According to the App Engine documentation, you can set system properties and environment variables for your app in the deployment descriptor and App Engine sets the following system properties when it initializes the JVM on an app server:
* file.separator
* path.separator
* line.separator
* java.version
* java.vendor
* java.vendor.url
* java.class.version
* java.specification.version
* java.specification.vendor
* java.specification.name
* java.vm.vendor
* java.vm.name
* java.vm.specification.version
* java.vm.specification.vendor
* java.vm.specification.name
* user.dir
Why would you want to set any of these system properties?
Might be used as a workaround to make existing libraries work which may depend on particular values for these properties.