MyApp.groovy

package com.example

import ratpack.guice.Guice
import ratpack.server.RatpackServer

public class MyApp {
  public static void main(String[] args) throws Exception {

    RatpackServer.start { spec ->
      spec.registry {
        Guice.registry { r ->
          r.bind(MyService.class)
        }
      }
      .handlers { chain ->
        chain.get { ctx ->
          MyService myService = ctx.get(MyService.class)
          myService.myMethod()
          ctx.render("Finished!")
        }
      }
    }
  }
}

MyApp.java

package example;

import ratpack.guice.Guice;
import ratpack.server.RatpackServer;

public class MyApp {

  public static void main(String[] args) throws Exception {
    RatpackServer.start(spec -> spec
      .registry(Guice.registry(r -> r
        .bind(MyService.class)
      ))
      .handlers(chain -> chain
        .get(ctx -> {
          MyService myService = ctx.get(MyService.class);
          myService.myMethod();
          ctx.render("Finished");
        })
      ));
  }
}

MyService.java

package com.example

class MyService {
  void myMethod() {
    System.out.println("calling myMethod");
  }
}

If I run the MyApp.java application everything works fine. I can see the message calling myMethod in the console and the Finished message rendered in the browser.

If I run the MyApp.groovy application I get this error:

java.lang.ClassCastException: ratpack.guice.Guice$$Lambda$57/1865859824 cannot be cast to ratpack.registry.Registry
  at ratpack.server.internal.DefaultRatpackServer$DefinitionBuild.lambda$new$242(DefaultRatpackServer.java:169)
  at ratpack.server.internal.ServerRegistry.buildUserRegistry(ServerRegistry.java:81)
  at ratpack.server.internal.ServerRegistry.serverRegistry(ServerRegistry.java:70)
  at ratpack.server.internal.DefaultRatpackServer.buildServerRegistry(DefaultRatpackServer.java:292)
  at ratpack.server.internal.DefaultRatpackServer.buildAdapter(DefaultRatpackServer.java:265)
  at ratpack.server.internal.DefaultRatpackServer$ReloadHandler.lambda$null$257(DefaultRatpackServer.java:494)
  at ratpack.exec.Blocking$1.lambda$get$122(Blocking.java:70)
  at ratpack.exec.Blocking.intercept(Blocking.java:241)
  at ratpack.exec.Blocking.access$000(Blocking.java:35)
  at ratpack.exec.Blocking$1.get(Blocking.java:68)
  at ratpack.exec.Blocking$1.get(Blocking.java:61)
  at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
  at ratpack.exec.internal.DefaultExecController$ExecControllerBindingThreadFactory.lambda$newThread$135(DefaultExecController.java:113)
  at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:137)
  at java.lang.Thread.run(Thread.java:745)

Any idea why this happens?

Yes, just change to

public class MyApp {
  public static void main(String[] args) throws Exception {
    RatpackServer.start { spec ->
      spec.registry(
        Guice.registry { r ->
          r.bind(MyService.class)
        }
      )
      .handlers { chain ->
        chain.get { ctx ->
          MyService myService = ctx.get(MyService.class)
          myService.myMethod()
          ctx.render("Finished!")
        }
      }
    }
  }
}

Excellent! :)