Accepted answer

Possibly, you have a test which is still running (in debug mode). Try finishing all tests (you can see them in the debug view: window->show view->debug) and try again...


I went to the same Problem,But my Debugging session ended hours ago.But still ECLIPSE error kept coming.
So I just simply restart the ECLIPSE IDE, then the problem just solved.


if you have multiple projects involved, and multiple jre's installed, make sure all dependent projects are using the same javac/jre


Make sure your application server JRE and Eclipse JRE version should be same or else it will throw unsupported class version and hot code debug will not work.


Whenever this error message appears it also gives options to terminate or restart. Select Terminate and it will terminate any running debug case which you are unable to locate.


I was running into this issues too. I found a build hiding in the backgound that was giving me fits. Check to make sure you have all tests/builds closed or completed. Nonetheless, you can still run the new code.


See this thread:

This means you changed a class while it was debugging an application and it could not update the class for the application while it was running.
The error suggests you may be running an older JVM, i.e. pre-1.4.2 but this error can occur with any JVM if the change is incompatible with the previous version of the class.

Check carefully what JRE you are using in your debug session.

Also check you are deploying classes compiled with the debug attribute set. (see this thread)

Finally, as indicated here:

did you switch "Project->Build Automatically" off?
Hot code replacement works (only?) if automatic build is switched on...

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