When you're working with a Java project in Eclipse and you need to add a new jar file to your project, you don't need to close down Eclipse or your current Eclipse project (as I've seen a few people do). In fact, here are all the steps you need to take to add a new jar file to your Eclipse build path:
libfolder for their jar files, so I'll assume that's where we're going to add it.
libfolder, and refresh it.)
libfolder you'll see your new jar file in there.
That's all you have to do to add your new jar file to your current Eclipse project build path.
A few other notes about adding a new jar file to your Eclipse build path:
libdirectory and refresh it.
As one other note, this same technique works just as well for other files you want to add to your Eclipse project - image files, XML files, whatever. In those cases you don't need to modify your build path, but you follow the same first two steps: Add the file to your folders outside of Eclipse, then press F5 or click Refresh to tell Eclipse to re-scan your project folders.