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This error occurs when a Visual Web project has already been deployed, and then the Clean and Build is done. The error  message  "Unable to  delete file ...\build\web\WEB-INF\lib\appbase.jar" can be seen in the output window:

Deleting directory C:\Documents and Settings\User1\My Documents\NetBeansProjects\WebApplication3\build
C:\Documents and Settings\User1\My Documents\NetBeansProjects\WebApplication3\nbproject\build-impl.xml:776: Unable to delete file C:\Documents and Settings\User1\My Documents\NetBeansProjects\WebApplication3\build\web\WEB-INF\lib\appbase.jar
BUILD FAILED (total time: 5 seconds)

This  is a known issue in NetBeans Visual Web Pack 5.5.1 (see release notes )  and also applies to NetBeans6.0

I was able to reproduce this error in NetBeans6.0 when a user reported it recently. The work around as suggested in the release notes is to stop the server first. To do this go to the services tab -> Servers -> GlassfishV2, right click and stop the server. Also, someone suggested closing the browser window first, as if an application had been deployed the browser will still use appbase.jar.  I did not need to close the browser window. It worked for me with browser window still open.

You can then go to your project and Clean and Build without any issues. Then redeploy your application as usual.

This already has been logged in as a bug. For more information on this see  bug#94247 .