generic type with the help of its JavaType. This JavaType can be constructed from a java.lang.reflect.Type.

This means that new methods (or the existing methods) of the Node can look like:

<T> T toType(Type type);
Object value(Type typeHint);

Everything in the end delegates to toType(Type) which is implemented in the following way:

@Override
public <T> T toType(Type type) {
	try {
		return objectMapper.treeToValue(node, objectMapper.constructType(type));
	}
	catch (JsonProcessingException e) {
		throw new UncheckedIOException("Failed to convert to type " + type, e);
	}
}

As a final step all the places that were invoking the toType and value methods can be changed from doing Parameter#getType to Parameter#getParameterizedType.

There won't be a need to have a custom argument converter and it would be possible to map into things like Map<String, Poem> or List<Map<String, Poem>> etc.

[filiphr]

The implementation looks as I would expect now @Michael1993.

I've left some comments about some more general confusions I have.

One thing that you should decide within the team is the breaking change in Node.java. From my point of view it is fine to do that change, since this is a new interface and I really doubt that someone has implemented an entire new JSON parsing on top of JUnit Pioneer without contributing it back.

[Michael1993]

One thing that you should decide within the team is the breaking change in Node.java.

This PR contains no breaking changes as all classes/interfaces modified are package-private.

[filiphr]

This PR contains no breaking changes as all classes/interfaces modified are package-private.

I didn't notice that. They used to be public when I wrote them initially

[Bukama]

As far as I understand all those reflection things it looks good to me, but I mostly trust @filiphr. When I have a look at the documentation I think there's no need in updating it. But please have a second look at it @Michael1993

[Bukama]

Adding merge-ready to trigger full build

[beatngu13]

Thanks a lot Michael for working on this!

I only have two "take it or leave it" comments. In addition, do you think it make sense to add an example to the docs/demos?