Google App Engine - Restrictions

Google offers a cloud computing infrastructure called Google App Engine (App Engine) for creating and running web Applications. App Engine allows the dynamic allocation of system resources for an application based on the actual demand. Currently App Engine supports Python and Java based applications.

But Google App engine have some Restrictions

GoogleApp Engine runs a version of Java 6 but does not provide all Java classes, for example Swing and most AWT classes are not supported.

You cannot use Threads or frameworks which uses Threads. You can also not write to the filesystem and only read files which are part of your application. Certain "java.lang.System" actions, e.g. gc() or exit() will do nothing. You can not call JNI code. Reflection is possible for your own classes and standard Java classes but your cannot use reflection to access other classes outside your application.
A servlet needs also to reply within 30 seconds otherwise a com.google.apphosting.api.DeadlineExceededException" is thrown.

See   JRE Class White List