Re: Unregistering the driver from DriverManager

Hello,

I'm trying out the PR branch just now.  Seems fine (starts as a bundle, except for remark below, connection available, can use new features such as connection.getSchema() => "$user") ; a few small remaining observations:

• The dependency on org.osgi.service.jdbc has "resolution:=optional" in the MANIFEST.MF file, but it should almost certainly *not* be optional (as this causes java.lang.NoClassDefFoundError: org/osgi/service/jdbc/DataSourceFactory).
  
  
• In the PGBundleActivator class, for stop-restart cases, the "start" method should almost certainly have a small conditional block, such as:
  
  if (!Driver.isRegistered()) { Driver.register(); }
  
  => this would mirror the deregistration in the "stop" method, and avoid breaking code that might rely on the current static initialization behavior.

I'm still building from the forked PR repository ; from your other messages, I assume this will soon be merged into the main "pgjdbc" repository.

--
Christopher

On 4 January 2015 at 17:37, Alexis Meneses <alexis.meneses@gmail.com> wrote:

Thanks for your feedback. All your observations make sense to me and I updated the PR branch accordingly.

Alexis

2015-01-03 15:30 GMT+01:00 Christopher BROWN <brown@reflexe.fr>:

Hi,

I've cloned your pull request locally (haven't forked it, even although I've got a github account and could do so...).

I don't (yet) have time to try it out fully (won't be able to do so until Tuesday at the earliest), but here's some initial observations.

1/ You might want to use a more recent version of "bnd" (you're using 1.5, the current is 2.4)

2/ You refer to Bundle-Activator: org.postgresql.osgi.PGBundleActivator in the manifest, and the source code is there, but it's not included in the resulting OSGi ".jar" (generated in "/jars").

3/ The changes to the Driver class (register(), unregister(), and isRegistered()) look good in source code.

4/ In PGBundleActivator::start, you should perhaps use:

Dictionary<String,Object> properties = new Hashtable<>(4);

// instead of "Properties", to avoid implying that values are strings (this isn't the case here)

5/ In PGBundleActivator::start, shouldn't "Postgresql" be written "PostgreSQL" in the OSGI_JDBC_DRIVER_NAME property?

6/ In PGBundleActivator::stop, you should set the _registration instance to null after unregistering because it's possible to restart a stopped bundle.

7/ As you're using Bundle-ManifestVersion: 2, you might also want to add Require-Capability: osgi.ee;filter:="(&(osgi.ee=JavaSE)(version=1.7))" (where 1.7 is derived from "ant.java.version" during build), as this enforces the JDBC 4.1 "level" indirectly by requiring the appropriate JavaSE version).

The choice of the DataSourceFactory API seems like a good idea to me, as opposed to just registering the "Driver" interface directly.

Is the 9.4 branch stable (as in safe to use as-is, even if not at "feature freeze" yet)?

--
Christopher

On 2 January 2015 at 17:08, Alexis Meneses <alexis.meneses@gmail.com> wrote:

Christopher,

(1) Would you mind testing if PR #241 on Github fits your needs and works in the OSGi environment you're using?

Note that it don't check presence of BundleContext via Class.forName() because this class could be erroneously present in the classpath even without an OSGi Framework being started.

(2) Driver.deregister() method (named after DriverManager method) has been exposed so as to be used in other contexts like Java EE.

Alexis

2014-12-29 14:17 GMT+01:00 Christopher BROWN <brown@reflexe.fr>:

There are two main situations where it would be useful to automatically unregister the driver:

1) OSGi - and the suggestion of using BundleActivator.stop() would be a good fit here (as long as care is taken to ensure resolution=optional for other dependencies)

2) Java EE web applications, using a ServletContextListener, perhaps using annotations as described in https://blogs.oracle.com/swchan/entry/servlet_3_0_annotations (but this would exclude older application servers)

With regards to (2), I generally place JDBC drivers in the main classloader of the application server, as opposed to embedding in WEB-INF/lib when working with webapps.  Also, not all of the webapps I have to deal with (from time to time, it's not my main focus) are up to Servlet 3.0, many as still stuck on 2.5.  And in any case, embedding JDBC drivers in webapps (without matching versions) then accessing them via DriverManager is may cause class lookup problems.

A good solution to (1) above to me would be like this then (building on the suggestion of Alexis):

- keep the static block in driver

- check -- via Class.forName("org.osgi.framework.BundleContext") -- if OSGi classes are visible, implying that the driver has been loaded as a bundle, and skip the DriverManager registration in the static block (unless forced via a system property, just in case something breaks for someone relying on current behavior)

- register the driver in DriverManager in the BundleActivator.start() method

- unregister it (same instance, kept as a reference) in the BundleActivator.stop() method

The only reason I mention (2) is because it might be useful to share some common code.  Or not.  In any case, (1) is the only requirement at this time and (2) isn't as much of a problem.

--

Christopher

On 29 December 2014 at 13:45, Alexis Meneses <alexis.meneses@gmail.com> wrote:

If the only concern is OSGi environments, I think that unregistering could be handled in a BundleActivator.stop() implementation bundled with the driver.

See pending issue #71 on Github.

2014-12-29 12:48 GMT+01:00 Dave Cramer <pg@fastcrypt.com>:

I have no objection to an unregister static method being added. It's not in the API so it would not effect anything really

Dave Cramer

dave.cramer(at)credativ(dot)ca
http://www.credativ.ca

On 29 December 2014 at 04:53, Christopher BROWN <brown@reflexe.fr> wrote:

Hello,

I'm starting to integrate the Postgresql JDBC driver into an OSGi environment, as an OSGi bundle.  I'm evaluating the different ways to avoid a classloader leak with DriverManager when hot-swapping the driver bundle without restarting the host application, and am seeking suggestions on best practice regarding the Postgresql JDBC driver.

Another bundle (which I provide, it's not third-party) will directly depend upon it (loading classes directly, namely org.postgresql.Driver); when the Postgresql JDBC driver classes are loaded, the other bundle will create a DataSource using a JDBC connection pool, and register the DataSource as an OSGi service.  Normally, that's all that will happen during the application lifecycle, but in principle, it's possible for the administrator to want to replace say the 9.3 driver with the 9.4 driver by removing the 9.3 ".jar" at runtime, and replacing it with the 9.4 ".jar", all at runtime; when the first ".jar" is deleted, the dependent bundle is knocked offline, unregistering the DataSource automatically, and notifying all clients; when the second is installed, the application is once again fully-functional (and all this normally occurs within a few hundred milliseconds).

Looking at the source code, I can see that the org.postgresql.Driver class registers itself in a "static" block with DriverManager (which is the correct behavior regarding the JDBC spec).  However, short of a brute-force loop -- like this one: http://stackoverflow.com/a/5315467 (enhanced to check the class name of each driver, to avoid clobbering unrelated driver registrations) -- is there any other approach possible or that could be added, say a NonRegisteringDriver (superclass of Driver, with all logic except for the static initializer) or an "unregister()" static method, or a field containing the registered Driver instance?

Thanks,

Christopher