My experiences with Java

You are never sure that the technical solution you are providing is the best (at least in my case,needless to say ...it is not ).I prefer logical arguments as well as constructive criticisms from others on technical ground ,rather than just mute and dumb agreements.At times just few comments from others do help to think differently and come out with a far better implementation. I am here to accumulate some of my technical experiences and have some discussions with others on these topics .

Sunday, December 18, 2011

GComparator,the Generic Comparator

I have recently created a project in code.google.com named GComparator (http://code.google.com/p/gcomparator/).

GComparator is an attempt to create a single generic comparator ,that will have the capability and flexibility to meet complex sorting requirements .

For GComparator , 'G' stands for the term 'Generic'.

It has following features

1.Can sort any java data type ,in any mode (ascending/descending) .

2.It can perform multi-field sort.

Its just an initial release .I will continue to add different features depending on the feedback I get .

Wednesday, June 29, 2011

Integrating Jmesa with Struts 1

After a long gap ,I am back .
Background:
Few months back , I used Jmesa with Struts 1 in my project .
Never heard the name Jmesa before .My requirement was bit complex and was searching for a custom API .Had a look at different java tags like DisplayTag etc , but was not happy .
Finally I found Jmesa !
It was more than enough for me then :)

The Jmesa site had no documentation on Struts1 and Jmesa integration ,so had to do it on my own and frankly speaking ... it took some effort .
But ,after the job was done , I was really impressed with this wonderful API .
It is simple,lucid,easy to use .

I was bit surprised that why Jmesa is not that popular as it should be ? May be lack of documentation ? I do not know . It is actually very well documented .
I wanted to give back something to the Jmesa community ,so ...decided to write a step by step guide doc about Struts1 and Jmesa integration and finally send it to Jmesa owner Jeff Johnston .
Jeff was very glad with it and published it in the Jmesa website itself. Here is the link Integrating Jmesa with Struts1

I have send a war file also that contains end to end integration of Struts and Jmesa .Go to the download location Jmesa related downloads and download the war file named 'Struts1WithAjaxExample.war'.

I see the download count increasing everyday and feel really happy .My purpose is met .
Hope this helps you .If you see the download count ....at least it has helped more than 1100 developers across the globe by now :)

Saturday, July 3, 2010

Creating pop ups with JQuery

Background:
I like Jquery as well as YUI .I have used some components of Jquery in few cases. But there are specific things which you should be careful while you are using this nice frameworks .

Sample:
Jquery dialogs are used for pop ups .Have a look at the code below :
// Dialog
$('#dialog').dialog({
autoOpen: false,
modal:true,
width: 600
});
As you can see "dialog" is the document id where a div based dialog will be created .It will look like

Up to this it is fine .Now if I have multiple pop ups in my page should I be creating multiple different div ids to create them ? Answer is no .
1. Create a Java script global variable , var popupDivId = "#panelDivId" and replace the line $('#dialog').dialog with $(popupDivId).dialog.
2.Similarly assign a closing function to the dialog ,like
$(popupDivId).dialog({
autoOpen: false,
modal:true,
width: 600,
beforeclose: function(event, ui) { performClosingTask(); }
});
This performClosingTask method will remove the div id ,when the dialog is closed .
var containerDiv = doc.getElementById(popupDivId );
containerDiv.parentNode.removeChild(containerDiv);

Note:
With this strategy, you should also have a check before you create open any pop up, whether already another popup is already opened or not .that is
if(docObj.getElementById(panelDivId))
{
window.status = 'Div with id ' + panelDivId + 'is already found ' ;
return;
}
If already one pop up is opened new pop up should not be opened , the already opened pop up should be closed ,then only new pop up should be opened .

Thursday, July 1, 2010

Generic code to detect browser close

Problem description:
You need to detect the browser close , irrespective of the fact how the user has closed it !

Background:
I had a code that was detecting whether the user has attempted to close the browser or not .If yes ,then there should be a message ,other wise not .The code that was working fine ,is was like
<html>
<head>
<title>Detecting browser close in IE</title>
<script type="text/javascript">
var message='Browser close attempt is detected';
function ConfirmClose()
{
if (event.clientY < 0)
{
event.returnValue = message;
}
}
</script>
</head>
<body onbeforeunload="ConfirmClose()" >
<h4>Close browser!</h4>
</body>
</html>
This works fine but with two exceptions .
1.If the user does right-click on the title bar , an option to close the browser appears.Now , if he clicks on that close option browser gets closed without any message .
2.If the browser is minimized and the user right-clicks on the minimized browser , an option to close the browser appears.Now also if close option clicked browser gets closed without any message .

Both of the cases do occur as the condition mentioned in my code i.e.
"event.clientY < 0 " is violated .Now that code above is not good, because I am considering on the mouse coordination!!! The mouse could be anywhere. An alternative would be

<html>
<head>
<title>Detecting browser close in IE</title>
</head>
<body onbeforeunload="return 'Browser close attempt is detected';">
<h4>Close browser!</h4>
</body>
</html>

Now the problem with the code is that 'onbeforeunload' will be fired even when a new page gets loaded ,if the user navigates from one page to another .In that case ,this message would appear irrespective of the situation whether the user wants to close the browser or wants navigate to other page .That is where the risk lies .

