Github user mattf-horton commented on a diff in the pull request:

    https://github.com/apache/metron/pull/530#discussion_r131822044
  
    --- Diff: 
bundles-lib/src/main/java/org/apache/metron/bundles/BundleClassLoaders.java ---
    @@ -0,0 +1,353 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *     http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +package org.apache.metron.bundles;
    +
    +import java.net.URISyntaxException;
    +import java.util.*;
    +
    +import org.apache.commons.collections.CollectionUtils;
    +import org.apache.commons.vfs2.FileObject;
    +import org.apache.commons.vfs2.FileSystemException;
    +import org.apache.commons.vfs2.FileSystemManager;
    +import org.apache.metron.bundles.bundle.Bundle;
    +import org.apache.metron.bundles.bundle.BundleCoordinate;
    +import org.apache.metron.bundles.bundle.BundleDetails;
    +import org.apache.metron.bundles.util.BundleProperties;
    +import org.apache.metron.bundles.util.BundleSelector;
    +import org.apache.metron.bundles.util.FileUtils;
    +import org.apache.metron.bundles.util.BundleUtil;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +/**
    + * A singleton class used to initialize the extension and framework 
classloaders.
    + */
    +public final class BundleClassLoaders {
    +
    +  private static volatile BundleClassLoaders bundleClassLoaders;
    +  private volatile InitContext initContext;
    +  private static final Logger logger = 
LoggerFactory.getLogger(BundleClassLoaders.class);
    +
    +  private final static class InitContext {
    +
    +    private final List<FileObject> extensionDirs;
    +    private final Map<String, Bundle> bundles;
    +
    +    private InitContext(
    +        final List<FileObject> extensionDirs,
    +        final Map<String, Bundle> bundles) {
    +      this.extensionDirs = extensionDirs;
    +      this.bundles = bundles;
    +    }
    +  }
    +
    +  private BundleClassLoaders() {
    +  }
    +
    +  /**
    +   * @return The singleton instance of the BundleClassLoaders
    +   */
    +  public static BundleClassLoaders getInstance() {
    +    BundleClassLoaders result = bundleClassLoaders;
    +    if (result == null) {
    +      synchronized (BundleClassLoaders.class) {
    +        result = bundleClassLoaders;
    +        if (result == null) {
    +          bundleClassLoaders = result = new BundleClassLoaders();
    +        }
    +      }
    +    }
    +    return result;
    +  }
    +
    +  public static void reset() {
    +    getInstance().unInit();
    +  }
    +
    +  private void unInit() {
    +    synchronized (this) {
    +      initContext = null;
    +    }
    +  }
    +
    +  /**
    +   * Initializes and loads the BundleClassLoaders. This method must be 
called before the rest of the
    +   * methods to access the classloaders are called and it can be safely 
called any number of times
    +   * provided the same framework and extension working dirs are used.
    +   *
    +   * @param fileSystemManager the FileSystemManager
    +   * @param extensionsDirs where to find extension artifacts
    +   * @param props BundleProperties
    +   * @throws FileSystemException if any issue occurs while exploding 
bundle working directories.
    +   * @throws java.lang.ClassNotFoundException if unable to load class 
definition
    +   * @throws IllegalStateException already initialized with a given pair 
of directories cannot
    +   * reinitialize or use a different pair of directories.
    +   */
    +  public void init(final FileSystemManager fileSystemManager, final 
List<FileObject> extensionsDirs,
    +      BundleProperties props)
    +      throws FileSystemException, ClassNotFoundException, 
URISyntaxException {
    +    if (extensionsDirs == null || fileSystemManager == null) {
    +      throw new NullPointerException("cannot have empty arguments");
    +    }
    +    InitContext ic = initContext;
    +    if (ic == null) {
    +      synchronized (this) {
    +        ic = initContext;
    +        if (ic == null) {
    +          initContext = ic = load(fileSystemManager, extensionsDirs, 
props);
    +        }
    +      }
    +    } else {
    +      boolean matching = CollectionUtils
    +          .isEqualCollection(initContext.extensionDirs, extensionsDirs);
    +      if (!matching) {
    +        throw new IllegalStateException(
    +            "Cannot reinitialize and extension/framework directories 
cannot change");
    +      }
    +    }
    +  }
    +
    +  /**
    +   * Should be called at most once.
    +   */
    +  private InitContext load(final FileSystemManager fileSystemManager,
    +      final List<FileObject> extensionsDirs, BundleProperties props)
    +      throws FileSystemException, ClassNotFoundException, 
URISyntaxException {
    +    // get the system classloader
    +    final ClassLoader systemClassLoader = 
ClassLoader.getSystemClassLoader();
    +
    +    // find all bundle files and create class loaders for them.
    --- End diff --
    
    This also has the unpleasant side effect of effectively loading all 
extensions at startup, doesn't it?  The NiFi folk were talking about fixing 
this, but I don't know if they have.  If so, that would certainly be one good 
reason for updating.  Please note it in METRON-1070.


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