I must setup your blog engine that "needs to be integrated having a java application". They are my needs, I am not kidding. The java application is really a web application that serves very dynamically transformed content.

Just what your blog which application are likely to exchange - I don't know, because my client does not know. (yes, I understand, my client is ..., but this is actually the situation).

I believe I've two options: a java blog engine and wordpress.

Wordpress is popular, stable, easy to customize, ..., however in PHP. Java blogs aren't popular, however in Java.

Inside a previous project I had been utilizing a .Internet blog, known as dasBlog, it wasn't popular and never easily easy to customize. I'd rather not make that mistake again.

Basically can have the ability to fully control a Wordpress blog (publish posts, edit sidebar contents, make wordpress request me for content for any sidebar, ...) via JAX-RPC or Relaxation and when that's straightforward, maybe I'd choose Wordpress.

What is your opinion?

It isn't that uncommon to mix a java-application having a wordpress installation (I've done this myself at least one time). Just front by having an apache, running php and wordpress and also have your java application running "behind" by itself context. Tricky parts are if you want to share periods and logins.

For wordpress-java integration I discovered these:

Quercus (runs PHP): http://wordpress.tv/2008/10/27/spreading-the-word-to-java-bring-wordpress-and-java-together-with-quercus-slides/

Java API for wordpress http://code.google.com/p/wordpress-java/ (This really is my personal favorite. however , I'm not sure how effective it's)

java port of wordpress: http://www.theserverside.com/news/thread.tss?thread_id=50890 (I do not like that one)

you may also try apache-roller, its no match towards the UI of wordpress. however it can complete the job...