How to change OS boot priority in Ubuntu

Login as root user.
edit the following file
/boot/grub/mentu.lst

you could see list of all installed OS here as follows:
Note: I have installed Ubuntu and XP in my machine

title Ubuntu 8.10, kernel 2.6.27-7-generic
uuid 1d034dd4-58c3-4ae9-abc0-c1b5e4591639
kernel /boot/vmlinuz-2.6.27-7-generic root=UUID=1d034dd4-58c3-4ae9-abc0-c1b5e4591639 ro quiet splash
initrd /boot/initrd.img-2.6.27-7-generic

title Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
uuid ....
kernel ....

title Ubuntu 8.10, memtest86+
uuid ....
kernel ....

title Other operating systems:
root ....

title Microsoft Windows XP Home Edition
root ....

The id start from 0.
i.e.
0 = Ubuntu 8.10, kernel 2.6.27-7-generic
1 = Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
2 = Ubuntu 8.10, memtest86+
3 = Other operating systems:
4 = Microsoft Windows XP Home Edition

The following line is taking care of which OS to boot by default

default 0

this means by default Ubuntu 8.10, kernel 2.6.27-7-generic will be booted

If we want to boot XP as default, change this line to

default 4

save the file. When you boot next time by default XP will be booted.

java UnsupportedClassVersionError

While deploy java code into tomcat, it thrown exception as follows

SEVERE: Exception starting filter struts
java.lang.UnsupportedClassVersionError: Bad version number in .class file (unable to load class jcaptcha4struts2.core.actions.support.CaptchaImageResult)
at org.apache.catalina.loader.WebappClassLoader.findClassInternal( WebappClassLoader.java:1851)
at org.apache.catalina.loader.WebappClassLoader.findClass( WebappClassLoader.java:890)
.......
.......
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:288)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)
17 Mar, 2009 1:29:25 PM org.apache.catalina.core.ApplicationContext log
INFO: Closing Spring root WebApplicationContext

The problem is version conflict [development machine java version is 1.5 but machine where i deploy has the 1.6 version]

To fix this problem we need to install java-1.6 version in the deployment machine.

JSP form getting submitted twice

When i was working in the struts2,hibernate and spring technology, strangely "null pointer exception" thrown for attribute which used in the form even i give valid input.

I dig this issue and found that, struts action called twice when i request for the URL. Again this also crazy for me. When debug the application, found that browser sending new request if any empty 'src' in the <img> tag present in the code.

code
<img src="">

In our case browser sent second request with empty form[no data] . so obviously "null pointer exception".

so take out or give the valid image src name and this will fix the problem.