Whenever You are creating a .properties files(ex displaytag.properties) or adding a external JARS(ex:struts.jar) after creating Project,You need to place it in classpath of the project to take immediate effect.
Methods to do this
1.Place the created file (say displaytag.properties) file in WEB-INF/classes folder,Eclipse will automatically add the file to its classpath
2.For JAR's place the jar file in WEB-INF/lib folder then it will automatically added in classpath.
So that you can avoid classnotfound exception
When adding external jars don't forget to check the jars in javaee dependencies option in buildpath in eclipse.
Happy coding