This thesis addresses the subject of developing extensible software in Java. In the introductory part, the analysis of the process of software development is presented as well as the main reasons why the majority in the IT business world decide to develop extensible software. In the following part we have the two types of extensible software and the most used design pattern in that area (plugins). Analysis of the widely used framework for developing extensible software (OSGi) were made and the facts behind the reasons for not using that framework and developing our own are presented. In the central part of the thesis a deep analysis of our newly developed framework with emphasis on the main problem in developing such framework - security has been made. In the final part we present the usage example to prove that the framework is almost invisible to the end user, and that it doesn't add complexity to the application.