Global variable in servlet & DBPooling questions

Question

Hello guys,
I used to develop PHP/ASP, and am new to servlet. I have been searching around for a solution ...
With Php, we can get the reference of a global variable in any classes->functions...
How do I do this with servlet ?
And second..I have developed the DB class as below... I set the datasource to be static, so it initializes only once. Is it a good idea? How would you like to improve this class? any comments?
package shop.database;
import javax.sql.DataSource;
import java.sql.*;
import org.apache.commons.dbcp.BasicDataSource;
import org.apache.log4j.Logger;
import shop.admin.exception.GeneralException;
public class DdManager {
     static protected Logger logger = Logger.getLogger(DdManager.class);
     private String userName = "root";
private String password = "";
private String hostName = "jdbc:mysql://localhost:3306/shop";
private String database="shop";
     static private DataSource ds;     // set this to be static so all threads share the same job in JVM
     private Statement stmt;
     private Connection conn;
     private ResultSet rs;
     private CallableStatement cs;
public DdManager() {}
* setup the data source and return it
     public static DataSource getDataSource(
          String sDrvName,
          String sUserName,
          String sPwd,
          String connectURI) {
          BasicDataSource ds = new BasicDataSource();
          ds.setDriverClassName( sDrvName );
          ds.setUsername( sUserName );
          ds.setPassword( sPwd );
          ds.setUrl( connectURI );
          ds.setMaxActive( 15 );
          ds.setMaxIdle( 10 );
          ds.setMaxWait( 10000 ); // 10 seconds
          return ds;
     * static init of the class
     * this class is will be called only once to initialize the DataSource
     static {
          try {
               Class.forName( "com.mysql.jdbc.Driver" );
               ds = getDataSource(     "com.mysql.jdbc.Driver",
                                        "root",
                                        "jdbc:mysql://localhost:3306/shop" );
               if (ds == null) {
                    String msg = "Connection Pool error";
                    logger.error(msg);
                    throw new GeneralException(msg);
               logger.info("DataSource has been initialized");
          } catch(Exception exception) {
               logger.error(exception.toString());
               try {
                    throw new GeneralException(exception.toString());
               } catch (GeneralException e) {
                    logger.error(e.toString());
     * get the connection from the pool (DataSource)
public void openConnection() throws GeneralException {
try {
     BasicDataSource bds = (BasicDataSource) ds;
     logger.info("NumActive: " + bds.getNumActive() + ", " + "NumIdle: " + bds.getNumIdle());
conn = ds.getConnection();
logger.info("Connection of " + database + " has been established");
} catch(Exception exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
* close the connection will actually return the connection to the pool (Must)
public void closeConnection() throws GeneralException {
     initResource();
try {
     if (conn != null){
               conn.close();
               logger.info("Connection of " + database + " has been closed");
} catch(SQLException exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
* prepare the calling stmt
public void prepareProcedure(String callStatement) throws GeneralException {
     initResource();
try {
     cs = conn.prepareCall(callStatement);
} catch(SQLException exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
* set the pass-in parameter for "String"
public void setParameter(int position, String parameter) throws GeneralException {
try {
     cs.setString(position, parameter);
} catch(Exception exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
* set the pass-in parameter for "Integer"
public void setParameter(int position, int parameter) throws GeneralException {
try {
     cs.setInt(position, parameter);
} catch(Exception exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
* execute the procedure and return the resultset
public ResultSet execProcedure() throws GeneralException {
try {
     rs = cs.executeQuery();
} catch(SQLException exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
return rs;
* close the statment and resultset
     private void initResource() throws GeneralException {
     try {
          if(rs != null) {
               rs.close();
          if(stmt!= null) {
               stmt.close();
          logger.info("Statement & Resultset have been free");
     } catch(Exception exception) {
     logger.error(exception.toString());
     throw new GeneralException(exception.toString());
Thanks mates!
myy

