1 CS6320 – Servlet Structure and Lifecycle L. Grewe

2 The Servlet Interface Java provides the interface Servlet Java provides the interface Servlet Specific Servlets implement this interface Specific Servlets implement this interface Whenever the Web server is asked to invoke a specific Servlet, it activates the method service() of an instance of this Servlet Whenever the Web server is asked to invoke a specific Servlet, it activates the method service() of an instance of this Servlet service(request,response) MyServlet (HTTP) request (HTTP) response

3 Example – Generic Servlet import java.io.*; import java.servlet.*; public class HelloWorldServlet extends GenericServlet { public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException{ throws ServletException, IOException{ PrintStream out = newPrintStream(res.getOutputStream()); out.println("Hello world!"); } public String getServletInfo() { return "Hello World Servlet"; }}

4 Servlet Hierarchy YourOwnServlet HttpServlet Generic Servlet Servlet service(ServletRequest, ServletResponse) doGet(HttpServletRequest, HttpServletResponse) doPost(HttpServletRequest HttpServletResponse) doPut doTrace …

5 HTTP Request Methods POST - application data sent in the request body POST - application data sent in the request body GET - application data sent in the URL GET - application data sent in the URL HEAD - client sees only header of response HEAD - client sees only header of response PUT - place documents directly on server PUT - place documents directly on server DELETE - opposite of PUT DELETE - opposite of PUT TRACE - debugging aid TRACE - debugging aid OPTIONS - list communication options OPTIONS - list communication options

6 Class HttpServlet Class HttpServlet handles requests and responses of HTTP protocol Class HttpServlet handles requests and responses of HTTP protocol The service() method of HttpServlet checks the request method and calls the appropriate HttpServlet method: The service() method of HttpServlet checks the request method and calls the appropriate HttpServlet method: doGet, doPost, doPut, doDelete, doTrace, doOptions or doHead This class is abstract This class is abstract

7 Creating a Servlet Extend the class HTTPServlet Extend the class HTTPServlet Implement doGet or doPost (or both) Implement doGet or doPost (or both) Both methods get: Both methods get: HttpServletRequest : methods for getting form (query) data, HTTP request headers, etc.HttpServletRequest : methods for getting form (query) data, HTTP request headers, etc. HttpServletResponse : methods for setting HTTP status codes, HTTP response headers, and get an output stream used for sending data to the clientHttpServletResponse : methods for setting HTTP status codes, HTTP response headers, and get an output stream used for sending data to the client Many times, we implement doPost by calling doGet, or vice-versa Many times, we implement doPost by calling doGet, or vice-versa

8 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class TextHelloWorld extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { PrintWriter out = res.getWriter(); out.println("Hello World"); } public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { doGet(req, res); } HelloWorld.java

9 The Response: Returning HTML By default, no content type is given with a response By default, no content type is given with a response In order to generate HTML In order to generate HTML Tell the browser you are sending HTML, by setting the Content-Type headerTell the browser you are sending HTML, by setting the Content-Type header Modify the printed text to create a legal HTML pageModify the printed text to create a legal HTML page You should set all headers before writing the document content. Can you guess why? You should set all headers before writing the document content. Can you guess why?

10 public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out = response.getWriter(); out.println(" Hello World \n"); out.println(" "); out.println(" " + new java.util.Date() + " \n"); out.println(" Hello World \n "); } } HelloWorld.java

11 Life of a Servlet  Birth: Create and initialize the servlet  Important method: init()  Life: Handle 0 or more client requests  Important methods: service(), doGet(), and doPost().  Death: Destroy the servlet  Important method: destroy()

12 Servlet Life Cycle Servlet Class Calling the init method Servlet Instance Deal with requests: call the service method Destroy the Servlet: call the destroy method Garbage Collection ServletConfig

13 The init() method  The init() method is called when the servlet is first requested by a browser request.  It is not called again for each request.  Used for one-time initialization. The method init is overloaded and has a parameter of type ServletConfig The method init is overloaded and has a parameter of type ServletConfig ServletConfig has methods to get external initialization parameters ServletConfig has methods to get external initialization parameters In Tomcat, these parameters are set in web.xmlIn Tomcat, these parameters are set in web.xml To make initializations, override init() and not init(ServletConfig) To make initializations, override init() and not init(ServletConfig) init() is automatically called by after performing default initializationsinit() is automatically called by after performing default initializations

14 Service() Method  Each time the server receives a request for a servlet, the server spawns a new thread and calls the servlet’s service () method. Browser Web Server Single Instance of Servlet service()

15 The Service Method  By default the service() method checks the HTTP Header.  Based on the header, service calls either doPost() or doGet().  doPost and doGet is where you put the majority of your code.  If your servlets needs to handle both get and post identically, have your doPost() method call doGet() or vice versa.

