Why does hot code replace fail in Eclipse while debugging a web application running in Tomcat 7?

Everything went well when I was using Tomcat 6. Now I am connecting the debugger to a web application deployed in Tomcat 7 usi开发者_开发百科ng Eclipse 3.6.1. The project's "Build automatically" feature is turned on. I only add a line like System.out.println("why does hot code replace not work?"); and the Eclipse error dialog appears saying hot code replace failed. Again, using Tomcat 6 I didn't experience this behavior. What am I doing wrong?