当使用带有JSON对象的$ .ajax作为数据时，request.getParameter（）返回null

I'm learning Java servlets and I wrote two separate servlets for "GET" and "POST". When a "GET" request is sent to the server, the servlet accesses the database and retrieves everything and converts the result to the format that can be recognized by Google Charts. And when a "POST" request is sent to the server, the servlet gets the parameters and adds them to a Java object and then the DAO adds the data to the database. However, when I hit the "add" button after the input, the web app cannot find the servlet at all. It simply just "skips" the ajax function and proceeds. So here's the servlet that does the insertion:I'm learning Java servlets and I wrote two sepa