HDDS-1587. Support dynamically adding delegated classes from to isolated class loader. Contributed by Junjie Chen. (#942)

hadoop-ozone/common/src/main/bin/ozone-config.sh

@@ -49,3 +49,8 @@ else
   exit 1
 fi
 
+# HADOOP_OZONE_DELEGATED_CLASSES defines a list of classes which will be loaded by default
+# class loader of application instead of isolated class loader. With this way we can solve
+# incompatible problem when using hadoop3.x + ozone with older hadoop version.
+#export HADOOP_OZONE_DELEGATED_CLASSES=
+

hadoop-ozone/ozonefs/src/main/java/org/apache/hadoop/fs/ozone/FilteredClassLoader.java

@@ -22,6 +22,8 @@ import java.net.URLClassLoader;
 import java.util.HashSet;
 import java.util.Set;
 
+import org.apache.hadoop.util.StringUtils;
+
 /**
  * Class loader which delegates the loading only for the selected class.
  *
@@ -57,6 +59,10 @@ public class FilteredClassLoader extends URLClassLoader {
     delegatedClasses.add("org.apache.hadoop.fs.ozone.OzoneFSStorageStatistics");
     delegatedClasses.add("org.apache.hadoop.fs.ozone.Statistic");
     delegatedClasses.add("org.apache.hadoop.fs.Seekable");
+
+    delegatedClasses.addAll(StringUtils.getTrimmedStringCollection(
+        System.getenv("HADOOP_OZONE_DELEGATED_CLASSES")));
+
     this.delegate = parent;
     systemClassLoader = getSystemClassLoader();
 

hadoop-ozone/ozonefs/src/test/java/org/apache/hadoop/fs/ozone/TestFilteredClassLoader.java

@@ -0,0 +1,63 @@
+/*
+ * 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.hadoop.fs.ozone;
+
+import java.net.URL;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.powermock.api.mockito.PowerMockito;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+import org.powermock.modules.junit4.PowerMockRunner;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.when;
+
+/**
+ * FilteredClassLoader test using mocks.
+ */
+@RunWith(PowerMockRunner.class)
+@PrepareForTest({ FilteredClassLoader.class, OzoneFSInputStream.class})
+public class TestFilteredClassLoader {
+  @Test
+  public void testFilteredClassLoader() {
+    PowerMockito.mockStatic(System.class);
+    when(System.getenv("HADOOP_OZONE_DELEGATED_CLASSES"))
+        .thenReturn("org.apache.hadoop.fs.ozone.OzoneFSInputStream");
+
+    ClassLoader currentClassLoader =
+        TestFilteredClassLoader.class.getClassLoader();
+
+    List<URL> urls = new ArrayList<>();
+    ClassLoader classLoader = new FilteredClassLoader(
+        urls.toArray(new URL[0]), currentClassLoader);
+
+    try {
+      classLoader.loadClass(
+          "org.apache.hadoop.fs.ozone.OzoneFSInputStream");
+      ClassLoader expectedClassLoader =
+          OzoneFSInputStream.class.getClassLoader();
+      assertEquals(expectedClassLoader, currentClassLoader);
+    } catch (ClassNotFoundException e) {
+      e.printStackTrace();
+    }
+  }
+}