Target runtime J2EE Runtime Library is not defined.

在工程目录下的.settings文件夹里，打开org.eclipse.wst.common.project.facet.core.xml文件，其内容是：

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
  <runtime name="J2EE Runtime Library"/>
  <fixed facet="java"/>
  <fixed facet="jst.utility"/>
  <installed facet="java" version="1.5"/>
  <installed facet="jst.utility" version="1.0"/>
</faceted-project>

删除中间几行，只留下下面三行就可解决此问题

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
</faceted-project>