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Some other notes:

1. Don't mix naming styles:

    String DataURL = "jdbc:db://localhost:5112/users";
    String loginName = "stackoverflow"
   

Variable names should be camelCase (with lowercase first letter).

2. Instead of checking that iUserID is -1 or not check the result of result.next() and set a boolean flag:

    final boolean hasUser = result.next();
    if (hasUser) {
    iUserID = result.getInt(1);
    loginUser = result.getString(2);
    } else {
    wr.println ("you cannot login in !Access Denied!");
    }
   

Or even better:

 final boolean hasUser = result.next();
 if (!hasUser) {
 wr.println ("you cannot login in !Access Denied!");
 return; // etc.
 }
 final int userID = result.getInt(1);
 final String loginUser = result.getString(2);

It removes magic strings from the code and makes it easier to read.

Note that I moved the variable declarations too. (Effective Java, Second Edition, Item 45: Minimize the scope of local variables has a good overview on this topic. (Google for "minimize the scope of local variables", it's on Google Books too.))

3. Instead of the ResultSet.get*(int columnIndex) methods use the ResultSet.get*(String columnLabel) ones:

    userID = result.getInt("UserID");
    loginUser = result.getString("Username);
   

It also removes some magic numbers from the code and makes it easier to read.

4. This:

    PrintWriter wr = response.getWriter ();
   

should be:

 PrintWriter wr = response.getWriter();

(From here):

Note that a blank space should not be used between a method name and its opening parenthesis. This helps to distinguish keywords from method calls.

5. I'd use longer variable names for readability:

    PrintWriter writer = response.getWriter();
   

Some other notes:

1. Don't mix naming styles:

    String DataURL = "jdbc:db://localhost:5112/users";
    String loginName = "stackoverflow"
   

Variable names should be camelCase (with lowercase first letter).

2. Instead of checking that iUserID is -1 or not check the result of result.next() and set a boolean flag:

    final boolean hasUser = result.next();
    if (hasUser) {
    iUserID = result.getInt(1);
    loginUser = result.getString(2);
    } else {
    wr.println ("you cannot login in !Access Denied!");
    }
   

Or even better:

 final boolean hasUser = result.next();
 if (!hasUser) {
 wr.println ("you cannot login in !Access Denied!");
 return; // etc.
 }
 final int userID = result.getInt(1);
 final String loginUser = result.getString(2);

It removes magic strings from the code and makes it easier to read.

Note that I moved the variable declarations too. (Effective Java, Second Edition, Item 45: Minimize the scope of local variables has a good overview on this topic. (Google for "minimize the scope of local variables", it's on Google Books too.))

3. Instead of the ResultSet.get*(int columnIndex) methods use the ResultSet.get*(String columnLabel) ones:

    userID = result.getInt("UserID");
    loginUser = result.getString("Username);
   

It also removes some magic numbers from the code and makes it easier to read.

4. This:

    PrintWriter wr = response.getWriter ();
   

should be:

 PrintWriter wr = response.getWriter();

(From here):

Note that a blank space should not be used between a method name and its opening parenthesis. This helps to distinguish keywords from method calls.

5. I'd use longer variable names for readability:

    PrintWriter writer = response.getWriter();
   

1, Don't mix naming styles:

String DataURL = "jdbc:db://localhost:5112/users";
String loginName = "stackoverflow"

Variable names should be camelCase (with lowercase first letter).http://stackoverflow.com/questions/2082729/to-camelcase-or-not-to-camel-case

2, Instead of checking that iUserID is -1 or not check the result of result.next() and set a boolean flag:

final boolean hasUser = result.next();
if (hasUser) {
  iUserID = result.getInt(1);
 loginUser = result.getString(2);
} else {
 wr.println ("you cannot login in !Access Denied!");
}

Or even better:

final boolean hasUser = result.next();
if (!hasUser) {
 wr.println ("you cannot login in !Access Denied!");
 return; // etc.
}
final int userID = result.getInt(1);
final String loginUser = result.getString(2);

Note that I moved the variable declarations too. (Effective Java, Second Edition, Item 45: Minimize the scope of local variables has a good overview on this topic. (Google for "minimize the scope of local variables", it's on Google Books too.))

3, Instead of the ResultSet.get*(int columnIndex) methods use the ResultSet.get*(String columnLabel) ones:

userID = result.getInt("UserID");
loginUser = result.getString("Username);

4,

PrintWriter wr = response.getWriter ();

should be

PrintWriter wr = response.getWriter();

From http://www.oracle.com/technetwork/java/codeconventions-141388.html#475

5, I'd use longer variable names:

PrintWriter writer = response.getWriter();

It's more readable.

1. Don't mix naming styles:

    String DataURL = "jdbc:db://localhost:5112/users";
    String loginName = "stackoverflow"
   

Variable names should be camelCase (with lowercase first letter).

2. Instead of checking that iUserID is -1 or not check the result of result.next() and set a boolean flag:

    final boolean hasUser = result.next();
    if (hasUser) {
    iUserID = result.getInt(1);
    loginUser = result.getString(2);
    } else {
    wr.println ("you cannot login in !Access Denied!");
    }
   

Or even better:

final boolean hasUser = result.next();
 if (!hasUser) {
 wr.println ("you cannot login in !Access Denied!");
 return; // etc. }
 final int userID = result.getInt(1);
final String loginUser = result.getString(2);

Note that I moved the variable declarations too. (Effective Java, Second Edition, Item 45: Minimize the scope of local variables has a good overview on this topic. (Google for "minimize the scope of local variables", it's on Google Books too.))

3. Instead of the ResultSet.get*(int columnIndex) methods use the ResultSet.get*(String columnLabel) ones:

    userID = result.getInt("UserID");
    loginUser = result.getString("Username);
   

4. This:

    PrintWriter wr = response.getWriter ();
   

should be:

 PrintWriter wr = response.getWriter();

(From here ):

5. I'd use longer variable names for readability:

    PrintWriter writer = response.getWriter();