inter-Extension dependency support

extensions-core/kafka-extraction-namespace/pom.xml

@@ -44,6 +44,7 @@
       <groupId>org.apache.druid.extensions</groupId>
       <artifactId>druid-lookups-cached-global</artifactId>
       <version>${project.parent.version}</version>
+      <scope>provided</scope>
     </dependency>
     <dependency>
       <groupId>org.apache.druid</groupId>

extensions-core/kafka-extraction-namespace/src/main/resources/druid-extension-dependencies.json

@@ -0,0 +1,3 @@
+{
+  "dependsOnDruidExtensions": ["druid-lookups-cached-global"]
+}

processing/src/main/java/org/apache/druid/guice/DruidExtensionDependencies.java

@@ -0,0 +1,49 @@
+/*
+ * 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.druid.guice;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+import javax.annotation.Nonnull;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class DruidExtensionDependencies
+{
+
+  @JsonProperty("dependsOnDruidExtensions")
+  private List<String> dependsOnDruidExtensions;
+
+  public DruidExtensionDependencies()
+  {
+    this.dependsOnDruidExtensions = new ArrayList<>();
+  }
+
+  public DruidExtensionDependencies(@Nonnull final List<String> dependsOnDruidExtensions)
+  {
+    this.dependsOnDruidExtensions = dependsOnDruidExtensions;
+  }
+
+  public List<String> getDependsOnDruidExtensions()
+  {
+    return dependsOnDruidExtensions;
+  }
+}

processing/src/main/java/org/apache/druid/guice/ExtensionFirstClassLoader.java

@@ -24,21 +24,20 @@
 
 import java.io.IOException;
 import java.net.URL;
-import java.net.URLClassLoader;
 import java.util.ArrayList;
 import java.util.Enumeration;
 import java.util.List;
 
 /**
  * The ClassLoader that gets used when druid.extensions.useExtensionClassloaderFirst = true.
  */
-public class ExtensionFirstClassLoader extends URLClassLoader
+public class ExtensionFirstClassLoader extends StandardURLClassLoader
 {
   private final ClassLoader druidLoader;
 
-  public ExtensionFirstClassLoader(final URL[] urls, final ClassLoader druidLoader)
+  public ExtensionFirstClassLoader(final URL[] urls, final ClassLoader druidLoader, final List<ClassLoader> extensionDependencyClassLoaders)
   {
-    super(urls, null);
+    super(urls, null, extensionDependencyClassLoaders);
     this.druidLoader = Preconditions.checkNotNull(druidLoader, "druidLoader");
   }
 
@@ -60,8 +59,13 @@ protected Class<?> loadClass(final String name, final boolean resolve) throws Cl
           clazz = findClass(name);
         }
         catch (ClassNotFoundException e) {
-          // Try the Druid classloader. Will throw ClassNotFoundException if the class can't be loaded.
-          return druidLoader.loadClass(name);
+          try {
+            clazz = loadClassFromExtensionDependencies(name);
+          }
+          catch (ClassNotFoundException e2) {
+            // Try the Druid classloader. Will throw ClassNotFoundException if the class can't be loaded.
+            clazz = druidLoader.loadClass(name);
+          }
         }
       }
 
@@ -76,20 +80,26 @@ protected Class<?> loadClass(final String name, final boolean resolve) throws Cl
   @Override
   public URL getResource(final String name)
   {
-    final URL resourceFromExtension = super.getResource(name);
+    URL resourceFromExtension = super.getResource(name);
 
     if (resourceFromExtension != null) {
       return resourceFromExtension;
-    } else {
-      return druidLoader.getResource(name);
     }
+
+    resourceFromExtension = getResourceFromExtensionsDependencies(name);
+    if (resourceFromExtension != null) {
+      return resourceFromExtension;
+    }
+
+    return druidLoader.getResource(name);
   }
 
   @Override
   public Enumeration<URL> getResources(final String name) throws IOException
   {
     final List<URL> urls = new ArrayList<>();
     Iterators.addAll(urls, Iterators.forEnumeration(super.getResources(name)));
+    addExtensionResources(name, urls);
     Iterators.addAll(urls, Iterators.forEnumeration(druidLoader.getResources(name)));
     return Iterators.asEnumeration(urls.iterator());
   }