Helpful Tips And Tricks

How to know the character set and collation of mysql database, table and column

2011-05-10T11:19:00.000-07:00

Here are the SQL commands to know the character set and collation of mysql database and tables/columns

mysql> use database_name;
mysql> show variables like "character_set_database";
mysql> select data_type,collation_name from information_schema.columns where table_schema='database_name' and table_name='table_name' and column_name='column_name';

Debug PHP project with IntelliJ Idea 10 and XDebug

2011-05-07T22:06:00.000-07:00

Recently, I was working on a PHP project. I am an ardent fan of IntelliJ Idea and chose to use it for this project as well. Everything went well but I was not able to debug my PHP source code.

After a little bit of googling I found this nice article about debugging PHP code using Idea 10. Since it contained different settings than mine, I modified the instructions to make it work. Here are the modified instructions to debug PHP project in intelliJ Idea 10.

1. Create and copy a file named info.php to your webserver root that contains the following code:

<?php phpinfo(); ?>

3. Open the http://localhost/info.php in a browser to get your PHP configuration.

4. Now view the source of the page in your browser. Copy the entire contents of the source to your clipboard.

5. Goto the XDebug Tailored Installation Instructions and paste your PHP configuration source into the text box then click “analyse my phpinfo() output”.

6. Follow the resulting instructions for adding XDebug to your PHP installation. Once you make those changes, make sure the new section of your php.ini looks like this:
[XDebug]
zend_extension=""
xdebug.remote_enable=true
xdebug.remote_port=9000
xdebug.profiler_enable=1
xdebug.profiler_output_dir="d:\wamp\logs"

7. Open IntelliJ Idea settings by clicking on the wrench icon or going to File->Settings menu. Choose PHP on left navigation tree and set the PHP home along with desired debugger support.

I chose to use XDebug.

8. On the same navigation menu choose servers and enter the details of your web server. Don't forget to mention your web server port. Click on Use path mapping check box and enter the absolute path location of your project files.

9. On the same navigation menu choose Debug and fill the correct value of Debug port. Make sure this entry is same as xdebug.remote_port in step no 6.

10. Now go to Run->Edit Configuration menu and add a PHP Web Application configuration. Choose your newly created server from the drop down in Server field. Also fill the value of Start URL field. Generally it is your index page where you want to start the debugging from.

11. That is all. Now you can start debugging your PHP project in IntelliJ Idea. Just apply a break point in your code and click on Debug button and if everything went well, it will open in your default browser and you
will be able to debug it in Idea.

Happy Debugging.

How to know free memory on linux

2010-12-21T23:46:00.000-08:00

Command to know free memory on Linux/Unix machines

$ free -m

It will show you status in MB. You can optionally use other filters as well

-b switch displays the amount of memory in bytes

-k switch (set by default) displays it in kilobytes

-m switch displays it in megabytes.

-t switch displays a line containing the totals.

-o switch disables the display of a "buffer adjusted" line. If the -o option is not specified, free subtracts buffer memory from the used memory and adds it to the free memory reported.

-s switch activates continuous polling delay seconds apart. You may actually specify any floating point number for delay, usleep(3) is used for microsecond resolution delay times.

You can also use /proc/meminfo to see the current status of memory

$ cat /proc/meminfo

Create subversion repository on Ubuntu

2010-12-12T01:19:00.000-08:00

Recently I had to setup a ubuntu box with subversion on it. I had to also provide web access from subversion with authentication. After few hiccups I managed to do this with installing few packages and configuring them.

Step 1: login to your ubuntu box

#ssh root@x.y.z.t

Step 2: Install required packages

#yum install subversion
#yum install mod_dav_svn httpd

Step 3: Create a parent directory where all your repositories will be set up.

#mkdir -p /srv/svn

Step 4: Create a sample repository using svnadmin tool

#svnadmin create /srv/svn/app

In this step we have created a repository named as app.

Step 5: Change the user and group of the created app repository to give the permission to apache user.

#chown -R apache.apache /srv/svn/app

This user is created when you installed httpd package in step 2. Without this step user will not be able to access it via webserver http protocal, they can however access it using command promp using ssh protocol.

Step6: Restart apache webserver.

#service httpd restart

Step 7: Create a password file for the authentication of users which will be accessing your app repository via http protocol.

#mkdir -p /srv/auth/svn/
#htpasswd -c /srv/auth/svn/app.htpasswd ahsan.javed

In this step we are creating a directory first to store the htpasswd file. then using the htpasswd tool we are creating an user name and password hash entry in created file. This htpasswd tool is by default installed when you perform step 2.

