Configuring firewalls to work with BlueJ

BlueJ uses TCP/IP socket communication as part of its normal operation. The communication occurs between two processes running on your computer - one is for BlueJ itself, and the other is for a "debug VM" which runs your program. Although this communication doesn't go over the network, some firewall software will block this communication which inhibits BlueJ operation.

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.

For BlueJ 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 BlueJ must be allowed to make outbound connections (or "act as a client") as well as receive incoming connections (or "act as a server").

If your firewall sets rules on a per-program basis, the program you should apply the rules to are actually the java program (java.exe on Windows), not the bluej launcher (bluej.exe). There are actually multiple copies of the java.exe executable installed as part of the JDK or J2SDK on Windows - you may need to change the rules for them independently.