Life Cycle

Servlets: The Servlet Life Cycle Ethan Cerami New York University

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Road Map      

Overview of the Life Cycle Birth of a Servlet Life of a Servlet Threading Issues Death of a Servlet Tips for Debugging Servlets

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Overview of Servlet Life Cycle

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Life of a Servlet 

Birth: Create and initialize the servlet 

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Life: Handle 0 or more client requests 

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Important method: init() Important methods: service(), doGet(), and doPost().

Death: Destroy the servlet 

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Important method: destroy()

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Birth of a Servlet

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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. 

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Simple Example The init() method is a good place to put any initialization variables.  For example, the following servlet records its Birth Date/time… 

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import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class Birth extends HttpServlet { Date birthDate; // Init() is called first public void init() throws ServletException { birthDate = new Date(); } 10/17/08

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// Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); out.println ("I was born on: "+birthDate); out.close(); } }

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Another Example This servlet initializes a set of Lottery Numbers…  Listing 3.7 in Text: LotteryNumbers.java 

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Life of a Servlet

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Life of a Servlet The first time a servlet is called, the Servlet is instantiated, and its init() method is called.  Only one instance of the servlet is instantiated.  This one instance handles all browser requests. 

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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 Browser

service()

Web Server

service() service()

Single Instance of Servlet

Browser

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Let’s Prove it…     

To prove that only one instance of a servlet is created, let’s create a simple example. The Counter Servlet keeps track of the number of times it has been accessed. This example maintains a single instance variable, called count. Each time the servlet is called, the count variable is incremented. If the Server created a new instance of the Servlet for each request, count would always be 0!

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import java.io.*; import javax.servlet.*; import javax.servlet.http.*;

Only one instance of the counter Servlet is created. Each browser request is public class Counter extends HttpServlet { therefore incrementing the // Create an instance variable same count variable. int count = 0; // 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(); } }10/17/08 Servlet Life Cycle 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.

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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…

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package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*;

This code is problematic. Can result in a race condition, where two users can actually get the same User-ID! For example:

public class UserIDs extends HttpServlet { User 1 makes request: private int nextID = 0; String id = "User-ID-" + nextID;

public void doGet( Gets nextId of 45. HttpServletRequest request, HttpServletResponse response) Now User 2 makes request, throws ServletException, IOException {and pre-empts user 1: response.setContentType("text/html"); PrintWriter out = response.getWriter(); String id = "User-ID-" + nextID; Gets nextId of 45 (same one!) String title = "Your ID"; String docType = … Admittedly, this case is rare, String id = "User-ID-" + nextID; but it’s especially problematic. out.println("

" + id + "

"); Imagine if user Id was tied to nextID = nextID + 1; credit card number! out.println(""); } }10/17/08

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How to Solve Synchronization Problems 

You have a few options for solving servlet synchronization issues: 1)

2)

3)

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Never use instance variables in your servlets. If you don’t have shared instance variables, you don’t have shared synchronization problems. Synchronize code explicitly with Java synchronization blocks. Use the SingleThreadInterface (not recommended)

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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; }

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SingleThreadModel Interface 

  

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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.

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Death of a Servlet

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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: 

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Updating log files. Closing database connections. Closing any socket connections.

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Example: Death.java This next example illustrates the use of the destroy() method.  While alive, the servlet will say “I am alive!”.  When the server is stopped, the destroy() method is called, and the servlet records its time of death in a “rip.txt” text file. 

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import import import import

java.io.*; java.util.*; javax.servlet.*; javax.servlet.http.*;

public class Death extends HttpServlet { // Handle an HTTP GET Request public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { response.setContentType("text/plain"); PrintWriter out = response.getWriter(); out.println ("I am alive!"); out.close(); Continued…. } 10/17/08

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/ This method is called when one stops / the Java Web Server public void destroy() { try { FileWriter fileWriter = new FileWriter ("rip.txt"); Date now = new Date(); String rip = "I was destroyed at: "+now.toString( fileWriter.write (rip); fileWriter.close(); } catch (IOException e) { e.printStackTrace(); } } }

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Example rip.txt file I was destroyed at: Thu Jan 12 11:10:58 CDT 2004

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Putting it all together

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A Persistent Counter 

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Now that we know all about the birth, life and death of a servlet, let’s put this knowledge together to create a persistent counter. The Counter.java example we covered earlier has a big problem: 

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When you restart the web server, counting starts all over at 0. It does not retain any persistent memory.

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Persistent Counter 

To create a persistent record, we can store the count value within a “counter.txt” file. 

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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

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port port port port

java.io.*; java.util.*; javax.servlet.*; javax.servlet.http.*;

At Start-up, load the blic class CounterPersist extends HttpServlet { counter from tring fileName = "counter.txt"; file. nt count; In the event of any exception, ublic void init () { initialize try { count to 0. FileReader fileReader = new FileReader (fileName);

BufferedReader bufferedReader = new BufferedReader (fileR String initial = bufferedReader.readLine(); count = Integer.parseInt (initial); } catch (FileNotFoundException e) { count = 0; } catch (IOException e) { count = 0; } catch (NumberFormatException e) { count = 0; }

Continued….

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andle an HTTP GET Request ic void doGet(HttpServletRequest request, HttpServletResponse re hrows 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 countContinued…. variable. 10/17/08

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// 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 Any problems

variable back to counter.txt.

with this code?

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Tips for Debugging Servlets     

Section 3.8 of our Text has a few tips for Servlet Debugging. Pay close attention! At this stage, the best debugging option is to use print statements. print statements will be output to the Tomcat console (very useful!) Let’s try it out… For production systems, a real logging library, such as Log4J, is a much better option.

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Summary 

Servlets have three distinct phases:   

  

birth: refer to the init() method life: refer to the service() method death: refer to the destroy() method

The first time a servlet is called, the Servlet is instantiated, and its init() method is called. Only one instance of the servlet is ever instantiated. This one instance handles all browser requests (don’t ever forget this!)

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