HTML menu DIV tag over a SELECT element

We are using a JS menu that used layers created by div tags. The problem was that this layer was hiding behind behind 'select' dropdowns in the page.

I googled around for a solution and found out 2 solutions:
- Hide the select boxes when the menu is clicked and unhide them when the menu rolls back.
- Use the zIndex property and create a transparent IFrame under the menu layer.

The following links were of great help to me:
BEA portal menu.
IFrame option.
Hiding option.

Heap size and Perm size

I was getting confused if the heap size specified by -Xmx included the Perm size or it did not? The GC document on Sun's site showed the Perm generation to be part of the Heap size, but I think this is wrong. It seems that the Perm size is different from the Heap size. I found this link that confirms this belief.

The "permanent" generation in the Sun JVM is for class objects, method objects, static fields for those classes (just the static fields, not the objects referenced from the static fields: those are in the regular Java object heap because Java code allocates them), and random other bits like any interned Strings etc. So if U are using a lot of reflection in you code, then a lot of method objects and class objects are created at runtime. For e.g. XML binding frameworks such as Castor etc. use reflection by default.

Hence the total heap size used by the JVM is the Permanent Generation size + the -Xms/Xmx size.

The total memory used by the JVM process will be = Memory used by the JVM for internal management + memory allocated in native code + memory allocated for the permanent space.
This is the memory that U see in Task-Manager in Windows or 'top' on Solaris.

What happens when we allocate more Java heap than the max physical memory available?
The OS would start swapping the process to disk (depending on the Virtual memory configuration) if the total memory reserved by all applications and the OS in the system exceeds the physical RAM installed. This would seriously degrade the performance of the system.

*IMP*: If your application is throwing an OutOfMemoryError then it could be because the PermSize is full, rather than the heap size.

HttpClient in a multi-threaded environment

Our application was using HttpClient library to connect to various third-party systems. I was proposing to develop a smart resuable component that could be used by other modules and also suffice the performance requirements of the application.

I was joyed to see how the design of HttpClient offers excellent support for multithreaded environments.
The HttpClient and ConnectionManager classes can be shared across the entire application. Each respective thread of execution must have a local instance of HttpMethod and can have a local instance of HttpState or/and HostConfiguration to represent a specific host configuration and conversational state.
Hence these must be local variables in all the methods. At the same time the HttpClient instance and connection manager are shared among all threads for maximum efficiency.

If an proxy configuration is required, then this information can be passed in the execute() method of PostMethod using the HostConfiguration class.

Finding the version of Solaris

Commands to find out the version of Solaris the box is running on:

uname -X
more /etc/release

Viewing large files on Linux/Solaris

Log files of J2EE applications can become very big. In our application, the log files were of the size ~ 700 MB to 1GB.
To search such big files for information the following unix commands can be used:
- less fileName.txt
To search a string in the forward direction
- /searchString
To move to the next search
- n
To search backwards
- ?searchString

Javascript goodies

Today, I spend quite some time refactoring messy Javascript code:

1.) I needed to extract a number appended to the end of the string - use the power of regular expressions to make this one clean and sleek.

var RowIdNumber = rowId.match(/\d+$/)[0];

2.) To obtain a 'anchor' element inside of a table row and invoke it's click (href) -

rows[i].getElementsByTagName("a")[0].click();

Enjoy :)

Java Applet Caching Issues

I spent the last 3 days in frustration trying to get Java Plug-in cache mechanisms to work.
Since JDK 1.5, the Java Plug-in has advanced caching options using the "cache_archive" and
"cache_version" tags.

One particular vexing issue that I faced was that inspite of adding all the cache directives in the applet's object tag, the jar files were not cached. A quick google showed 2 bugs:
1. Unfortunately, if the servers do not return Last-Modified or Expires in HTTP response headers, it would disable plugin caching for the connection, and plugin would try to redownload the file again without caching. This results in two HTTP GET requests per file.
2. If a running applet makes a call to URLConnection.setDefaultUseCaches(false), then all subsequent loads from this plugin will exhibit the multiple-load-request behaviour.

