Thanks Peter, you're right, 'simpler' is not always less complex.

Serialized circular object graphs are not required for RMI either (in our version of RMI, called JERI, which stands for Jini Extensible Remote Invocation), we found only Throwable used it and were able to work around it. Another unnecessary complexity is annotation of serialization streams with codebase URL strings (again it appears a simple solution at first). Effectively every class has it's ClassLoader located from a map using a URL string and Thread context ClassLoader and any class can be loaded given an annotation string. This duplication of class resolution systems between RMIClassLoader and ClassLoader visibility, leads to incredible complexity in debugging ClassNotFoundException and ClassCastException's.

I've eliminated circular object graphs and duplicate class resolution. Instead ClassLoader's are assigned to each Endpoint, and ClassLoaders are then responsible for class visiblity. This allows us to support OSGi, let it perform class resolution and make some very difficult problems dissapear. I've limited dynamic class download to what we call smart proxy's (used for remote services), where previously any class was allowed to be dynamically loaded. Smart proxy's are serialized independantly of the serialization stream from which they're obtained, the service the proxy represents is first authenticated, then if necessary a bundle is provisioned and used for deserialization of the proxy classes, the proxy relies on the bundle to determine class resolution and the bundles at each endpoint (the proxy and that of the service) will have identical bundle versions as well.



On 14/04/2018 10:38 PM, Peter Kriens wrote:
Very nice example of something I see too often: choosing a bad solution that just appears to be ’simpler’ to use. I think the Java world suffers from this terribly.

We also had a big discussions about circularity in the OSGi for the DTO’s. Initially they were circular but in the we agreed to not make them have inner references. It is slightly more work for the receiver but it makes life so much simpler overall …

Kind regards,

Peter Kriens

On 14 Apr 2018, at 05:35, Peter via osgi-dev < <>> wrote:

On 13/04/2018 6:32 PM, Neil Bartlett via osgi-dev wrote:

On Thu, Apr 12, 2018 at 10:12 PM, Mark Raynsford via osgi-dev < <><>> wrote:

   On 2018-04-12T20:32:13 +0200
   Peter Kriens < <>
<>> wrote:

> Caught between a rock and a hard place with only one way forward …

   I should make the point that I don't hate the JPMS. I do think that
   it's just barely the minimum viable product, though.

   The JVM really did need a module system, both for the maintenance of
   the JDK itself and the future features that the system enables.

> Oracle’s strategy is a mystery to me.

I think their strategy is fairly explicable, but I think they did make
   some mistakes with some of the specifics (filename-based
   There's a pattern that Oracle tend to follow: They solicit
   opinions from
   everyone vigorously, and then they implement the smallest possible
   subset such that the fewest possible people are pissed off by it. If
   there's a possibility of doing something wrong, nothing is done

While I've seen that principle operate at other times (remember how controversial erasure was in Java 5?), I'm not sure it's worked that way in the JPMS case. In fact JPMS does far more than it needed to.

The key feature of JPMS that could not be achieved before, even with ClassLoaders, was strictly enforced isolation via the accessibility mechanism, as opposed to the visibility mechanism that is employed by OSGi. That strict isolation was needed primarily to allow Oracle to close off JVM internals from application code and thereby prevent a whole class of security vulnerabilities. Remember that Oracle was being absolutely slaughtered in the press around 2011-12 over the insecurity of Java, and most corporates uninstalled it from user desktops.

Java deserialization vulnerabilties.

Ironically, Java serialization was an exception, rather than a minimalist approach, it was given advanced, if not excessive functionality, including the ability to serialize circular object graphs.

Circular relationships generally tend to be difficult to manage.

Due to the support for circular object graphs, it wasn't possible to to use a serialization constructor, so all invariant checks had to be made after construction, when it was too late. Making matters worse, an attacker can create any serializable object they want, and because of the way deserialized objects are created, child class domains aren't on the call stack during super class deserialization. An attacker can take advantage of the circular object graph support, and caching to obtain a reference to any object in a deserialized graph.

In essence, they needed to have an alternative locked down implementation of serialization.

There's nothing wrong with the java serialization protocol. I wrote a hardened implementation of java serialization, refactored from Apache Harmony's implementation, implementing classes use a serialization constructor, that ensures an object cannot be created unless it's invariants were satisfied, this includes the ability to check inter class invariants as well. It doesn't support circular object graphs and has limits on how much data could be cached, limits on array size etc.

I submitted the api to the OpenJDK development mail list, there was interest there, but they decided they needed to support circular object graphs.

In the end Oracle decided to use white listing.



But they could have achieved this with a thin API, comparable to ProtectionDomain. If they had done that then OSGi (and other module systems like JBoss) could have chosen to leverage the API to enforce strict separation between OSGi bundles.

But they didn't do that. Instead they implemented a whole new, incompatible module system with its own metadata format, including changes to the Java language spec. Then they restricted the ability to apply strict isolation to artifacts that are JPMS modules. With the thin API they could have still built their own module system on top, following their own ideas of how modules should work, and competed with OSGi on a fairer playing field.

   Being incomplete and "too strict" is considered preferable to
   any kind of maintenance burden or making a mistake that people then
   come to depend upon. Then, after a version of something is released,
   the dust settles and the people that are still complaining after a
   or so of implementation are asked for comments again. The process
   repeats! You can see this going on with quite a few of their
   A good example of this with the JPMS is that there was a vigorous
   insistence that the module graph be a DAG. Now, some way into the
   year, it's going to be allowed to be cyclic but only at run-time. I
think the process does eventually produce results, but it takes a long
   time to get there and demands a lot of patience from the people
   involved. Most VM features seem to start off utterly spartan and then
   grow to support the use-cases that people wish they'd supported right
   from the start.

   Java in particular has awful press, and a userbase that seems to
   incomprehensibly enraged over all good news and all bad news
   indiscriminately, so that doesn't help the perception of the process
   (or the results).

   I think the key will be to continue complaining for as long as it
   to get better interop between OSGi and the JPMS. :)

Interop already works just fine in one direction: OSGi bundles depending on JPMS modules, with a combination of the changes in R7 to export java.* packages from the system bundle and some creative use of Provide/Require-Capability. But bidirectional interop will likely always be impossible or very hard, because JPMS modules are only allowed to depend on JPMS modules. This was clearly a deliberate strategy to tilt the table towards JPMS, but it may be backfiring since -- as you've pointed out -- applications can only migrate to modules when all of their dependencies are modules, including third party libraries, and the migration of libraries has been exceedingly slow.


   --     Mark Raynsford |

   OSGi Developer Mail List <><>

OSGi Developer Mail List <>

OSGi Developer Mail List <>

OSGi Developer Mail List

Reply via email to