Force installer to fail when ports are not available? - install4j

Question

I am using install4j to install an intranet application which requires an HTTP and HTTPS port. I would like to test that these ports are available and warn the user/block the installation until they select unavailable ports.
The only avenue I see for this (besides custom code) is to ensure the windows service fails if the application cannot bind to needed ports, and use a Failure Strategy "Ask user whether to retry or quit on failure". In the web server startup code, I use System.exit(1) if the server cannot bind to ports. However, this does not appear to register as a failure to the installer - the installation proceeds without invoking the failure strategy.
What is the proper approach for signaling failure to the "Start a service" action? Have other people taken an alternate approach to guaranteeing the installation uses available ports?

Answer 1

A good alternate approach I've since found: add a custom code action:
List<Integer> takenPorts = new ArrayList<Integer>();
for (int port : Arrays.asList(80, 443)) {
java.net.ServerSocket socket = null;
try {
socket = new java.net.ServerSocket(port);
} catch (IOException e) {
takenPorts.add(port);
} finally {
if (socket != null) socket.close();
}
}
if (takenPorts.isEmpty()) {
return true;
}
else {
String msg;
if (takenPorts.size() == 2) {
msg = "Ports 80 and 443 must be available for uDiscovery";
}
else {
msg = "Port " + takenPorts.get(0) + " must be available for uDiscovery";
}
context.setVariable("portErrorMessage", msg);
return false;
}
Good explanation of how to wire this up here

Answer 2

All who are just getting an error dialog telling
"com.install4j.runtime.beans.action.control.RunScriptAction failed"
I think that also was the one, Adam got:
In the srciptlet above the variable named portErrorMessage was set.
I also didn't realise that first. The trick is quite simple: You have to insert ${installer:portErrorMessage} in the field "Error message". Doing so, you don't need the Util.showOptionDialog described by Ingo, since that method call opens another, second dialog that has to be acknowledged first, after that the user would have to acknowloedge the dialog from install4j, too.