Answer 1

Thanks Saish,
Your response is really appreciated. Sorry about that
as i didnt know there is 'code' formatting button,
and I will look into the Singleton pattern.
As I'm still in the learning stage. Therefore, i
still have a lot of thing do not understand.
... use it in a method signature ...What is "a method signature" ?
A method signature is basically the method's parameters, return value, name and any access or other modifiers. The following is a method signature:
static final public void main(final String[] args)Between the braces of the method body is the implementation (or as I already alluded to, the method body).
Consider using an already-developed connection poolimplementation, such as Jakarta Commons DBCP ...
I'm trying to implement the Jakarta DBCP. Did I go
into the wrong way?
Sorry, did not read the imports. Yes, you are. However, I am confused about what you are trying to implement. You have a static method getDataSource(). You also have a static variable 'ds'. Use one or the other. I would be that there are seemingly random errors cropping up based on whether you remember to call getDataSource() or not.
You do not, generally, want the data source to be static. Multiple threads might use the class. And if there is only a static data source, you will either need to synchronize the methods that use the data source (resulting in a scaling bottleneck) or not synchronize them (which will totally destroy any concept of a logical unit of work or database transaction).
.. A static datasource, as in your class, can onlysafely be used by one thread at a time, potentially
introducing scaling bottlenecks (or race conditions)
in your system ...
So, you mean there is no need for the DataSource to
be static ?
No, in fact, IMO, it should not be. That is why you are pooling. Use instances. The pool will manage the connections and their availabilty for you.
Why are you throwing GeneralException everywhere?Here's a question: can someone using your class (a
caller) realistically be expected to handle a
database exception?
When there is a database error, I just want to stop
the process and redirect the user to an error page. I
will look into the unchecked exceptions. Thanks.
Unchecked exceptions do not need to be declared in a method signature or caught within the method body. Checked exceptions do. As such, an added benefit is that unchecked exceptions de-clutter your code.
In your initResources() method, what happens if theclose() on ResultSet throws an exception
Oh, yes. I'm so stupid.
Now I only have ...
     private static DataSource ds;     // set this to
be static so all threads share the same obj in JVM
     private Connection conn;
     private CallableStatement cs;
private void initResource() throws GeneralException
n {
     try {
          if(cs != null) {
               cs.close();
logger.info("CallableStatement has been
as been free");
     } catch(Exception exception) {
     logger.error(exception.toString());
throw new
throw new GeneralException(exception.toString());
You still have issues.
public void initResources() {
   if (rs != null) {
     try { rs.close(); } catch (SQLException ignore) { ignore.printStackTrace(); }
   if (stmt != null) {
     try { stmt.close(); } catch (SQLException ignore) { ignore.printStackTrace(); }
}Normally, this type of method would not be called initResources() but rather closeResources() or freeResources(). It would be called from within the 'finally' block of another method using the CallableStatement or ResultSet.
This is really is problem, would you mind to tell me
how to handle this(close the connection) if the
close() on either CallableStatement or Resultset
throws an exception ?
See above. Simply log the exception (there is usually nothing meaningful you can do if a close() fails, and it is up to you as a developer if this is an error or just a warning). Another option is to 'chain' exceptions. In your own exception, add a method 'addException(Throwable)'. This would add another exception to a List of exceptions. When you print the stack trace, iterate through the chained exceptions to print them all out. One place where I find this useful is rollback() code. If the original SQL statement fails AND the rollback fails, I definitely want to see that rollback() exception as well in my logs.
The DB thing makes me headache. What I actually
wanted is a solution for:
Let say I have a class "HelloAction.class" contains
the code:
public ActionForward XXX() {
     DbManager DB = new DBManager();
     ... do some DB thing here...
     SecondClass SC = new SecondClass();
     SC.doSomeOtherDbThing();
     ... do something else...
     ThirdClass TC = new ThirdClass();
     SC.doMoreOtherDbThing();
}There are some functions in SecondClass.class and
ThirdClass.class that will need database connection.
I consider 'global variable' is because I want these
two classes are able to use the same
connection(DbManager) from the function -
ActionForward XXX().
What is the best way to implement the above situation
(sharing the same connection in different classes &
sub-classes?
I also just realize that the problem of multi-threads
with these two class variables..
     private Connection conn;
     private CallableStatement cs;Really headache. I really appreciate any comments.
Thanks.
- myyPass the Connection or DataSource to each method or constructor. Call commit() or rollback() from the method that created the transaction.
- Saish