Java Class File Byte Sequence

Java Class File Byte Sequence

Java class files can be recognized by a magic byte sequence that is
required at the beginning of every class file. The pitfall associated with
this is that Java class files may come as part of a compressed archive.
Due to the nature of compression, nothing in the archive (even its name
can be changed) exposes the fact that it contains Java class files. Class
files that are part of an archive cannot be detected by this technique. In
addition, class files may be passed via an encrypted connection, which
will make them indistinguishable from ordinary files to the firewall.

Java Class Extension

Java class files can be recognized by their .class filename extension. The
pitfall associated with this is that depending on the browser, this may
either not be the case, or, again can be circumvented by sending applets
as part of an archive.