Session and Cookies 2

程序员文章站 2022-06-10 11:47:00

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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("<HEAD><TITLE>Current Shopping Cart Items</TITLE></HEAD>");
    out.println("<BODY>");

    // 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:<BR>");
    if (items == null) {
      out.println("<B>None</B>");
    } else {
      out.println("<UL>");
      for (int i = 0; i < items.length; i++) {
        out.println("<LI>" + items[i]);
      }
      out.println("</UL>");
    }

    // Ask if they want to add more items or check out.
    out.println("<FORM ACTION=\"/servlet/ShoppingCart\" METHOD=POST>");
    out.println("Would you like to<BR>");
    out.println("<INPUT TYPE=SUBMIT VALUE=\" Add More Items \">");
    out.println("<INPUT TYPE=SUBMIT VALUE=\" Check Out \">");
    out.println("</FORM>");

    // Offer a help page.
    out.println("For help, click <A HREF=\"/servlet/Help"
        + "?topic=ShoppingCartViewerCookie\">here</A>");

    out.println("</BODY></HTML>");
  }

  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"};  
  }
}

Session and Cookies 2

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