Step 8: Create a virtual host for hosting your subversion access
create a new virtual host file at /etc/httpd/conf.d/svn.conf with the content

    LoadModule dav_svn_module     modules/mod_dav_svn.so
    LoadModule authz_svn_module   modules/mod_authz_svn.so

    <Location /repos>
       DAV svn
       SVNParentPath /srv/svn
    </Location>
    <Location /repos/app>
       AuthType Basic
       AuthName "Subversion Repository"
       AuthUserFile /srv/auth/svn/app.htpasswd
       Require valid-user
    </Location>

Step 9: Restart apache webserver

#service httpd restart

Step 10: Access your setup repository in a browser by typing

http://x.y.z.t:80/repos/app.

This will ask a user name and password, provide the username and password created in step 7 to access the repository.

Enable mysql to connect from desired remote IP

2010-12-09T02:49:00.000-08:00

Recently I had to work on mysql server on Linux platform.

All our development was done on windows machine and everything went fine.

When we tried to migrate the same code to Linux platform, everything worked fine except that our program was not able to connect to mysql. It was continuously throwing this exception.

Caused by: java.net.ConnectException: Connection refused
        at java.net.PlainSocketImpl.socketConnect(Native Method)
        at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
        at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
        at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
        at java.net.Socket.connect(Socket.java:529)
        at java.net.Socket.connect(Socket.java:478)
        at java.net.Socket.(Socket.java:375)
        at java.net.Socket.(Socket.java:218)
        at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:254)
        at com.mysql.jdbc.MysqlIO.(MysqlIO.java:292)

However we were able to connect via mysqladmin as follows

#mysql -u username -h localhost -p password

After doing some google I found that mysql was actually not allowing remote connections from our program. It was only allowing the Unix socket connections in case of mysql admin, thats why it was able to connect to it.

I had to change few configuration parameters in mysql configuration files as follows to allow remote connections to mysql and restart it.

# vi /etc/my.cnf
Once file opened, locate line that read as follows

[mysqld]

Add the following line at the end of this block

bind-address=YOUR-SERVER-IP
#skip-networking

Where,

    * bind-address : IP address to bind to.
    * skip-networking : Don’t listen for TCP/IP connections at all. All interaction with mysqld must be made via Unix sockets. This option is highly recommended for systems where only local requests are allowed. Since you need to allow remote connection this line should be removed from my.cnf or put it in comment state

Restart the mysql server, enter:
# /etc/init.d/mysql restart

How to know which port is blocked by which process on Windows?

2010-11-24T21:30:00.000-08:00

Recently I was trying to use my apache server for my web development. However every time, I tried it showed me that port 80 is being used already. So I took a dig and found out the solution as follows.

C:\>netstat -aon | findstr 0.0:80
-a means list all active connections and their ports. -o means include their process IDs. -n means display the port numbers numerically.

I got this output.
TCP 0.0.0.0:80 0.0.0.0:0 LISTENING 4504

It means that process 4504 is currently listening on port 80. So to start apache on this default port we need to kill this process, you can do it by Task Manager and find process by the id 4504 and kill that process.

You can also identify the process by using command line as follows.
C:\>tasklist | findstr 4504

I found that skype is using the port 80.
Skype.exe 4504 Console 1 107,170K

So we need to kill the process as follows.
taskkill /F /PID 4504

Now we can start apache as usual. Even if we start skype again it will work, I think it choses some other random port this time.

Facebook Page for GreenDeewali

2010-10-28T09:38:00.000-07:00

This is the page for Green Deewali.
http://www.facebook.com/pages/Green-Deewali/100290780040511?v=app_4949752878

Please visit, this is very nice initiative to make environment greener and cleaner.

A nice initiative by webshrub to make environment cleaner

2010-10-28T08:31:00.000-07:00

Web shrub has launched a new initiative to make environment greener and cleaner.
Please have a look at their another web site Green Deewali and take a pledge this year.

http://www.greendeewali.com/take-pledge.html

Helpful Tips And Tricks: My letter to Airtel...

2010-06-11T06:46:00.000-07:00

Facebook page for my problem.

My letter to Airtel...

2010-06-10T22:14:00.001-07:00

My letter to Airtel...

Hi,
I have been facing problems with my internet connection for last 5 days.

