Session and Cookies 2

转载自：http://www.java-programming.info/tutorial/pdf/csajsp2/08-Session-Tracking.pdf

http://www.java2s.com/Code/Java/Servlets/Usecookietosavesessiondata.htm

Session Tracking

 

HttpSession session = request.getSession();
synchronized(session) {
SomeClass value =
(SomeClass)session.getAttribute("someID");
if (value == null) { 
value = new SomeClass(...);
}
doSomethingWith(value);
session.setAttribute("someID", value);
}

 

•  The J2EE blueprints say not to bother
  • There are no race conditions when multiple different users access the page simultaneously
  • On the face of it, it seems practically impossible for the same user to access the session concurrently
• The rise of Ajax makes synchronization mportant
  • With Ajax calls, it is actually quite likely that two requests from the same user could arrive concurrently
• Performance tip
  • Don’t do “synchronized(this)”!
    • Use the session or perhaps the value from the session as the label of the synchronized block

 

      HttpSession Methods:

• getAttribute
  

  Extracts a previously stored value from a session object. Returns null if no value is associated with given name.

• setAttribute
  Associates a value with a name. Monitor changes: values 

  implement HttpSessionBindingListener.

• removeAttribute
  Removes values associated with name.
• getAttributeNames
  Returns names of all attributes in the session.

• getId
  Returns the unique identifier.

• isNew
  Determines if session is new to client (not to page)

• getCreationTime
  Returns time at which session was first created
• getLastAccessedTime
  Returns time at which session was last sent from client
• getMaxInactiveInterval, setMaxInactiveInterval
  

  Gets or sets the amount of time session should go without access before being invalidated

• invalidate
  Invalidates current session

Use cookie to save session data:

 

import java.io.IOException;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ShoppingCartViewerCookie extends HttpServlet {

  public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException,
      IOException {
    res.setContentType("text/html");
    PrintWriter out = res.getWriter();

    String sessionid = null;
    Cookie[] cookies = req.getCookies();
    if (cookies != null) {
      for (int i = 0; i < cookies.length; i++) {
        if (cookies[i].getName().equals("sessionid")) {
          sessionid = cookies[i].getValue();
          break;
        }
      }
    }

    // If the session ID wasn't sent, generate one.
    // Then be sure to send it to the client with the response.
    if (sessionid == null) {
      sessionid = generateSessionId();
      Cookie c = new Cookie("sessionid", sessionid);
      res.addCookie(c);
    }

    out.println("");
    out.println("");

    // Cart items are associated with the session ID
    String[] items = getItemsFromCart(sessionid);

    // Print the current cart items.
    out.println("You currently have the following items in your cart:
");
    if (items == null) {
      out.println("None");
    } else {
      out.println("
");
      for (int i = 0; i < items.length; i++) {
        out.println("
" + items[i]);
      }
      out.println("
");
    }

    // Ask if they want to add more items or check out.
    out.println("
");
    out.println("Would you like to
");
    out.println("");
    out.println("");
    out.println("
");

    // Offer a help page.
    out.println("For help, click here");

    out.println("");
  }

  private static String generateSessionId() throws UnsupportedEncodingException {
    String uid = new java.rmi.server.UID().toString(); // guaranteed unique
    return URLEncoder.encode(uid,"UTF-8"); // encode any special chars
  }

  private static String[] getItemsFromCart(String sessionid) {
    return new String[]{"a","b"};  
  }
}