Servlet Life cycle

1
Servlet Life cycle

• If a client request for a servlet is the first
  request and the servlet is not loaded, then the
  init method is called otherwise, the init method
  is not called
• Then the service method is called
• The service method automatically calls doGet,
  doPost, or another HTTP request method
• The server automatically calls the destroy method
  before shutting down the servlet

2
Servlet init method

• Use the init method to initialize instance
  variables, or perform other initialization tasks,
  in particular some that might be cpu intensive
  such as accessing files,

3
Servlet threads

• Each client request to a servlet generates a new
  thread ? multithreaded environment
• ? potential concurrency issues when accessing
  common resources
• Instance variables of the servlet
• Files
• Databases

4
Instance variables of a Servlet

• A servlet can have instance variables, for
  instance a variable keeping track of the visitor
  count
• Since we can have multiple threads of a single
  servlet running concurrently, any instance
  variable can be accessed simultaneously by
  several threads
• ? Concurrency is an issue
• We can synchronize access to instance variables
  to deal with any concurrency issue

5
Instance variables of a Servlet

• public class MyServlet extends HttpServlet
• private int visitorCount
• / initialize the instance variables inside the
  init
• method /
• public void init( ) throws ServletException
• visitorCount 0
• // code rest of the class here

6
Instance variables of a Servlet

• // code rest of the class here
• public void doGet( HttpServletRequest request,
• 
  HttpServletResponse response)
• throws IOException,
  ServletException
• synchronized( this )
• // only 1 thread at a time will access
  this block
• visitorCount
• // display the visitor count

7
Instance variables of a Servlet

• public void doGet( HttpServletRequest request,
• 
  HttpServletResponse response)
• throws IOException,
  ServletException
• int localVisitorCount 0
• synchronized( this )
• // only 1 thread at a time will access
  this block
• visitorCount
• localVisitorCount visitorCount
• // Process localVisitorCount here

8
Concurrency issues - Servlet

• You can also resolve concurrency issue by coding
  synchronized methods
• For instance access to a file or a database can
  be done via a synchronized method
• public synchronized void foo( )
• // thread-safe code here