Now we can get the thumbnail images of a user. key1 to key4 point to the
same image as I still don't know how to operate imageMagick. Also, I
change the sturts2.xml. Now only the action "enterUserUI" can access
User.jsp. Other actions have to redirect to enterUserUI to enter the
User Page. The reason why I changed this is that enterUserUI will be the
only action used to update the data, including user name and thumbnails
list in the User Page.