what is the usage of Set<Class<?>> resources = new java.util.HashSet<>();

Question

What is the usage of this code line in the rest api Set<Class<?>> resources = new java.util.HashSet<>();

@ApplicationPath("/service")
public class ApplicationConfig extends Application {
@Override
public Set<Class<?>> getClasses() {

    Set<Class<?>> resources = new java.util.HashSet<>();

    System.out.println("REST configuration starting: getClasses()");            

    //features
    //this will register Jackson JSON providers
    resources.add(org.glassfish.jersey.jackson.JacksonFeature.class);



    //more code.....
}

Answer 1

The Application class defines the components of a JAX-RS application. Subclasses of Application can override the getClasses() to register a set of root resource, provider and feature classes used by the application.

The simplest implementation possible is as following:

@ApplicationPath("api")
public SampleApplication extends Application {

}

In the example above no resources classes or providers are registered, so the JAX-RS runtime will scan the classpath for JAX-RS components annotated with @Path and @Provider and will register them automatically.

See this answer for details.

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Set<Class<?>> means a Set that holds Classes of unknown types (it's expressed by the ? wildcard). In The Java Tutorials from Oracle you'll find a whole section about generics. I encourage you to have a look.