Earlier they told me that my DSL link was not stable and they registered my complaint with service request no 33593502.

They told me that this problem will be resolved soon. However when I tried it after a day time, I found that they have closed the ticket without taking any appropriate action.

After that, I fisrt contacted Mr. Sachin Purti(Backend Admin) 0120-4367827 and he said that it will be resolved by few hours. Nothing happened. Some guy visited me and he told me that he cannot do anything in this regard as that was fault with my main line MDF and the guy responsible for maintenance of it was on leave so it could not be fixed at that time as well. They however gave me one more SRN 33635706.

Then I contacted Mr. Yadvendra Kumar Yadav (09810184236) Sector 18 Noida, who said that the problem will be fixed by a day time. I again waited for a day but was not able to get any positive response. My internet connection was still not up, however my DSL Link problem was gone automatically. Fixing the initial DSL problem took them 3 days to fix.

After not getting any resolution to my problem for 4 days I contacted Mr Ali Afsar (Escalation Supervisor at Bhopal Centre) who assured me that my problem will be resolved on 5th day. However I still dont see that my problem is getting resolved any soon.

I have been very disappointed by Airtel service and seeing such service to customers, I am sure that it will be MOST DESPISED BRAND despite of Sunil mittal's vision to be the most admired brand by 2010. Your services is just HORRIBLE...! no other service provider can beat this!! Airtel will be crowned the Worst service provider for years to come...!

If this problem is not going to be fixed in near future I am going to take some drastic action and that will of course not be going to consumer forum rather than I will tarnish your brand image via powerful internet/social network media.

Thanks
A customer who has been tortured by Airtel.

Managing content with Role and Permissions in dotCMS

2010-03-17T23:58:00.000-07:00

In my previous post I mentioned how to create a blog site using templates, containers and contents in dotCMS. After the site has been created you need to add posts to it so that it can go publishing all its posts.

By default the content created contains the permissions of their parent structure. However in most of the cases it is required to give certain permissions to certain group of users/roles. In our case we will create a user mysiteuser and assign it a group mysitegroup. We will also create a role called mysiterole and this role will be granted to mysitegroup.

Using this as an example we will allow users having mysiterole only to add content/post to the blog just created.

1. Add New Role
Click on CMS Admin link and chose New -> Role from the drop down. This will lead to a screen where you can add a new role named as mysiterole.

2. Add New Group

Click on CMS Admin link and chose New -> Group from the drop down. This will lead to a screen where you can add a new group named as mysitegroup.

3. Add New User

Click on CMS Admin link and chose New -> User from the drop down. This will lead to a screen where you can add a new user named as mysiteuser by filling all its details.

4. Assign Role to mysiteuser.

Click on CMS Admin link and chose Users. This will show a screen containing search button. Click on Search button. It will populate the newly created user mysiteuser. Select the user and click on User Roles tab from the screen. Now there will be a screen showing available and current roles.

Chose CMS User Role from Available to Current list. This role is important because of this is the parent role of all dotCMS editing roles.

5. Assigning Group to mysiteuser.

Click on CMS Admin link and chose Users. This will show a screen containing search button. Click on Search button. It will populate the newly created user mysiteuser. Select the user and click on User Groups tab from the screen. Now there will be a screen showing available and current group.

Chose CMS Users group from Available to Current list.

6. Adding Role mysiterole to mysitegroup.

Click on CMS Admin link and chose Permissions -> Groups. This will show a screen containing newly created group mysearchgroup in the left side. Click on mysitegroup. It will enable few options downwards. Click on Edit Roles button. It will open a screen containing available and current roles for th group. Select mysiterole from the available list and click update button.

7. Editing pages for the role

Click on CMS Admin link and chose Permissions -> Groups. This will show a screen containing newly created group mysearchgroup in the left side. Click on mysitegroup. It will enable few options downwards. Click on Edit Pages button. It will populate a screen as follows.

Inside the Add New Page text box write the name of content "Content" and click on Update Page Settings button. It will now reflect a new screen downwards as follows.

Click on Selected Page "Content" from the drop down menu and click on Content Management link under categories tab. Now select Content Search Manager(n) from the list of checkboxes. Click on Update Content Settings button to make changes permanent.

8. Change the permissions of Blog structure.

Click on Structures link and chose structures. This will show a screen containing all structures. Click on Blog. Then click on Permissions tab and check mysiterole and CMS Anonymous role under View column.

