Question

Instead of writing too many sql statements in each servlet I just want to write one sql statement in a java class and want to call that class from each servlet but I have not found the correct way to do so.

The sql statement is: select distinct username from table where userid=?

while (rs.next){
    username=rs.getString(1);
}

I want to return username to each servlet
so I created a java class in netbeans:

public class Username {
     //How should I proceed here?        
}

Answer 1

Don’t embed that code in a servlet.

Create a separate POJO that does the work for you. Write it, test it, and give a reference to any servlet that needs its services. Let the servlet call the one method that executes that logic.

Start with an interface:

package persistence;

public inteface UserDao {
    String getUsername(Long userId);
}

Then give it an implementation:

package persistence; 

public class UserDaoimpl implements UserDao {
    public String getUsername(Long userId) {
        // All the SQL stuff here
    }
}

Your servlet will have a reference to it:

private UserDao userDao = new UserDaoImpl();

The servlet should acquire the Connection from a pool and give it to the DAO.

If you have write operations, I’d recommend that you create a separate service layer. Services know about units of work. They’ll outlive your web UI and those servlets.