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Error message when performing a connection the postgreSQL database:

java.sql.SQLException: No suitable driver
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at com.asip.servlet.ProcLogin.doPost(ProcLogin.java:73) // source code line that throws this error

Possible Root Causes & Solutions

The jdbc jar file is not yet added into the project.

Right click the project
Properties > Libraries > Add External JARs (Applicable for Eclipse)
Add the jar file (postgresql-x.x-xxx.xxx.jar)
Restart [...]

When I enter the password to access into the database, a failure message prompts out:-
An error has occurred:
Error connecting to the server: server closed the connection unexpectedly
This probably means the server terminated abnormally before or while processing the request.
Steps to Identify the Problem
1. Check postgresql log in C:\Program Files\PostgreSQL\8.2\pg_log
2. The log message shows the following:
2008-05-31 [...]

Notes:

1. The primary key only can be set constraint during the creation of the table. Else it will display an error that the field returns null value and primary key cannot be created.
2. If you want to change the primary key after the table has been created, you have to drop the table and creates the table [...]