Greenfoot launch problems

Greenfoot uses TCP/IP socket communication as part of its normal operation. The communication occurs between two processes running on your computer - one is for Greenfoot itself, and the other is for a "debug VM" which runs your scenario and most of the Greenfoot user interface. Although this communication doesn't go over the physical network, some firewall software will block this communication which inhibits Greenfoot's operation. When that happens, you will see an error message that says something like:

Greenfoot was unable to create a virtual machine (VM) to execute scenarios. Often this is caused by overly restrictive firewall settings. Please see [ http://www.bluej.org/help/faq.html#socket-exc ] for more information, or contact support@greenfoot.org.

or:

Greenfoot failed to launch. For help go to http://greenfoot.org/programming/support/index.html

This page contains information on how you can resolve this problem yourself. If you try the solutions described here without success, you should contact the support team for additional assistance.

Cause of the problem

Usually, this problem is caused by specific network settings on your machine. Greenfoot uses TCP/IP and sockets to communicate with the debug VM. The most common cause for this problem is an overly restrictive firewall that does not allow TCP/IP connections from your local host. Switch off the firewall (if you have one) to test this. If this solves it, try to find a firewall setting that allows connections to and from the 127.0.0.1 "loopback" address.

Why does Greenfoot need to access the internet to run?

It doesn't. Greenfoot uses the TCP/IP mechanisms for communicating between two processes which both run on your computer. Some security software actually blocks this communication, which prevents Greenfoot from being able to start.

Greenfoot also sends some anonymous data (such as Operating System, Java version and Greenfoot version) to a server controlled by the Greenfoot team, for planning purposes. Blocking this transmission does not prevent Greenfoot from running.

Configuring firewall software to resolve the problem

Firewalls generally take one of two approaches to blocking network traffic. The first approach is to block traffic based solely on the source/destination IP address (and/or port number). The second approach is to block traffic on a per-program basis. A common combination approach is to allow setting specific source/destination rules on a per-program basis. Some firewalls only block incoming connection attempts while other firewalls may also block outbound connections.

For Greenfoot to work, communication must be allowed when both the source and destination IP address is 127.0.0.1, which is the "loopback" address (i.e. it refers to the local machine, not a machine on the network). Note that Greenfoot must be allowed to make outbound connections (or "act as a client") as well as receive incoming connections (or "act as a server").

Further assistance

If you are getting one of the error messages above but are certain that you don't have any firewall software, you can contact the support team. Be aware however that in most cases these errors are due to external software or configuration, and in these cases the support team will be unable to help.