I want to know how to integrate a Java web application with OpenCms contents. Is it possible to integrate a Java application with OpenCms?
Yes, it's possible. For example you can integrate SpringMVC or Struts into OpenCms, or other tools like JForum, an open source java forum, which also supports single-sign-on (SSO).
You can find information here & a small sample OpenCms+Spring integration project:
Since OpenCms is open source and based on Java EE standards, you are pretty free.