16 Thread Synchronization  By default, multiple threads are accessing the same servlet object at the same time.  You therefore need to be careful to synchronize access to shared data.  For example, the code on the next slide has a problem…

17 package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class UserIDs extends HttpServlet { private int nextID = 0; public void doGet( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String title = "Your ID"; String docType = … String id = "User-ID-" + nextID; out.println(" " + id + " "); nextID = nextID + 1; out.println(" "); } This code is problematic. Can result in a race condition, where two users can actually get the same User-ID! For example: User 1 makes request: String id = "User-ID-" + nextID; Gets nextId of 45. Now User 2 makes request, and pre-empts user 1: String id = "User-ID-" + nextID; Gets nextId of 45 (same one!) Admittedly, this case is rare, but it’s especially problematic. Imagine if user Id was tied to credit card number!

18 How to Solve Synchronization Problems  You have a few options for solving servlet synchronization issues: 1)Never use instance variables (or protect them) in your servlets…use local variables. If you don’t have shared instance variables, you don’t have shared synchronization problems. 2)Synchronize code explicitly with Java synchronization blocks. 3)Note recommended - Use the SingleThreadInterface

19 Java Synchronization  Use a synchronization block whenever accessing/modifying a shared variable.  For example: synchronized (this) { String id = "User-ID-" + nextID; out.println(" " + id + " "); nextID = nextID + 1; }

20 SingleThreadModel Interface  To prevent multi-threaded access, you can have your servlet implement the SingleThreadModel: public class YourServlet extends HttpServlet implements SingleThreadModel { …}  This will guarantee that your servlet will only process one browser request at a time.  It therefore addresses most synchronization issues.  Unfortunately, however, it can result in severe slowing of performance, and most people strongly recommend against using it.  In fact, the SingleThreadModel interface is now deprecated in the Servlet 2.4 API.

21 Death of a Servlet  Before a server shuts down, it will call the servlet’s destroy() method.  You can handle any servlet clean up here. For example:  Updating log files.  Closing database connections.  Closing any socket connections. The server may remove a loaded Servlet, Why?: The server may remove a loaded Servlet, Why?: asked to do so by administrator(e.g. Server shutdown)asked to do so by administrator(e.g. Server shutdown) Servlet was idle a long timeServlet was idle a long time server needs to free resourcesserver needs to free resources The server removes a Servlet only if all threads have finished or a grace period has passed The server removes a Servlet only if all threads have finished or a grace period has passed

22 Example: Persistent Counter  To create a persistent record, we can store the count value within a “counter.txt” file.  init(): Upon start-up, read in the current counter value from counter.txt.  destroy(): Upon destruction, write out the new counter value to counter.txt

23 import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class CounterPersist extends HttpServlet { String fileName = "counter.txt"; int count; public void init () { try { FileReader fileReader = new FileReader (fileName); BufferedReader bufferedReader = new BufferedReader (fileReader); String initial = bufferedReader.readLine(); count = Integer.parseInt (initial); } catch (FileNotFoundException e) { count = 0; } catch (IOException e) { count = 0; } catch (NumberFormatException e) { count = 0; } } At Start-up, load the counter from file. In the event of any exception, initialize count to 0. Continued….

24 // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); count++; out.println ("Since loading, this servlet has " +"been accessed "+ count + " times."); out.close(); } Each time the doGet() method is called, increment the count variable. Continued….

25 // At Shutdown, store counter back to file public void destroy() { try { FileWriter fileWriter = new FileWriter (fileName); String countStr = Integer.toString (count); fileWriter.write (countStr); fileWriter.close(); } catch (IOException e) { e.printStackTrace(); } When destroy() is called, store new counter variable back to counter.txt. Any problems with this code?