Is there a central javadoc repository online?

I'm aware this is a very old question, but sometimes old questions warrant new answers.

http://www.javadoc.io/

Free javadoc hosting for any open source Central Maven Repository project.

Seems http://www.jarvana.com/jarvana/doc is currently down. You can try to use JavaDoc.io:

http://www.javadoc.io/doc/$org/$artifact/$version

For example. Spring-aop version 4.0.5.RELEASE

http://www.javadoc.io/doc/org.springframework/spring-aop/4.0.5.RELEASE

But the limitation is it only hosts JavaDocs for Open Source project released to central maven repository.

Jarvana Javadoc

EDIT:
As you might notice all the examples pointed out by you and others sites mentioned in answers don't do theirs job because it's hard to find all new api and track all api changes and present it in reasonable manner (plethora of versions). Even dzone.com child jdocs.com serves javadoc for out of date versions. Jarvana as a Maven focused search engine offers a great solution to find appropriate javadoc if it deployed to Maven repositories. We as developers have to influence library creators to deploy their software to Maven repositories with javadoc generated and support creator of Jarvana :)