I am working on a proprietary java application. I want to use a java library licensed LGPL 2.1. One condition of the license is to disclose source code. What does it exactly mean? Do I need to disclose the source code of my full application?
Yes, if the LGPL-covered JAR is linked in a way so that users are free to modify it, then you don't have to disclose your own source code. You do need to publish the source code for the library though, see the license for options.
The easiest way to comply is if you provide the source code of the LGPL-covered JAR alongside the JAR file. Also, you should not prevent the JAR from being changed by users, so it would be best not to sign the JAR file.
Note also that the GPL and LGPL only trigger their requirements when you give someone else a copy of (L)GPL covered software. These licenses do not impose any requirements on software that you use purely internally, or if the software provides services to others but is only executed on your servers. So for a Java Servlet or for an internall app you wouldn't have to do anything, but you would have to follow the license for an app or program that you make available to other businesses or people.