To detect theft of Java class files efficiently, we have so far proposed a concept of Java birthmarks. Since the birthmarks are unique and native characteristics of every class file, a class file with the same birthmark of another can be easily suspected as a copy. However, performance and tolerance of the birthmarks against sophisticated attacks had not been evaluated well. To clarify these issues, this paper conducts two experiments. In the first experiment, we demonstrate that the proposed birthmarks successfully distinguish non-copied files in practical Java application (97.8005%). The second experiment shows that the proposed birthmarks are quite tolerant of attacks with automatic program optimizers/obfuscators (93.3876%)