Error about import javax.servlet.* in eclipse

Today in order to debug the servlet I wrote, all of a sudden, found that my site all the servlet is wrong, look carefully, the original is import javax.servlet.* here error.

Then I went online to look up some information, only to find that the original was I reinstalled the Tomcat server a few days ago, is this in addition to the problem?

So I reconfigured to add my newly installed Tomcat server in Eclipse, but the error still occurs, is it not related to the server? But out of careful me, I was re-establishing a project to rewrite a servlet, but this time I was surprised, this time there is no mistake, but the previous site file errors are a lot of. Why is there such a situation?

I thought about it, may be the previous configuration of the server has not been deleted, really did not think wrong, and finally found that all of my previous servers are red forks, only the server just configured is normal. I then replaced all my sites with new servers. All forks are removed, hehe, isn't it very simple?

Let me take a look at the following diagram:

See the whole red fork.

Step Two:

Choose a server without a red fork, you can, is it simple?

Error about import javax.servlet.* in eclipse