Re: Dynamic classpath

[wizard of oz]

Can you post an example of Connection.connect (String, properties)?

My java doc says java.sql.Connection is an interface and doesn't mention a
connect method.

TIA

"Thomas Kellerer" <YQ**********@spammotel.comwrote in message
news:68*************@mid.individual.net...

wizard of oz, 07.05.2008 10:41:

>I have a need to have a classpath that is determined by the set of jars a
user places into a directory.

By way of example, placing jar's into certain directories in a tomcat web
server will cause tomcat to include them into the web applications class
path. An example of this might be a charting package used by the web app
to generate charts.

In my specific example, I am building a query tool and I want to be able
to tell users to simply drop the JDBC drivers into a directory and my app
will "pick them up". Thus there would be no need to edit the classpath.

My target environment is Java 6.0

You will need to use a URLClassLoader to load the driver(s)
The only problem is then doing the connection as the DriverManager will
refuse to use a driver that was loaded by a different classloader.

You have two options to resolve this:

1) use Connection.connect(String, Properties) directly (bypassing
DriverManager) 2) create a wrapper class that pretends to be a driver but
delegates everything to the real instance (loaded by your own
classloader).
I am using option 1) without any problems.

Thomas

[Thomas Kellerer]

Sorry, I meant Driver.connect()

Once you have loaded the driver class using a URLClassLoader, you can create a new instance and cast that to a Driver and then ask the driver to connect.

Something like this:

URLClassLoader l = new URLClassLoader(...);
Class drvClass = l.loadClass("org.postgresql.Driver");
java.sql.Driver drv = (java.sql.Driver)drvClass.newInstance();
Properties props = new Properties();
props.put("user", "postgres");
props.put("password", "password");

java.sql.Connection conn = drv.connect("jdbc:postgresql:localhost/mydb", props);

Regards
Thomas

wizard of oz, 07.05.2008 13:38:

Can you post an example of Connection.connect (String, properties)?

My java doc says java.sql.Connection is an interface and doesn't mention a
connect method.

TIA

"Thomas Kellerer" <YQ**********@spammotel.comwrote in message
news:68*************@mid.individual.net...

>wizard of oz, 07.05.2008 10:41:

>>I have a need to have a classpath that is determined by the set of
jars a user places into a directory.

By way of example, placing jar's into certain directories in a tomcat
web server will cause tomcat to include them into the web
applications class path. An example of this might be a charting
package used by the web app to generate charts.

In my specific example, I am building a query tool and I want to be
able to tell users to simply drop the JDBC drivers into a directory
and my app will "pick them up". Thus there would be no need to edit
the classpath.

My target environment is Java 6.0

You will need to use a URLClassLoader to load the driver(s)
The only problem is then doing the connection as the DriverManager
will refuse to use a driver that was loaded by a different classloader.

You have two options to resolve this:

1) use Connection.connect(String, Properties) directly (bypassing
DriverManager) 2) create a wrapper class that pretends to be a driver
but delegates everything to the real instance (loaded by your own
classloader).
I am using option 1) without any problems.

Thomas

[wizard of oz]

Cool, thanks.

The only problem is then doing the connection as the DriverManager will refuse to use a driver that was loaded by a different classloader.

While I was awaiting your reply, I thought I would have a go at it myself. For what it is worth, the following also works. So the "standard method" of connecting to a database (via DriverManager) seems to work now (at least for my JDBC driver).
As mentioned I'm working in Java 6.0.
// For some reason my driver is in three parts
URL jdbc1 = new URL ("file:/classpath/jars/JDBC/tdgssconfig.jar");
URL jdbc2 = new URL ("file:/classpath/jars/JDBC/tdgssjava.jar");
URL jdbc3 = new URL ("file:/classpath/jars/JDBC/terajdbc4.jar");

// Either of the following seem to work.
// ClassLoader loader = new URLClassLoader (new URL [] { path, jdbc1, jdbc2, jdbc3 }, this.getClass().getClassLoader());
ClassLoader loader = new URLClassLoader (new URL [] { path, jdbc1, jdbc2, jdbc3 });

Class jdbcClass = loader.loadClass ("com.ncr.teradata.TeraDriver");
// The following is required otherwise I get a "No suitable driver" SQLException.
Object jdbcDriver = jdbcClass.newInstance ();

Connection c = DriverManager.getConnection("jdbc:teradata://dbc/", "uid", "pass");
Statement s = c.createStatement();
ResultSet r = s.executeQuery("select * from t1;");
while (r.next()) {
System.out.println (r.getString(1) + ", " + r.getString (2));
}
r.close ();
s.close ();
c.close ();

Thanks again for your post Thomas, it really helped point me in the right direction.

