A minimal and modern web framework for Java

What's the advantage of Jooby over Spark[1]? The latter seems more concise and doesn't require the use of initializer blocks[2] (a not-very-well-known feature of Java).

[1] http://sparkjava.com/

[2] https://docs.oracle.com/javase/tutorial/java/javaOO/initial....

How does it compare to Ratpack http://ratpack.io ?