Target runtime Apache Tomcat v7.0 is not defined.

1. According to the literal meaning, "Apache Tomcat v7.0" is undefined;
2. Baidu found that the problem was in the file "org.eclipse.wst.common.project.facet.core.xml", and it was found in the file. "<runtime name="Apache Tomcat v7.0"/>";
3. Since "Apache Tomcat v7.0" is a name attribute, does it mean that this attribute cannot find the corresponding information;
4 , Try to enter runtime in eclipse->Window->Preferences, but find the "Runtime Environment" property, the content is empty; try it with the idea of ​​​​no matter what, Add—>Apache->Apache Tomcat v7.0->Next->Tomcat installation directory (select the tomcat installation directory)->Complete, no error was reported, and it was successfully resolved.

Solution 2: In the "Problems" tab, right-click in the error, select "Quick Fix", and then follow step 4 above to solve the problem.

If you know your own tomcat name, enter "org.eclipse.wst.common.project.facet.core.xml" and modify <runtime name="tomcat name"/>, then refresh the project and it will be ok