In our case, the Applet was setting the cache as false in the code. Hence the cache directives were not working as expected. More information on this can be found here and here.

Search functionality in Textpad and EditPlus

A lot of people use text editors such as Textpad and EditPlus. My favourite is Textpad; it can handle large files and also has a very good interface.

Many times we need to search for muliple words in a line. For e.g. in a log file, search for all lines having words 'abc' and 'xyz' on a single line. Using the regular expression search facility available in these editors, this task becomes a piece of cake.

RegEx for the above task: abc.*xyz
We can keep on adding more words - abc.*xyz.*123

A simple feature, but very powerful while analysing logs etc.

Favicon is a bit messy

Recently when I was analysing the log files on the server, I came across many reqeuests for a resource - favicon.ico
This was strange, since I knew that there was no such file on the server - so why were browsers making a request for this file.

I did a quick google and came to know about this interesting things:
- Favicon stands for "Favourites Icon". Its the small image U see in front of the URL on the addressbar.
- The icon looks good, but the problem is that some browsers keep on making a request to this file everytime the page is loaded !

Here are some links having more info:
http://www.favicon.co.uk/
http://www.htmlkit.com/services/favicon/

Cool collection of JS code

Found this link that contains a cool collection of Javascript code. Worth a perusal.

I liked the fee menu javascript code available here:

http://javascriptkit.com/script/cutindex23.shtml

Interpreting 2 digit years

As a good practice, it is always recommended to use 4 digits to specify the year. But a lot of applications may have a front-end that accepts 2 digit year as the input from the user. Or there could be a intergration with a third-party that requires us to parse 2 digit years.

We all understand the ambiguity in parsing 2 digit years. The logic that you would need to use depends on the business requirement. For e.g. if you are accepting birth-year then it cannot be greater than today and we can go 100 yrs back.

So how do different API's handle it?

The SimpleDateFormat class follows the following logic:
"For parsing with the abbreviated year pattern ("y" or "yy"), SimpleDateFormat must interpret the abbreviated year relative to some century. It does this by adjusting dates to be within 80 years before and 20 years after the time the SimpleDateFormat instance is created."

The Joda Time API takes a different approach:
"What is particularly interesting about this format is the two digit year. Since the interpretation of a two digit year is ambiguous, the appendTwoDigitYear takes an extra parameter that defines the 100 year range of the two digits, by specifying the mid point of the range. In this example the range will be (1956 - 50) = 1906, to (1956 + 49) = 2005. Thus 04 will be 2004 but 07 will be 1907. This kind of conversion is not possible with ordinary format strings, highlighting the power of the Joda time formatting architecture"

The default setting in Windows 2000 m/cs:
"Under Windows 98 or Windows 2000, two-digit years for the Year argument are interpreted based on user-defined computer settings. The default settings are that values from 0 through 29 are interpreted as the years 2000–2029, and values from 30 through 99 are interpreted as the years 1930–1999. For all other Year arguments, use a four-digit year; for example, 1924."

The Internet mail standard uses the following:
"Two digit years are treated as valid in the loose translation and are translated up to a 19xx or 20xx figure. By default, following the specification of RFC2822, if the year is greater than '49', it's treated as being in the 20th century (19xx). If lower, or equal, then the 21st (20xx). That is, 50 becomes 1950 while 49 is 2049."

JS library for dates

I was looking for a reusable JS function that would allow me to format and compare dates just the way it is done in Java using classes such as DateFormat, Date etc.

I found this JS file on the web - simple and easy to use. And quite powerful too:
http://www.javascripttoolbox.com/lib/date/

Difference between empty string and null in Oracle Database

In Oracle, when we store a empty string i.e. "" through JDBC, then Oracle stores it as a NULL value. This causes a lot of confusion to Java developers. In Java a null and a "" are 2 different entities.

Hence even if you store an "" string when inserting into the database, when you retrieve the results, you would get the string as null.

If we do a getInt(), getFloat() etc. then we would get 0.

