Support Assisted Injection via Factory Codegen

[SentryMan]

• when a bean is annotated with @AssistFactory a factory class is generated which will handle all managed injections while accepting user dependencies
• supports constructor, field, and method assisted injection

Note the example below has been edited to reflect the built behaviour.

Given a factory interface with a single method:

public interface CssFactory {

  Scanner scanner(String path);
}

A class with @AssistFactory(CssFactory.class):

@AssistFactory(CssFactory.class)
class CssScanner implements Scanner {

  private final Path path;
  private final MyInjectedComponent myComponent;

  CssScanner(@Assisted Path path, MyInjectedComponent myComponent) {
    this.path = path;
    this.myComponent = myComponent;
  }

  @Override
  public String scan() {
    return "scanWith|path=" + path + " | myComponent =" + myComponent.hi();
  }
}

An assist factory class will be generated. (Naturally, it too will be processed and have a DI class created)

@Generated("io.avaje.inject.generator")
@Named("css")
@Component
final class CssScanner$AssistFactory implements CssFactory {

  private final MyInjectedComponent myComponent;

  CssScanner$AssistFactory(MyInjectedComponent myComponent) {
    this.myComponent = myComponent;
  }

  /**
   * Fabricates a new CssScanner.
   */
  @Override
  public CssScanner scanner(Path path) {
    var bean = new CssScanner(path, myComponent);
    return bean;
  }
}

Then the CssFactory can be used like:.

@Component
public class CssThing {

  private final CssFactory factory;

  CssThing(CssFactory factory) {
    this.factory = factory;
  }

  public String scan(Path path) {
    Scanner scanner = factory.scanner(path);
    return scanner.scan();
  }
}

will resolve #466

[SentryMan]

Hmm, I can do better

[rbygrave]

Adding tests. Noting that when there is no target factory the generated AssistedFactory can't be injected into other components (with compilation error I believe because it does not exist in the first compilation round).

Thinking that we should make the target factory required at this stage - that feels like the correct thing to do at this stage. Later on if there is demand for not having a target factory interface then that can be revisited / looked at again.

[SentryMan]

it does not exist in the first compilation round

hmm, this highlights another potential problem in that inject doesn't know how to handle beans that directly use generated code.