[GitHub] [accumulo] dlmarion commented on a diff in pull request #3136: Remove deprecated AccumuloVFSClassLoader

["dlmarion (via GitHub)" <gi...@apache.org>]

dlmarion commented on code in PR #3136:
URL: https://github.com/apache/accumulo/pull/3136#discussion_r1104510866


##########
core/src/main/java/org/apache/accumulo/core/classloader/URLContextClassLoaderFactory.java:
##########
@@ -0,0 +1,81 @@
+/*
+ * 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
+ *
+ *   https://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.accumulo.core.classloader;
+
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.Arrays;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
+
+import org.apache.accumulo.core.spi.common.ContextClassLoaderFactory;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import com.google.common.cache.Cache;
+import com.google.common.cache.CacheBuilder;
+
+/**
+ * The default implementation of ContextClassLoaderFactory. This classloader returns a
+ * URLClassLoader based on the given context value which is a CSV list of URLs. For example,
+ * file://path/one/jar1.jar,file://path/two/jar2.jar
+ */
+public class URLContextClassLoaderFactory implements ContextClassLoaderFactory {
+
+  private static final AtomicBoolean isInstantiated = new AtomicBoolean(false);
+  private static final Logger LOG = LoggerFactory.getLogger(URLContextClassLoaderFactory.class);
+  private static final String className = URLContextClassLoaderFactory.class.getName();
+
+  // Cache the class loaders for re-use
+  // WeakReferences are used so that the class loaders can be cleaned up when no longer needed
+  // Classes that are loaded contain a reference to the class loader used to load them
+  // so the class loader will be garbage collected when no more classes are loaded that reference it
+  private final Cache<String,URLClassLoader> classloaders =
+      CacheBuilder.newBuilder().weakValues().build();
+
+  public URLContextClassLoaderFactory() {
+    if (!isInstantiated.compareAndSet(false, true)) {
+      throw new IllegalStateException("Can only instantiate " + className + " once");
+    }
+  }
+
+  @Override
+  public ClassLoader getClassLoader(String context) {
+    if (context == null) {
+      throw new IllegalArgumentException("Unknown context");
+    }
+
+    try {
+      return classloaders.get(context, () -> {

Review Comment:
   What happens with the Caffeine cache entry when the ClassLoader is garbage collected and the WeakReference is cleared? Are both the key and value removed? Is a new ClassLoader created after the WeakReference is cleared out? Just curious if there is a possibility of null being returned here.



##########
start/src/main/java/org/apache/accumulo/start/Main.java:
##########
@@ -38,20 +37,17 @@ public class Main {
 
   private static final Logger log = LoggerFactory.getLogger(Main.class);
   private static ClassLoader classLoader;
-  private static Class<?> vfsClassLoader;
   private static Map<String,KeywordExecutable> servicesMap;
 
   public static void main(final String[] args) throws Exception {
     // Preload classes that cause a deadlock between the ServiceLoader and the DFSClient when
     // using the VFSClassLoader with jars in HDFS.
     ClassLoader loader = getClassLoader();
-    Class<?> confClass = null;
+    Class<?> confClass;
     try {
-      @SuppressWarnings("deprecation")
-      var deprecatedConfClass = org.apache.accumulo.start.classloader.AccumuloClassLoader
-          .getClassLoader().loadClass("org.apache.hadoop.conf.Configuration");
-      confClass = deprecatedConfClass;
-      Object conf = null;
+      confClass =
+          ClassLoader.getSystemClassLoader().loadClass("org.apache.hadoop.conf.Configuration");
+      Object conf;

Review Comment:
   Going back and looking at ACCUMULO-4341 and ACCUMULO-3923, it looks like this was added to get around an issue, that's been resolved, in VFS not loading resource files in jars correctly. I think this can be removed. If you test this with the new VFS classloader and we run into an issue, then we can put it back.



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