Dynamically making a field as readOnly using Javascript

I was looking for a sleek solution that would make all the fields of my form read-only dynamically - or atleast make them appear to be readonly.

I decided to use the powerful JQuery library to make this happen. Here is the code that can be put in a JSP/ASP page and included in any page that needs to have all fields as readonly.
$(document).ready(function(){
$(":input,checkbox,radio").addClass("readonlytextbox");
$(":input").focus(function() {this.blur();} );
$("input[@type=checkbox]").click(function()
{alert("Cannot change this field.");
return false;} );
$("input[@type=checkbox]").keydown(function()
{return false;} );
$("input[@type=radio]").click(function()
{return false;} );
$("input[@type=radio]").keydown(function()
{return false;} );
$("select").focus(function()
{alert("Cannot change this field.");
return false;} );
//-- end of code
});

ReadOnly and Disabled fields

In HTML, we have two options if we want non-editable fields. Mark the fields as readonly or make them disabled.

The difference between the two is that a diabled field is not send as a HTTP parameter to the server when the form is submitted, whereas a readonly field is send back to the server.
This may be important when you use frameworks such as Struts where the formbeans may expect nested properties to come back to the server.

Snoop on Solaris and Ethereal

I am a big fan of Ethereal tool and use it frequently for analysis of network traffic.
The GUI is great and we can decode HTTP traffic from the TCP packet frames.

Recently I had to capture the traffic on a remote production box running on Solaris and did not have the time to install Ethereal there. I knew about the 'snoop' command on Solaris and had used it in the past to capture network calls.
But I did not know that the snoop command can write all captured network packets to a file that is RFC 1761- compliant. What that means is that I can write the packets to a file and then open that file anywhere in Ethereal :)

This was just what I needed for my scenario. Quick listing of snoop commands:

- To capture packets to a file and only those on port 9080
snoop -o fileName.cap port 9080

- To format a captured file in ASCII
snoop -i fileName.cap -x0

Time complexity and Space complexity of algorithms

I had just downloaded the SAP Memory Analyser and was impressed with its performance.
More information on this tool can be found here. Going thru their wiki, I read how they had taken pains to make critical operations have a time complexity of O(1).

Time complexity and Space complexity are terms used when dealing with algorithms. If the input size (problem size) for a algorithm increases, then how does it affect the time taken for the algorithm to complete and how how much more memory does it take?

If the time consumed by the algorithm is independant of the input size then the algorithm is said to be a complexity of O(1); i.e. a constant-time method. If the time taken is linear then it is known as linear-time method - O(n).

More information can be found at the following links:
http://pages.cs.wisc.edu/~hasti/cs367-common/
notes/COMPLEXITY.html
http://www.leda-tutorial.org/en/official/ch02s02s03.html

Generating GUIDs on the client side web browser

If your application requires that a GUID be created to identify the client, then we have 2 options:

- If we are sure that the end users use only IE, then we can use the ActiveX function:

return new ActiveXObject("Scriptlet.TypeLib").
GUID.substr(1,36);

For other browsers, we can write a JS function as shown here.
Basically the function uses the random numbers and padding to generate a unique number.
Snippet of the JS function:

function generateGuid()
{
var result, i, j;
result = '';
for(j=0; j<32; j++)
{
if( j == 8 || j == 12|| j == 16|| j == 20)
result = result + '-';
i = Math.floor(Math.random()*16).
toString(16).toUpperCase();
result = result + i;
}
return result
}

4 GB memory limitation for 32 bit machines

What is the maximum heap size that U can allocate to a JVM or a .NET runtime?
The answer is dependant on the operating system and hardware. On a 32-bit machine, the CPU can use only 32 bits to refer to a memory pointer. Hence 2^32 = 4 gb.

Out of this, 2gb is reserved for the kernel and 2 gb for the applications. Hence we can allocate only 2gb memory to an application. More info on this can be found here.