9. Change the permission of the Blog Entry structure.

Click on Structures link and chose structures. This will show a screen containing all structures. Click on Blog Entry. Then click on Permissions tab and check View, Modify and Publish permissions to mysiterole. Also give View permission to CMS Anonymous role.

This now completed the assigning permissions and role to a blog site. Now only user mysiteuser will be able to create a blog post when logged into the system. After you login to the system as mysiteuser it will look like as follows.

By clicking on Add New Content and selecting Blog Entry from the drop down you will be able to go to following screen where you can write your contents of post.

After you are done with writing your changes, you need to relate this blog post with one of the blogs. You can do it by going to relationship tab and clicking on Relate Content button. This will populate a search screen. You need to search for the relevant blog and relate content.

Creating a blog Site with dotCMS(A Open Source Java CMS)

2010-03-17T02:10:00.000-07:00

In my organization I had to create a blog site for one of my International team. The obvious options were to chose from vast range of PHP based CMS. But due to some technical issues we could not proceed to them and we were asked to use Open Source Java based CMS. After few days analysis we found that dotCMS matched our requirements for the blog site.

I searched on the net and found very little documentation on how to work with dotCMS however. It took me couple of days to figure out how to make a customized site only for blogging using dotCMS.

Here are few building blocks of dotCMS architecture.

1.Structure
2.Content
3.Category
4.Relationship
5.Container
6.Template

For any site we need to understand these properly and the initial documentation is given at this url

dotCMS comes bundled with a starter site that contains a blog module in itself. We will use this blog module to create a blog site of us named as MySite.

We will first login as admin user(test@dotcms.org/test) to http://localhost/c url.
We see that there are bunch of links available there for managing whole site. However initially we will be interested only in Container and Template section.

In every website we have few elements common to all like Header, Footer, Left Side Navigation, Right Side Navigation and Center Body. There also needs to be a template file which will contain all these elements.

Hence we will create/modify few files for things to work.

1.Create a container MySite-Header by clicking on Add/Edit Container link and write down the html that will go to your header

2.Create a container MySite-Footer by clicking on Add/Edit Container link and write down the html that will go to your footer

3.Create a container MySite-CenterBody by clicking on Add/Edit Container link and write down the html that will go to your center body(Make sure you have selected max contents to be non zero). Chose WebPageContent as content's structure. Normally the code for the body will be $!{Body} because it will be replaced with the content you will add to the center body container.

4.Create a container MySite-LeftSideNavBar by clicking on Add/Edit Container link and write down the html that will go to your Left Side Navigation Bar

5.Create a container MySite-RightSideNavBar fby clicking on Add/Edit Container link and write down the html that will go to your Right Side Navigation Bar

6.Create a template MySite-Template by clicking on Templates and then Add/Edit Templates link. You will write here the overall layout of the site. While creating the template code you also need to include the headers, footer, center body, left side navigation bar, right side navigation bar container just created above. For this you need to click on Add Container button and search for all your containers, this selected container will now be included in the code of your template created.

Click on Save and Publish to make your template active.

Now click on Browser section and browse to /global/images and create a new folder called mysite by right clicking on images->New->Folder

Now upload all your css/images files there. You may also create a separate folder for images and css in the same way.

Now Go to Browser section again and disable all shown folders to be shown on Main Menu by right clicking on each folder and selecting Edit. Make sure you have unchecked the Show on Menu checkbox.

Now we will modify the pages in /blog directory to customize them. First of all we will modify the index.dot file by double clicking on /blog/index.dot. When you click on the index.dot you will see a link Page Properties on the top left side of the page. By Clicking on this link we will change the page template to our created MySite-Template.

We will change the templates of all the files present in /blog folder so that every page is in consistency with the site template.

We will now start adding actual content to all the pages modified.

For doing this we need to click on Add Content link and selecting Reuse content from the drop down. We need to search for "Widget-blog listing" content from the search window.

We need to add the content "widget - Entries by tag" in tags.dot.

Now click on add content button and chose widget - Blog Entry Detail (with audio catcha) to /blog/detail.dot

Add the content "Contributor Blog Entries" in contributor_blog_entries.dot

Add the content "Blog Archived List" in archived_blog_list.dot

Add the content "Widget - Blog listing" again to entries.dot

This now completes the blog site creation tutorial. I will update the role and user creation tutorial in next few days.

awk in multiline records

2009-12-18T03:47:00.000-08:00

I had a database creation script that looks like spanning thousands of lines. However I was only interested to see only few of them which were only containing name pattern TEST1 and TEST2.

