Open the .metadata.pluginsorg.eclipse.core.runtime.settings folder in the project directory and delete org.eclipse.wst.server.core.prefs and org.eclipse.jst.server.tomcat.core.prefs file and restart eclipse, it should be fine. If it still doesn't work, it may be that the jdk version selected for your project is not consistent with tomcat7.0. Just adjust it.
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Open the .metadata.pluginsorg.eclipse.core.runtime.settings folder in the project directory and delete org.eclipse.wst.server.core.prefs and org.eclipse.jst.server.tomcat.core.prefs file and restart eclipse, it should be fine. If it still doesn't work, it may be that the jdk version selected for your project is not consistent with tomcat7.0. Just adjust it.