Abstract: With the fast iteration of software version and the rapid development of software scale,software design and quality issues have aroused wide spread concern in the IT field.Using complex network theory to study the characteristics of software systems has become an important way to solve these problems.Characterizing software source code dependencies as a network,with the complex network method,the structure of the macroscopic level of the code can be deeply understood and the overall evolution trend can be grasped,which help the developers optimize the software architecture and make it better.As an open-source mainstream Java EE application server,Tomcat has been widely used in industry applications.Based on the complex network method,the conclusion comes out that Tomcat’s class dependency networks of all history versions satisfy the nature of small-world network and scale-free network.By deeply analyzing the evolution process of the nine versions,this paper found that Tomcat shows preferential attachment phenomenon,so it can maintain the robustness of software.

Key words: Complex network theory, Preferential attachment, Robustness, Software evolution, Tomcat