Servlet interview questions

servlet Life cycle of

1. servlet initialization —init() Method
   ① First creation Servlet Called when the , Initialization is performed only once
   ② The default is to create... The first time a request is processed Servlet, It can also be set to create... The first time the server starts Servlet

   public void init() throws ServletException {
         
   	//  Initialization code ...
   }
   

2. servlet Processing requests ：service() Method
   ① Processing requests from browsers , Respond the processed results to the browser
   ②servlet After receiving the request , call service Methods to deal with ,service For different requests , Select call doPost or doGet Other methods
   ③ Writing servlet when , Need to achieve doPost/doGet Other methods

   protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         
       String method = req.getMethod();
       long lastModified;
       if (method.equals("GET")) {
         
   		// Handle get request 
           }
       } else if (method.equals("HEAD")) {
         
          
       } else if (method.equals("POST")) {
         
           this.doPost(req, resp);
       } else if (method.equals("PUT")) {
         
           this.doPut(req, resp);
       } else if (method.equals("DELETE")) {
         
           this.doDelete(req, resp);
       } else if (method.equals("OPTIONS")) {
         
           this.doOptions(req, resp);
       } else if (method.equals("TRACE")) {
         
           this.doTrace(req, resp);
       } else {
         
       	// other 
       }
   }
   

3. servlet It has not been called for a long time or the server has shut down , Would call destroy() Method The destruction servlet, from JVM Garbage collector to recycle servlet
   destroy Method only executes once
   After execution , The servlet Marked as garbage , Waiting for recycling

servlet Thread safety of

servlet Not thread safe

servlet Is a singleton , After the initialization , Only when the request is not accepted for a long time or the container is closed , This servlet Will destroy , This one is accessed by multiple request threads servlet

One servlet Instantiate before processing the first request , initialization . Or initialize when the container starts ( Need to set up )
This is called later servlet Request , Are called by independent threads service Method , There are thread safety issues

How to solve ？

1. be not in servlet Using member variables in , There is no resource competition between threads , Thread safety

2. Use ThreadLocal Provide an independent variable for each thread

3. Lock the method that handles the request ; Cause obstruction

   protected synchronized void doGet(HttpServletRequest req, HttpServletResponse resp) 
   								throws ServletException, IOException {
         
   }
   

4. Realization SingleThreadModel Interface ; One will be created for each user servlet example , Poor performance ( Have been abandoned )

   public class DormServlet extends HttpServlet implements SingleThreadModel
   

get And post difference

HTTP Request default use get

get	post
Parameter in url in	Parameter in resquest body Request body
Limited length	There is no length limit
Exposure to the url in , unsafe	Hidden in the request body , Security
Can only url code	Support for multiple encodings
Produce a TCP Data packets	Produce two TCP Data packets
Get data from the server	Modify the data on the server