If you use Adobe Flex Web applications that connect to Plain Old Java Objects on the server side, chances are you use a popular, robust, and freely available server called Apache Tomcat. If you use Eclipse-based Flex Builder, you can smoothly debug both Flex and Java code without leaving Eclipse. Flex Builder debugger does not need any special configuration. But we need to add a couple of parameters to the startup routine of Tomcat so it'll engage the Java Platform Debugger Architecture (JPDA), which will allow other applications attach to JVM that runs Tomcat and debug deployed Java classes remotely.