myeclipse中增加user libraries时出现“ One or more selected libraries are already on the

    myeclipse中增加user libraries时出现了“ One or more selected libraries are already on the build path”：

    很奇怪，明明已将相应jar包导入，系统却报了这个错。后来发现将jar放入lib文件夹后还要在myeclipse中lib文件夹右键刷新1下，否则系统辨认不到。