It can be very convenient when developing to server based application to run them using "localhost" in order to maintain consistency between developer machines. This is normally a good idea but there is a small case where this can cause problems.

Consider if you are running a local http proxy on your machine in order to capture your HTTP traffic. (Cough perhaps even the one in JDeveloper I work on). Then you might run into Java bug 6737819. Basically by default JDK 1.6 was hard coded not to send any request to localhost via a proxy which of course was a bit of a pain. Luckily a workaround was put in where you could put the string "~localhost" in your nonProxyHosts entry to turn of this feature:

java -client -classpath classes -Dhttp.proxyHost=localhost 
-Dhttp.proxyPort=8099 -Dhttp.nonProxyHosts=~localhost 
-Dhttps.proxyHost=localhost 
-Dhttps.proxyPort=8099 client.Example 

Now moving forward to JDK 1.7 this workaround no longer works; but you need to take care to define nonProxyHosts as an empty string:

 java -client -classpath classes -Dhttp.proxyHost=localhost -Dhttp.proxyPort=8099
 -Dhttp.nonProxyHosts= -Dhttps.proxyHost=localhost 
-Dhttps.proxyPort=8099 client.Example  

If you define this any anything other than an empty string the DefaultProxySelector though beware because internally it will append / or use the http.nonProxyHosts value from ../jre/lib/net.properties".

Just a minor complication that is not obvious from the published API.