Accumulo VFS ClassLoader

This module contains a ClassLoader implementation that can be used as the JVM System ClassLoader or a ClassLoader for Accumulo table contexts.

System Class Loader

To use this ClassLoader as the System ClassLoader you must set the JVM system property java.system.class.loader to the fully qualified class name (org.apache.accumulo.classloader.vfs.AccumuloVFSClassLoader). This jar and it's dependent jars must be on the java.class.path.

NOTE: When used in this manner the reloading feature is disabled because the JVM caches classes created from the standard JVM hierarchy and thus reloading has no effect.

Configuration

To set the classpath for this ClassLoader you must define the system property vfs.class.loader.classpath and set it to locations that are supported by Apache Commons VFS.

The ClassLoader monitors the classpath for changes at 5 minute intervals. To change this interval define the sytem property vfs.classpath.monitor.seconds.

This ClassLoader follows the normal parent delegation model but can be set to load classes and resources first, before checking if the parent classloader can, by setting the system property vfs.class.loader.delegation to “post”.

Accumulo Table Context ClassLoader

To use this ClassLoader as the ContextClassLoader you must set the JVM system property vfs.context.class.loader.config to a valid JSON configuration file and set the Accumulo configuration property general.context.class.loader.factory to the fully qualified class name of the ContextClassLoaderFactory implementation (org.apache.accumulo.classloader.vfs.context.ReloadingVFSContextClassLoaderFactory). This jar and it's dependent jars must be on the java.class.path.

Configuration

You will need to define the supported contexts and their configuration in a JSON formatted file. For example:

{
  "contexts": [
    {
      "name": "cx1",
      "config": {
        "classPath": "file:///tmp/foo",
        "postDelegate": true,
        "monitorIntervalMs": 30000
      }
    },
    {
      "name": "cx2",
      "config": {
        "classPath": "file:///tmp/bar",
        "postDelegate": false,
        "monitorIntervalMs": 30000
      }
    }
  ]
}

Additional Configuration

Finally, this ClassLoader keeps a local cache of objects pulled from remote systems (via http, etc.). The default location for this cache directory is the value of the system property java.io.tmpdir. To change this location set the system property vfs.cache.dir to an existing directory.

Implementation

This ClassLoader maintains a VFSClassLoader delegate that references the classpath (as specified by vfs.class.loader.classpath). The ReloadingVFSClassLoader implements FileListener and creates a DefaultFileMonitor that checks for changes on the classpath at the interval specified by vfs.classpath.monitor.seconds and creates a new VFSClassLoader delegate. Future requests to load classes and resources will use this new delegate; the old delegate is no longer referenced (except by the classes it has loaded) and can be garbage collected.