So far I have thought about handling two things
1.Catch any kind of window termination (formal close)
2. Do not catch navigations to other pages of the application.
[But wait there can be other variations also :).For example if the user has a google toolbar and tries to search somthing with it in my application, or even directly open another URL in the window of your page...that makes 2 kind of navigation. internal & external. (and some externals like the google one, will result in coming back to your page again).So by having this in mind, do you want to "Dont Catch" all type of navigations? or only internal ones? ]
3.POWER LOSS !!
4.Ending the IExplorer.exe from task manager and so on ...
Confusing.Is'nt it ? Lets re-frame the entire thing little bit .First ask ourselves why we want to detect a browser close ? why ? In most of the cases it is detected , to log out the user who has closed the browser .Say there is a logout url in the page for a logged in user , but he has not clicked the ,instead he has just closed the browser.But now an updation in the database should be done when the user logs out .So while the user logs out (or CLOSES THE BROWSER!!) I need to trap that event and fire an updated to the DB.
For the time if I can trap following three events it will be sufficient for me .
1.If the user closes the browser using the 'X' button at the top right at the corner.
2.If the user does right-click on the title bar , an option to close the browser appears.Now if he clicks on that close option browser gets closed without any message .
3.If the browser is minimized and the user does right-click on the minimized browser , an option to close the browser appears.Now the user clicks that option.

First I need to need to display a message to the user while closing the browser ,in these cases.The code below shows a Message on the above conditions I mentioned earlier.The message MUST NOT APPEAR when:
1. User hits a hyperlink (by mouse or keyboard).
2. User submits a form.

How does it work:
If you hit on a hyperlink or submit a form, a variable called "Navigating" turns "true".
It's because all links.onclick & forms.onsubmit events are registered with this line of code (Navigating=true;) without loosing the old registered event handler of them (if there is any).So, afterwards the event "body.beforeunload" fires, then it only shows you the message if "Navigating" is not "true". That on this point the message will not appear.But if you only use those 3 kinds of closing options you've mentioned, the "Navigating" variable doesn't change (remains "false") and the message will appear.

Some issues ,fixes and enhancements:
Currently this code defines 2 kinds of navigations that could be trapped by links.onclick & forms.onsubmit events.If you have any other kinds of navigations on your pages, that you can trap them on any events, just register the line (Navigating=true;) with that event. use the function "ChargeHandler" to do that without loosing the old event handlers...
and just use a code like the last lines of javascript codes, to cover all instances of that event on the page...
Please note, if you dynamically add links or forms (or even other defined types of navigations that you added) after the page is loaded, the message may appear for navigations launched by these new elements.For fixing this issue you should call "ChargeHandler" after adding such element to the page.So the final code is ....
<html>
<body onbeforeunload="if(!Navigating){return 'Browser close attempt is detected';}else{Navigating=false;}">

<a href="http://www.google.com/">Navigate Away 1 (No Event Handler)</a><br>
<a href="http://www.google.com/" onclick="alert('Link Event Handler 1');">Navigate Away 2 (Event Handler 1)</a><br>

<form action="http://www.google.com/">
<input type="submit" value="Navigate Away Submit Case 1 (No Event Handler)">
</form>
<form action="http://www.google.com/" onsubmit="alert('Form Event Handler');">
<input type="submit" value="Navigate Away Submit Case 2(Event Handler)">
</form>

<script language="javascript">
var Navigating=false;
function ChargeHandler(obj,eve)
{
obj.oldHandler=obj[eve];
obj[eve]=function()
{
if(obj.oldHandler!=null) obj.oldHandler();
Navigating=true;
};
}
//Add Navigating=true; to all links.onclick & all forms.onsubmit event handlers...
for(i=0;i<document.links.length;i++) ChargeHandler(document.links[i],'onclick');
for(i=0;i<document.forms.length;i++) ChargeHandler(document.forms[i],'onsubmit');
</script>
</body>
</html>

Notes:
The final solution that I have mentioned above is a result derived from the valuable feed backs ,opinions as well as sample codes of my several colleagues and specifically some experts from different forums . Many thanks to them.
My role was of merely an observer and coordinator and nothing more.

Blob fails with merge on Hibernate 3.x

Problem description :
Blob fails with merge on Hibernate 3.x.
Blobs are saved correctly using saveOrUpdate(). Few days back I downloaded the latest version of Hibernate ( i.e. 3.5.3) and had a look at the org.hibernate.type.BlobType class .I found out the replace method written as ...