I could have used grep to find the valid lines of the file but my records were spread across the lines means each valid record consisted of multiple lines. So I googled for the problem and found that awk can also do a multi line search given proper inputs.

Suppose I have a file input.sql that contains many multiline database object creation queries. I now have to find all queries which only contains TEST1 and TEST2. All of these queries/records are separated by separator ";".

Write a awk input script as follows.

#scrip.awk
BEGIN {
RS=";"
}
{
if (($0 ~ /TEST1/ )||($0 ~ /TEST2/)) {
print $0 ";"
}
}

Here I am specifying that my record separator RS is equal to ";". Also In the awk execution line I specified that if each of my record field which is represented by $0 contains either TEST1 or TEST2 then dump that field to console using print command of awk. Also I have appended a additional string ";" so that my output file output.sql contains actual queries when run on sqlplus prompt

Now I can execute the command as follows on command prompt.
awk -f script.awk input.sql > output.sql

This should give you the all matching record containing TEST1 and TEST2 in the file output.sql

A very nice awk Related tutorial can be found at
http://www.ibm.com/developerworks/library/l-awk1.html
http://www.ibm.com/developerworks/library/l-awk2.html

EXPDP/IMPDP example in oracle for database copy

2009-12-18T03:20:00.000-08:00

Normally we need to have a database copy to export/import on our other database and separate schema. Here are the steps to do the schema copy from one database to another. You need to be admin user for performing some tasks.

$ mkdir datapump

$ chmod 777 datapump/

$ sqlplus adminuser1/adminpassword1@dbsid1

SQL> create directory datapump as '/path/to/folder/datapump';

SQL> grant read, write on directory datapump to public;

$ expdp user1/password1@dbsid1 dumpfile=datapump:myschema.dmp logfile=datapump:myschema_exp.log schemas=myschema

$ sqlplus adminuser2/adminpassword2@dbsid2

SQL> create directory datapump as '/same/path/to/folder/datapump';

SQL> grant read, write on directory datapump to public;

$ impdp user2/password2@dbsid2 dumpfile=datapump:myschema.dmp logfile=datapump:myschema_imp.log remap_schema=myschema:yourschema remap_tablespace=mytablespace:yourtablespace

You can remove remap_schema and remap_tablespace clauses in case your schema name and tablespace name are same on both the databases.

You also need to have same datapump location accessible from both the databases.

Browsing svn revisions in a web browser

2009-11-25T03:58:00.000-08:00

Browsing svn revisions in a web browser

http://{repository url}/!svn/bc/{revision}/{mymodule}/{trunk or branch}

for example
I want to see the revision 80121 of module testmodule's trunk
and my repository is http://svn/repository

Then the URL will be like

http://svn/repository/!svn/bc/80121/testmodule/trunk

Configuring resin for remote debugging

2009-10-26T02:42:00.000-07:00

I am using resin application server in my development environment. At times I need to debug my applications using resin container. I did some search and found a way to integrate resin to my IDE as follows.

For Resin-3.1.8.
I added few lines to my resin.conf file as follows
<jvm-arg>-Xdebug</jvm-arg>
<jvm-arg>-Xnoagent</jvm-arg>
<jvm-arg>-Djava.compiler=NONE</jvm-arg>
<jvm-arg>-Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005</jvm-arg>

For versions prior to Resin-3.1.8.
I added few lines to my resin startup script as follows
httpd.exe -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5005 -Xnoagent -Djava.compiler=NONE -conf D:\resin\conf\resin.conf

Using sed to search and replace a token without opening a file

2009-10-06T23:13:00.000-07:00

#!/bin/bash

i=0; while [ $i -le 47 ]

do

echo " Creating temp file for slice : $i"

sed s/slicenum/$i/g adspace_buy.ctl > adspace_buy.ctl.tmp

echo " Loading Started ..."

sqlldr userid=tfr_rep/welcome@datagen1-aswain:1521/tfrdb control=adspace_buy.ctl.tmp data=adspace_buy_data.$i.csv log=logssqlldr_log_adspaceBuy.log bad=logssqlldr_bad_adspaceBuy.log direct = true

echo "Loading Done for slice : $i"

i=`expr $i + 1`

done

echo "Script completed"

This script will search for a token slicenum in adspace_buy.ctl file and replace it with $i value of the loop counter. Also it redirects the output to a new file adspace_buy.ctl.tmp.
After that this script uses sql loader utility to load the data to oracle database using this newly generated file. This whole process continues for 48 times.

