Target runtime Apache Tomcat v7.0 is not defined.

打开项目，找到.settings--->org.eclipse.wst.common.project.facet.core 修改这个文件中：

<?xml version="1.0" encoding="UTF-8"?>
<faceted-project>
  <runtime name="Apache Tomcat v7.0"/>
  <fixed facet="java"/>
  <fixed facet="wst.jsdt.web"/>
  <fixed facet="jst.web"/>
  <fixed facet="dorado7.core"/>
  <installed facet="java" version="1.6"/>
  <installed facet="jst.web" version="2.5"/>
  <installed facet="wst.jsdt.web" version="1.0"/>
  <installed facet="dorado7.core" version="1.0"/>
</faceted-project>

将<runtime name="Apache Tomcat v7.0"/>删除掉保存即可