Re: Unregistering the driver from DriverManager

Alexis,

Referring to (1):

- The code would be more verbose (either reflection or other
constructions to handle problems with classloading) if it was conditional.
- If the OSGi Enterprise APIs aren't available, then the service would
not be registered in the service registry, which is more like a silent
failure and perhaps not immediately apparent.
- Anyway, the org.osgi.service.jdbc package is part of "compendium", of
which "enterprise" is more or less a superset; referring to the OSGi
Compendium specifications PDF, each package is pretty much independent and
has its own specification and version number; they're grouped for
convenience and it's therefore straightforward to pick and mix APIs. For
example, the implementation of EventAdmin with Apache Felix is supplied as
a ".jar" file including a specific API package only, plus the
implementation.

To get the code working (PostgreSQL JDBC driver), I simply derived an
org.osgi.service.jdbc_1.0.0.jar (that version number isn't arbitrary, it's
what's in the spec) by pointing "bnd" at the full compendium ".jar" and
telling it to derive a bundle with that specific package only. It's very
simple (contents below) and I'd recommend that you provide such a ".jar"
(or a "how to") to ensure that the driver can work for anyone who doesn't
already have the API (and OSGi deals nicely with avoiding duplicates and
sharing existing class loader definitions); if you have the relevant
compendium or enterprise bundles loaded by the OSGi framework, the driver
would "just work" (registering the DataSourceFactory), and if not, just
drop in the package

Here's the contents of my "org.osgi.service.jdbc.bnd" file:
________________________________________

# Usage:
# 1) run the command (with Java SE 7):
# java -jar biz.aQute.bnd.jar org.osgi.service.jdbc.bnd
# 2) you should now have your output bundle, check it like this:
# java -jar biz.aQute.bnd.jar print --impexp
org.osgi.service.jdbc_1.0.0.jar
# 3) remove the original compendium JAR file, you don't need it now

symname=org.osgi.service.jdbc
version=1.0.0

-classpath: org.osgi.compendium-5.0.0.jar
-output: ${symname}_${version}.jar

Bundle-SymbolicName: ${symname}
Bundle-Version: ${version}
Bundle-Vendor: OSGi Alliance
Bundle-Copyright: Copyright (c) OSGi Alliance (2000, 2013). All Rights
Reserved.
Bundle-License: http://opensource.org/licenses/apache2.0.php; link="
http://www.apache.org/licenses/LICENSE-2.0"; description="Apache License,
Version 2.0"
Specification-Title: JDBC Service Specification
Specification-Vendor: OSGi Alliance
Specification-Version: ${version}
Import-Package: *
Export-Package: org.osgi.service.jdbc.*;version=${version}
________________________________________

Christopher

On 6 January 2015 at 20:54, Alexis Meneses <alexis(dot)meneses(at)gmail(dot)com> wrote:

> Christopher,
>
> Thanks again for your tests on the subject.
>
> (1) Another thought would be to register the DataSourceFactory service
> only if the class is available to make an OSGi bundle that does not
> necessarily rely on OSGi Enterprise.
>
> (2) Indeed. This was done with the stop-uninstall or stop-update in mind.
> You're right to point out the simple stop-start case.
>
> Alexis
>
> 2015-01-06 11:33 GMT+01:00 Christopher BROWN <brown(at)reflexe(dot)fr>:
>
>> 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(dot)meneses(at)gmail(dot)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(at)reflexe(dot)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(dot)meneses(at)gmail(dot)com>
>>>> wrote:
>>>>
>>>>> Christopher,
>>>>>
>>>>> (1) Would you mind testing if PR #241
>>>>> <https://github.com/pgjdbc/pgjdbc/pull/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(at)reflexe(dot)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(dot)meneses(at)gmail(dot)com> wrote:
>>>>>>
>>>>>>> If the only concern is OSGi environments, I think that unregistering
>>>>>>> could be handled in a BundleActivator
>>>>>>> <http://www.osgi.org/javadoc/r4v43/core/org/osgi/framework/BundleActivator.html>.stop()
>>>>>>> implementation bundled with the driver.
>>>>>>>
>>>>>>> See pending issue #71 <https://github.com/pgjdbc/pgjdbc/issues/71>
>>>>>>> on Github.
>>>>>>>
>>>>>>>
>>>>>>> 2014-12-29 12:48 GMT+01:00 Dave Cramer <pg(at)fastcrypt(dot)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(at)reflexe(dot)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
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>