To get the top 1000 lines of all the files in a folder in another folder keeping the original file name same

2009-10-06T23:05:00.000-07:00

To copy files stripped from one folder to another
cd dir;
for file in *
do
head -1000 $file > ../dir_temp/$file
done

Export sqlplus results to a csv file

2009-08-07T02:40:00.000-07:00

Create the file buy_export.sql:
set echo off

set feedback off

set linesize 100

set pagesize 0

set sqlprompt ''

set trimspool on

spool buyInfo.csv

select buyid||','|| name
from dbbuy;

spool off

Run it using

sqlplus -S [username]/[password]@[SID] @buy_export.sql

Change the permission of table space

2009-08-04T02:55:00.000-07:00

To change the permission of table space to read write use following sql command

alter tablespace TFR_REP read write;

To change the oracle user tablespace

2009-08-03T22:34:00.000-07:00

In case if you have assigned wrong tablespace while creating a user, you can change tablespace of that user by following command.

first connect with the system on the database where you have created this user.

alter user AHSAN_TFR default tablespace TFR_REP quota unlimited on TFR_REP;

must assign quota otherwise you cannot run any SQL statement.

To know tablespaces of oracle user

2009-07-14T05:27:00.000-07:00

To know the tablespace assigned to a user run following query on sqlplus

select  USERNAME,
 CREATED,
 PROFILE,
 DEFAULT_TABLESPACE,
 TEMPORARY_TABLESPACE
from  dba_users
order  by USERNAME

Cut nth field of a csv file and sort

2009-07-12T23:52:00.000-07:00

Using cut command you can print out the nth field of a csv input file in.csv.

cut -d"," -f3 in.csv | sort |uniq > ~/out.csv

Above command cuts the 3rd field and then sorts and then take a unique of sorted output and then finally output in out.csv

Killing a defunct process

2009-07-12T10:32:00.000-07:00

A defunct (or "zombie") process is a process that isn't running, isn't eligible to run, and takes up no system resources. It's actually a process that has exited, but its parent has not called wait() in order to find out its exit status.

defunct processes can't be killed since they are already dead. To make them disappear you have to kill their parent process.

Find the parent process id of the defunct process and then kill that parent process:

ps -fe | grep defunctprocess | awk '{print $3}'

kill -9 parentprocessid

Class Loading in Java

2009-07-04T08:14:00.000-07:00

Class Loading in Java

Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of

primordial class loader. Let us look at the class loaders created by the JVM.

Bootstrap (primordial) - Not reloadable – Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path system property, typically loads rt.jar and i18n.jar)
Extensions – Not reloadable – Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system property – usually lib/ext directory of the JRE)
System – Not reloadable – Loads classes from system classpath (as defined by the java.class.path property, which is set by the CLASSPATH environment variable or –classpath or –cp command line options)

Classes loaded by Bootstrap class loader have no visibility into classes loaded by its descendants (ie Extensions and Systems class loaders). The classes loaded by system class loader have visibility into classes loaded by its parents (ie Extensions and Bootstrap class loaders).
If there were any sibling class loaders they cannot see classes loaded by each other. They can only see the classes loaded by their parent class loader.

Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true.

Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton.

Static class loading:
Classes are statically loaded with Java’s “new” operator.
class MyClass {
public static void main(String args[])
{
Car c = new Car();
}
}
A NoClassDefFoundException is thrown if a class is referenced with Java’s “new” operator (i.e. static loading) but the runtime system cannot find the referenced class.

Dynamic class loading:
Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.
Class.forName (String className); //static method which returns a Class
The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Unlike the static loading, the dynamic loading will decide whether to load the class Car or the class Jeep at runtime based on a properties file and/or other runtime conditions. Once the class is dynamically loaded the following method returns an instance of the loaded class. It’s just like creating a class object with no arguments.
class.newInstance (); //A non-static method, which creates an instance of a class (ie creates an object).
Jeep myJeep = null ;
//myClassName should be read from a properties file or Constants interface.
//stay away from hard coding values in your program.
String myClassName = “au.com.Jeep” ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();
myJeep.setFuelCapacity(50);
A ClassNotFoundException is thrown when an application tries to load in a class through its string name using the following methods but no definition for the class with the specified name could be found:

The forName(..) method in class – Class.
The findSystemClass(..) method in class – ClassLoader.
The loadClass(..) method in class – ClassLoader.