deployment.system.maxAppletThreadCount is not documented and sometimes it has to be changed to accommodate applications that start many threads when migrating to Java 7 or later.

On JDK 7, deployment.system.maxAppletThreadCount is enforced and if an applet starts more than 50 threads (the default value) a new JVM is started. This causes performance issues for applications that can't handle applets in multiple JVMs and the work around until the application is fixed is to increase deployment.system.maxAppletThreadCount for the application to run in the same JVM.