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Posts Tagged ‘Servlet’

Concurrency, Thread Safety and Local Variables

August 12, 2016 by kiranbadi1991 | Comments Off on Concurrency, Thread Safety and Local Variables | Filed in Development, Performance Center, Performance Engineering, Performance Test Tools

One of the most of the common reason for concurrency issues I often see in web application is due to concurrent access of data stored in variables. Generally in servlets , data in variables are often stored as Local variables, Instance Variables, Class Variables , request attributes, session attributes and context attributes.

Below example simplest I can think of for storing data as local variable and accessing it in a thread safe manner

public class MyServlet extends httpServlet {

  // mylocalage is localvariable here for this servlet.

    public void printAge(){
              int mylocalage = 0;
      mylocalageage = mylocalage - 10;
      System.out.println("My age 10 years earlier was: " + mylocalage);
      

    }
}

Its considered that by design that data stored in local variable is thread safe.

Every thread accessing the above servlet will have their own values and they will not interface with each other.

image

Local variables are stored in stack in Java. So data stored in these variables are thread safe.

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Log the Servlet Processing Time

January 12, 2014 by kiranbadi1991 | Comments Off on Log the Servlet Processing Time | Filed in Development, Environment, Performance Engineering

Recently one of my friend who had a Java based web application which was was recently developed and deployed in the production environment was facing some performance issues contacted me. For some reasons in that application couple of functionality was taking more time to show up than few other functionalities, after doing some investigation, we realized that couple of servlets were taking more time than others for processing on the server side. First we identified servlets, then identified DAO associated with those servlets and then finally queries associated with that DAO.Once queries were identified we did some load testing for those queries using different data sets.

We used the below program to gather the required information about the servlet processing time,

import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
 
@WebListener
public class logprocessingtime implements ServletRequestListener {
    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        ServletRequest servletRequest = sre.getServletRequest();
        servletRequest.setAttribute("start", System.nanoTime());
    }
 
    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        ServletRequest servletRequest = sre.getServletRequest();
        Long start = (Long) servletRequest.getAttribute("start");
        Long end = System.nanoTime();
        HttpServletRequest httpServletRequest =
                (HttpServletRequest) servletRequest;
        String uri = httpServletRequest.getRequestURI();
        System.out.println("time taken to execute servlet " + uri + ":" + ((end - start) / 1000) + "microseconds");
    }
}

The above java program is basically a web listener which we registered it to the servlet container and it listens for every request that web application receives. For every request the application receives we log the response time taken for it to process the request.

This program meets the servlet 3.0 Specs and in case if you want it to port to 2.4 , then probably you need to register the listener with web.xml.

More information about Servlet Request listener can be found here. Its powerful interface and it helps to get lot of troubleshooting information.

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Dealing with Browser’s back Button with Headers and Javascript History Object

August 28, 2012 by kiranbadi1991 | Comments Off on Dealing with Browser’s back Button with Headers and Javascript History Object | Filed in Browser, Development, Environment, Others, Testing

Quite often while coding application which has lot of forms in it, there comes a requirement where in developers needs to deal with back button functionality of the browser.Disabling the back button with Javascript is one of the options used by many sites to deal with duplicate submission of forms.

Browsers maintain information about pages visited in the browser’s history and Javascript can be used to manipulate the history using windows.history object.

Some of the methods which we can use to know more about history are.

window.history.back();
This works exactly as back button of the browser.Goes 1 page back.
window.history.forward();
This works as exactly as forward button of the browser.Goes 1 page forward.
The number of pages in the history stack of the browser can be determined reading its length property,
window.history.length
We can go back and forth in the history  identified by using current position of the page,
 
window.history.go(-1);
window.history.go(1);
go function is used to load relevant pages from the history. go(-1) loads the 1 page backwards from the current page and go(1) moves the browser 1 page ahead from the current page.
 
HTML 5 History object also provides good way to deal with History management of the browsers, some of the functions in HTML 5 are as below,(Reference: Opera Dev Library)
  • window.history.length: Returns the number of entries in the joint session history.
  • window.history.state: Returns the current state object.
  • window.history.go(n): Goes backwards or forwards by the specified number of steps in the joint session history. If the value you specify is zero, it will reload the current page. If it would cause the target position to be outside the available range of the session history, then nothing happens.
  • window.history.back(): Goes backwards by one step in the joint session history. If there is no previous page to go to, it does nothing.
  • window.history.forward(): Goes forwards by one step in the joint session history. If there is no next page to go to, it does nothing.
  • window.history.pushState(data, title [, url]): Pushes the data specified in the arguments onto the session history, with the given title and URL (the URL is optional).
  • window.history.replaceState(data, title [, url]): Updates the current entry in the session history, with the given data, title and URL (the URL is optional)

History object of HTML5 gives us the tool to push/replace the url in the browser’s history and its this feature which I believe is somewhat in secure in nature.Maybe we can have some warning message whenever some scripts wants to read or write the history of the user’s browser exactly the way geolocation api’s work.

Anyways coming back to our topic, we can also use the javascript to control the behavior of the back button,one of the ways quite often used is

function disablebackbutton()
    {window.history.forward()}
disablebackbutton();
window.onload=disablebackbutton;
window.onpageshow=function(evt){if(evt.persisted)disablebackbutton()}
window.onunload=function(){void(0)}
This is most ugly way of dealing back button problem.Just disable it on onload and onpageshow events.

Another clean way of dealing with back button issue is tell browsers not the cache or store any information of the page in its history and this can achieved by setting appropriate headers.With Servlets and JSP , this can done  by

    response.setHeader("Pragma", "no-cache");
    response.setHeader("Cache-Control", "no-store");
    response.setHeader("Expires", "0");
    response.setDateHeader("Expires", -1);

These headers can be added to any page which requires that it should not be cached or reused in any way.I did go via header’s approach did resolve the back button issue.Now whenever I use back button I get Page expired message and for some pages form values are not pulled out from cache.

I would suggest going via header’s approach as this can be done at server side and has very limited or low dependency of the clients and most browsers honors those headers.

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