/**
* {@inheritDoc}
*/
public Object replace(
Object original,
Object target,
SessionImplementor session,
Object owner,
Map copyCache) throws HibernateException {
//Blobs are ignored by merge()
return target;
}

It is already commented out there,that Blobs are ignored :(

Analysis :
First of all, I am not sure it is a bug or not. If you check Hibernate forum here
It is clearly mentioned by one of the Hibernate team members ...
'As per the EJB3 spec
"Fields or properties of type java.sql.Blob and java.sql.Clob are ignored by the merge operation"
But we should not throw any exception and we should explicit it in the doc if not yet..'

But then, if it is not a bug then what about this JIRA entry ?

Workaround :
For the time being use saveOrUpdate or save for Blob,Clob etc.

Tuesday, June 29, 2010

Generic solution to prepare the application URL from server end.

Problem description :
I was quite annoyed few days back when I got stuck for sometime while doing a very small task .
The requirement was to display an URL on a JSP page and the situation was such that, the URL value should be a DTO property value (may be populated by an Struts action class or whatever ) .
Say your application URL is "http://xyz.com:9080/PayrollApp".Now you want to display an URL which is like "http://xyz.com:9080/PayrollApp/edit.do".How to create such URL String ( without any sort of hard coding ) from an action class ? Well apparently it is quite simple , in fact it is , specially after you read this post , but trust me otherwise it is not.

The first thing that comes to our mind is "request.getContextPath()" will serve the purpose . Yes off course , but wait a minute ....it will do your job only partially . You will only get the application name (in this case the "PayrollApp" portion ) using request.getContextPath().

What about the previous parts ? The schema,host name ,port number ? How you will populate their value ? Confused ? Honestly speaking , I was .

Lets discuss about URLs in general . Generally what are the different parts of an URL ?
1.The first part is the protocol (here it is http).
2.The protocol is followed by ://
3.Next comes the host (in our example it is "xyz.com" )
4.You may have a port number also after the host name (9080 ) .
5. After the domain you may have your application name (as we have PayrollApp) .But always that is not the case.

Analysis and solution:
Well , the following code is self explanatory .Just use it ...

Sample Code:

String applicationPath = request.getScheme()+"://"+request.getServerName()+":"+
request.getServerPort()+request.getContextPath();

String finalURI=applicationPath+"/edit.do"

Notes :
Please note I have kept the "request.getServerPort()" , but that is not always the case .

Create a DB2 Data Source in JBOSS access it programmetically outside JBOSS context.

Problem Description:

It was a long lasting pain which only sufferers understand!!

Solution:
Just follow the steps mentioned one by one ...

1.We begin by adding its driver to the CLASSPATH: copy db2java.zip to the /server/default/lib directory. To configure the JBoss server with the DB2 data source, copy /docs/examples/jca/db2-ds.xml to the /server/default/deploy directory.

2.Next we modify the db2-ds.xml configuration file, by setting to

COM.ibm.db2.jdbc.app.DB2Driver and to jdbc:db2:, where is the DB2 database name.

Here it is start of db2-ds.xml

################################

test

false

jdbc:db2:tset

COM.ibm.db2.jdbc.app.DB2Driver

db2admin

SELECT * FROM TEST.USER_TABLE

DB2

####################################

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

PLEASE NOTE :In the above file I have one entry

false

It is kept false for connecting to JBoss Datasource from a standalone client.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

3.Remove hsqldb-ds.xml from /server/default/deploy

4.Go to jboss-4.2.0.GA\server\default\conf\standardjbosscmp-jdbc.xml

Then we modify standardjaws.xml (or jaws.xml) to set and .

java:/lkrgt

DB2

5.Now We need to test the data source from a Java class .

Copy all jar files from jboss-4.2.0.GA\client and place to lib or add to class path

Sample code:

package com.ayan;

import java.sql.Connection;

import java.sql.ResultSet;

import java.sql.Statement;

import java.util.Hashtable;

import javax.naming.InitialContext;

import javax.sql.DataSource;

public class TestConnect

{

public static void main(String[] args)

{

try

{

final String sql = "select * from LKRGT.TRAINING_REQUEST ";

Hashtable ht=new Hashtable();

ht.put(InitialContext.INITIAL_CONTEXT_FACTORY,

"org.jnp.interfaces.NamingContextFactory");

ht.put(InitialContext.PROVIDER_URL,"jnp://localhost:1099");

ht.put(InitialContext.URL_PKG_PREFIXES,"org.jboss.naming:org.jnp.interfaces");

InitialContext ic=new InitialContext(ht);

if(ic!=null)

{

System.out.println("Initial Context success");

}

DataSource ds=(DataSource)ic.lookup("lkrgt");

if(ds!=null)

{

System.out.println("Data Source success");

}

Connection con = null;

Statement stmt = null;

ResultSet rs = null;

try

{

con = ds.getConnection();

stmt = con.createStatement();

rs = stmt.executeQuery(sql);

while(rs.next()) {

System.out.println("Query '" + sql + "' returned " + rs.getString(1));

}

finally

{

if(rs != null)

rs.close();

if(stmt != null)

stmt.close();

if(con != null)

con.close();

}

catch (Exception e)

{

e.printStackTrace();

}