fix #43: Prevent ClassCastExceptions after plugin gets reloaded in a cluster

[guusdk]

This plugin caches instances of classes that are defined in the code of the plugin. This causes ClassCastExceptions, when the plugin gets unloaded (removing the class definitions) and updated/reloaded (replacing the class definitions). The cached values at that time will refer to classes that no longer exist, causing the exceptions.

https://igniterealtime.atlassian.net/browse/OF-2239 introduces a new type of Cache to work around this problem, the org.jivesoftware.util.cache.SerializingCache which is available since Openfire 4.7.0. This implementation does not cache the instance directly, but caches a serialized version instead. This way, there no longer is a reference to the class, which means that any (compatible) future class definition can be used to instantiate the cache entry again.

[guusdk]

With Openfire 4.9.0-SNAPSHOT, this generates the following runtime error, when the plugin gets initialized:

java.lang.ClassCastException: class org.jivesoftware.util.cache.CacheWrapper cannot be cast to class org.jivesoftware.util.cache.SerializingCache (org.jivesoftware.util.cache.CacheWrapper and org.jivesoftware.util.cache.SerializingCache are in unnamed module of loader org.jivesoftware.openfire.starter.JiveClassLoader @1da2cb77)
        at org.jivesoftware.util.cache.CacheFactory.createSerializingCache(CacheFactory.java:518) ~[xmppserver-4.9.0-SNAPSHOT.jar:4.9.0-SNAPSHOT]
        at org.igniterealtime.openfire.plugins.httpfileupload.OpenfireSlotProvider.<init>(OpenfireSlotProvider.java:36) ~[httpfileupload-1.4.1-SNAPSHOT.jar:?]
        at org.igniterealtime.openfire.plugins.httpfileupload.HttpFileUploadPlugin.initializePlugin(HttpFileUploadPlugin.java:413) [httpfileupload-1.4.1-SNAPSHOT.jar:?]
        at org.jivesoftware.openfire.container.PluginManager.loadPlugin(PluginManager.java:637) [xmppserver-4.9.0-SNAPSHOT.jar:4.9.0-SNAPSHOT]
        at org.jivesoftware.openfire.container.PluginMonitor$MonitorTask$4.call(PluginMonitor.java:380) [xmppserver-4.9.0-SNAPSHOT.jar:4.9.0-SNAPSHOT]
        at org.jivesoftware.openfire.container.PluginMonitor$MonitorTask$4.call(PluginMonitor.java:368) [xmppserver-4.9.0-SNAPSHOT.jar:4.9.0-SNAPSHOT]
        at java.util.concurrent.FutureTask.run(FutureTask.java:264) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
        at java.lang.Thread.run(Thread.java:840) [?:?]

[guusdk]

I believe that the SerializableCache implementation is just ... broken? Is this being used anywhere?

I have raised these new tickets:

• https://igniterealtime.atlassian.net/browse/OF-2781
• https://igniterealtime.atlassian.net/browse/OF-2782

PR with fixes: igniterealtime/Openfire#2397