For a JVM, if we need to allocate more than 2 gb of heap, then we need to install the 64 bit version of the JDK. Also on a solaris box, start the java process with the -d64 option.

servletContext.getRealPath() issue

Our application was using getRealPath() method to obtain the base path of the web application on the server. I noticed that the method was not behaving consistently across JEE containers.
On Windows/Tomcat5.5, getRealPath returned the path with a '/' at the end.
But on Solaris/Websphere, getRealPath returned the path without a '/' at the end.

The best way to handle this is to have a utility method that would handle all cases :-

String realPath = 
getServletContext.getRealPath(path);

if ((realPath != null) 
&& !realPath.endsWith("/"))
     realPath += "/";

Also check if using getRealPath is really required. The Servlet API specifies a temp directory that can be accessed using the following code:

File dir=(File)getServletContext().getAttribute
("javax.servlet.context.tempdir");

Silently print PDFs on the browser

If your web application needs to print PDF's silently on the browser, then check out this blog.

Of all options given, I found the iText trick the simplest to use. The concept is quite simple:
- Create a PDF on the fly using iText
- Embed Acrobat Javascript into the PDF that would print it automatically to the default printer.
- Open the PDF document in a iFrame that is hidden (size 0*0)

A live example can be found here.

Modal Dialog submit issue

In our application we were using a modal window pop-up. The user has to enter some fields and submit the form. The strange thing was that whenever I submitted the form, a new window was getting poped up.
I knew there was nothing wrong with the javascript code...so I googled around a bit and found the soultion here.

All that is needed is to add the following line inside the "head" tag of the modal page :
<base target=_self>

Generating PDFs on the fly

If we need to generate PDFs on the fly in a JEE web application , then we have 2 open-source options: using Apache FOP or iText.

The approach used by both the leading projects is different. Apache FOP uses XML/XSL to create a XSL-FO file using XSLT Transformation. This XSL-FO is then rendered into a PDF using the PDF FO processor. This is a pretty neat solution as there is a clear separation of concerns. But the only caveat is that U need to have a pretty good hold over the XSL language, which in my personal opinion is very arcane and difficult to learn. But if you gain mastery over XSL, then there is pretty much nothing U cannot do with FOP :)

iText is a pure Java API that allows developers to create PDFs on the fly. For e.g. the iText API would have methods such as 'addPara','addBlock','addImage' etc. etc. So U end up embedding the presentation logic in Java code...but it may not be a big deal for anyone except design purists. It is also possible to use absolute coordinates while creating your PDF's, but I would advice against if there is a possibility of the look&feel of reports changing often.

iText is hugely popular and has been ported to .NET (C-Sharp port known as iTextSharp and the J-Sharp port known as iText.NET)
The popular JasperReports open-source reporting tool uses iText in the background to make PDF documents. Also iText boasts of superb speed - (Creating a 1000 page PDF takes only a few seconds). Even on the FOP site, they recommend iText as a post-processor tool for merging, encrypting, changing PDF documents.

If someone is looking only for a PDF manipulation tool then they should have a look at pdftk. It is also based on iText.

Setting Cache headers for Gifs, Css and JS files

By default, browser and servers use the "if-modified-since" header to check if the file should be downloaded from the server or given from the cache.
Though this default behaviour is good for performance, we can increase the performance by giving a cache expiry period. This would make the browser cache the content for that period - i.e. the browser won't even make "if-modified-since" conditional fetch requests.

The best place to set these headers in a J2EE application is thru a filter.
Suppose you want to cache GIFs, CSS and JPEG for 2 hours, just add this header in the response:
Cache-Control: max-age=120

Setting nocache headers in Struts Applications

In older versions of struts, it was a common practise to extend the ActionServlet and set the no-cache response headers there, so that they are applied to all requests.

In Struts 1.2.x, there is a easier way - without writing a single line of Java code.
Just add the "nocache" attribute to the controller element in struts-config.xml file.