Glenn Mc
"Thomas Kellerer" <YQ**********@spammotel.comwrote in message news:68*************@mid.individual.net...

Sorry, I meant Driver.connect()

Once you have loaded the driver class using a URLClassLoader, you can create a new instance and cast that to a Driver and then ask the driver to connect.

Something like this:

URLClassLoader l = new URLClassLoader(...);
Class drvClass = l.loadClass("org.postgresql.Driver");
java.sql.Driver drv = (java.sql.Driver)drvClass.newInstance();
Properties props = new Properties();
props.put("user", "postgres");
props.put("password", "password");

java.sql.Connection conn = drv.connect("jdbc:postgresql:localhost/mydb", props);

Regards
Thomas

[Lew]

wizard of oz wrote:

Cool, thanks.

Please post to Usenet in plain text, not HTML.

--
Lew

[wizard of oz]

Spoke too soon. The following code *does* work in a test project, but not my
main project. Both projects are built using NetBeans 6.0.

The only difference I can pick is that the main project is a Swing project,
whereas the test project doesn't use Swing. Thomas's use of Driver.connect
does however work as he advertised.

Thanks again for all of the replies.

"wizard of oz" <no****@gtajb.comwrote in message
news:AF*****************@news-server.bigpond.net.au...
Cool, thanks.

The only problem is then doing the connection as the DriverManager will
refuse to use a driver that was loaded by a different classloader.

While I was awaiting your reply, I thought I would have a go at it myself.
For what it is worth, the following also works. So the "standard method"
of connecting to a database (via DriverManager) seems to work now (at
least for my JDBC driver).
As mentioned I'm working in Java 6.0.
// For some reason my driver is in three parts
URL jdbc1 = new URL
("file:/classpath/jars/JDBC/tdgssconfig.jar");
URL jdbc2 = new URL
("file:/classpath/jars/JDBC/tdgssjava.jar");
URL jdbc3 = new URL
("file:/classpath/jars/JDBC/terajdbc4.jar");

// Either of the following seem to work.
// ClassLoader loader = new URLClassLoader (new URL [] { path,
jdbc1, jdbc2, jdbc3 }, this.getClass().getClassLoader());
ClassLoader loader = new URLClassLoader (new URL [] { path,
jdbc1, jdbc2, jdbc3 });

Class jdbcClass = loader.loadClass
("com.ncr.teradata.TeraDriver");
// The following is required otherwise I get
a "No suitable driver" SQLException.
Object jdbcDriver = jdbcClass.newInstance ();

Connection c =
DriverManager.getConnection("jdbc:teradata://dbc/", "uid", "pass");
Statement s = c.createStatement();
ResultSet r = s.executeQuery("select * from t1;");
while (r.next()) {
System.out.println (r.getString(1) + ", " + r.getString
(2));
}
r.close ();
s.close ();
c.close ();

Thanks again for your post Thomas, it really helped point me in the right
direction.

Glenn Mc
"Thomas Kellerer" <YQ**********@spammotel.comwrote in message
news:68*************@mid.individual.net...

Sorry, I meant Driver.connect()

Once you have loaded the driver class using a URLClassLoader, you can
create a new instance and cast that to a Driver and then ask the driver
to connect.

Something like this:

URLClassLoader l = new URLClassLoader(...);
Class drvClass = l.loadClass("org.postgresql.Driver");
java.sql.Driver drv = (java.sql.Driver)drvClass.newInstance();
Properties props = new Properties();
props.put("user", "postgres");
props.put("password", "password");

java.sql.Connection conn = drv.connect("jdbc:postgresql:localhost/mydb",
props);

Regards
Thomas