HDDS-4104. Provide a way to get the default value and key of java-based-configuration easily

hadoop-hdds/config/src/main/java/org/apache/hadoop/hdds/conf/ConfigurationReflectionUtil.java

@@ -23,6 +23,8 @@
 import java.lang.reflect.Modifier;
 import java.util.Deque;
 import java.util.LinkedList;
+import java.util.Optional;
+import java.util.stream.Stream;
 
 /**
  * Reflection utilities for configuration injection.
@@ -240,4 +242,49 @@ private static <T> void updateConfigurationFromObject(
       }
     }
   }
+
+  public static Optional<String> getDefaultValue(Class<?> configClass,
+      String fieldName) {
+    return findFieldConfigAnnotationByName(configClass, fieldName)
+        .map(Config::defaultValue);
+  }
+
+  public static Optional<String> getKey(Class<?> configClass,
+      String fieldName) {
+    ConfigGroup configGroup =
+        configClass.getAnnotation(ConfigGroup.class);
+
+    return findFieldConfigAnnotationByName(configClass,
+        fieldName).map(
+            config -> configGroup == null ? config.key()
+                : configGroup.prefix() + "." + config.key());
+  }
+
+  public static Optional<ConfigType> getType(Class<?> configClass,
+      String fieldName) {
+    return findFieldConfigAnnotationByName(configClass, fieldName)
+        .map(Config::type);
+  }
+
+  private static Optional<Config> findFieldConfigAnnotationByName(
+      final Class<?> configClass, String fieldName) {
+    Class<?> theClass = configClass;
+    while (theClass != null) {
+      Optional<Config> config = Stream.of(theClass.getDeclaredFields())
+          .filter(f -> f.getName().equals(fieldName))
+          .findFirst()
+          .map(f -> f.getAnnotation(Config.class));
+
+      if (config.isPresent()) {
+        return config;
+      }
+
+      if (!theClass.getSuperclass().equals(Object.class)) {
+        theClass = theClass.getSuperclass();
+      } else {
+        theClass = null;
+      }
+    }
+    return Optional.empty();
+  }
 }

hadoop-hdds/config/src/test/java/org/apache/hadoop/hdds/conf/TestConfigurationReflectionUtil.java

@@ -0,0 +1,111 @@
+/**
+ * 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
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * 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.hdds.conf;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Optional;
+
+/**
+ * Test the configuration reflection utility class.
+ */
+@RunWith(Parameterized.class)
+public class TestConfigurationReflectionUtil {
+
+  private final ConfigType type;
+  private final String key;
+  private final String defaultValue;
+  private final Class testClass;
+  private final String fieldName;
+  private final boolean typePresent;
+  private final boolean keyPresent;
+  private final boolean defaultValuePresent;
+
+  @Parameterized.Parameters
+  public static Collection<Object[]> data() {
+    return Arrays.asList(new Object[][]{
+        {ConfigurationExample.class, "waitTime",
+            ConfigType.TIME, true,
+            "ozone.scm.client.wait", true,
+            "30m", true},
+        {ConfigurationExampleGrandParent.class, "number",
+            ConfigType.AUTO, true,
+            "number", true,
+            "2", true},
+        {ConfigurationExample.class, "secure",
+            ConfigType.AUTO, true,
+            "ozone.scm.client.secure", true,
+            "true", true},
+        {ConfigurationExample.class, "no-such-field",
+            null, false,
+            "", false,
+            "", false},
+        {ConfigFileAppender.class, "document",
+            null, false,
+            "", false,
+            "", false},
+    });
+  }
+
+  @SuppressWarnings("checkstyle:ParameterNumber")
+  public TestConfigurationReflectionUtil(
+      Class testClass, String fieldName,
+      ConfigType type, boolean typePresent,
+      String key, boolean keyPresent,
+      String defaultValue, boolean defaultValuePresent) {
+    this.testClass = testClass;
+    this.fieldName = fieldName;
+    this.typePresent = typePresent;
+    this.type = type;
+    this.key = key;
+    this.keyPresent = keyPresent;
+    this.defaultValue = defaultValue;
+    this.defaultValuePresent = defaultValuePresent;
+  }
+
+  @Test
+  public void testForGivenClasses() {
+    Optional<ConfigType> actualType =
+        ConfigurationReflectionUtil.getType(
+            testClass, fieldName);
+    Assert.assertEquals(typePresent, actualType.isPresent());
+    if (typePresent) {
+      Assert.assertEquals(type, actualType.get());
+    }
+
+    Optional<String> actualKey =
+        ConfigurationReflectionUtil.getKey(
+            testClass, fieldName);
+    Assert.assertEquals(keyPresent, actualKey.isPresent());
+    if (keyPresent) {
+      Assert.assertEquals(key, actualKey.get());
+    }
+    Optional<String> actualDefaultValue =
+        ConfigurationReflectionUtil.getDefaultValue(
+            testClass, fieldName);
+    Assert.assertEquals(defaultValuePresent, actualDefaultValue.isPresent());
+    if (defaultValuePresent) {
+      Assert.assertEquals(defaultValue, actualDefaultValue.get());
+    }
+  }
+}