<controller
processorClass="MyRequestProcessor"
nocache="true" />

Disabling keep-alive connections in Tomcat

I was using TcpTrace utility to dump my HTTP requests/responses. But the problem was that my Server (Tomcat) was using keep-alive connections and bcoz of this, the output in TcpTrace was all muddled up.
Disabling keep-alive connections in Tomcat 5.5 was quite simple:

Just add the maxKeepAliveRequests="1" attribute to the Connector tag in server.xml

CSS caching in IE

A strange problem regarding CSS caching had frustrated me for a whole 2 hours today.

The problem was reported by one of the developers that even if the CSS on the server was changed, IE was still used the cached version. I checked the application and found that there were no Cache headers that were set. This meant that IE should have made a GET "if-modified-since" request everytime for the CSS.

I even dumped the requests using the TcpTrace sniffer. I could see that IE did send a request for the CSS file and also the server responding with the new CSS file. Still no change on the page ?? This really baffled me ...If I hit F5/refresh, then the new CSS was loaded, but if I revisited the page, then still the old CSS was being used..

Then I learned from some site, that IE behaves a bit oddly here : IE usually caches CSS files by browser session (until you exit the browser). With a new session, the browser does a head request to see if the file has been modified, if so it reloads. So if I logged off the application and logged in again, the CSS would be reloaded. Strangely, this would not always happen...But if I close the browser and open the application again, I always get the new CSS..
It seems that IE is a bit erratic in this respect...so to be absolutely sure that U get the new CSS, you may have to close the browser and open it again :)

Magic of JQuery

We know that in IE, a form gets submitted whenever we press 'Enter' in a text-field. To disable it would require trapping the event in Javascript and disabling the default behaviour.

<input onkeydown="if (event.keyCode == 13){returnfalse;}" name="email">

But to do this in all text fields across hundreds of html pages would be a impossible task. Then suddenly I got the idea of using JQuery's powerful selector concept to apply this to all textfields across all pages. I just needed to add the following lines to the JS file that was present in all pages.

<script src="jquery-1.1.2.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("input[@type=text]").keypress( function() {
if (event.keyCode == 13){return false;}
} );
});
</script>

Mean, Median, Mode, Midrange and Standard Deviation

A definition of terms to help my frail memory when I need it most :)

The arithmetic mean is the sum of the numbers, divided by the quantity of the numbers.

The geometric mean of two numbers is the square root of their product.The geometric mean of three numbers is the cubic root of their product.

The quantity obtained by adding the largest and smallest values and dividing by 2, statisticians call the midrange.

The median is the central point of a data set. To find the median, you would list all data points in ascending order and simply pick the entry in the middle of that list. If there are an even number of observations, the median is not unique, so one often takes the mean of the two middle values.

The standard deviation is the most common measure of statistical dispersion, measuring how widely spread the values in a data set are. If the data points are close to the mean, then the standard deviation is small. Conversely, if many data points are far from the mean, then the standard deviation is large. If all the data values are equal, then the standard deviation is zero.It is defined as the square root of the variance.

For lists, the mode is the most common (frequent) value. A list can have more than one mode. For histograms, a mode is a relative maximum ("bump"). A data set has no mode when all the numbers appear in the data with the same frequency. A data set has multiple modes when two or more values appear with the same frequency.A distribution with two modes is called bimodal. A distribution with three modes is called trimodal.

DST implementations in Operating Systems and Java

I often wondered how operating systems knew about Day light saving offsets. Recently USA decided to change its DST settings and it immediately crossed my mind as to how Operating Systems and Applications would react to this.

Some basics first - In what format does a OS store date/time information ? All machines keep their clocks in synch with the GMT using Network Time Protocol. Then the machine calculates it local time using the local time zone settings that are stored on the machine.
Local time is always computed as = GMT + timezone offset + DST offset

But where does the OS get information about DST. All DST information is stored in a binary database called ZoneInfo database or TimeZone DB. On Solaris this database can be found at "/usr/share/lib/zoneinfo"

In Java, the JRE picks up DST information from the Timezone database stored at "%JRE_HOME%/lib/zi"

Tip: In Java, U can change the default timezone that the application uses by setting the user.timezone property
e.g. java -Duser.timezone=Area/City
or
java -Duser.timezone=GMT[+5.30]