Set a system property with ant

One way to set such a property is the ANT_OPTS system variable. You have to be very carefully to not simply skim over answers on google that state that options can be set that way, because it sounds so much like not what it does:

The documentation says:

ANT_OPTS - command-line arguments that should be passed to the JVM. For example, you can define system properties or set the maximum Java heap size here.

Who what have expected that? ANT_OPTS are options for the JVM and not for ant like the name suggests. The var which is used for ant options is called ANT_ARGS.

Now I can launch ant like this: ANT_OPTS="-Djavax.net.debug=all" ant myTarget and can see tons of log output.

(However this leaves the question open whether such a variable can be set using XML).

You can declare system properties in the xml with <sysproperty key="key" value="value"/>.

http://www.java2s.com/Code/Java/Ant/SetsystempropertiesinAntbuildscript.htm