The problem is building a webapp with maven using Eclipse.
I encountered 404 in tomcat 8.0
After searching online, I found out the solution.
Click your project -> properties -> deployment assemebly
Add folder “/src/main/webapp”
then it works !
However, if you are using Struts2 like me, there is an advance option beneath it “Folder for deployment descriptor” select the folder we just created. (This option won’t be showed if you have already the folder in the above list)