code: /*
   * 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.skywalking.oal.rt.parser;
  
  import com.google.common.collect.ImmutableSet;
  import com.google.common.reflect.ClassPath;
  import java.io.IOException;
  import java.util.HashMap;
  import java.util.Map;
  import lombok.SneakyThrows;
  import org.apache.skywalking.oap.server.core.analysis.metrics.Metrics;
  import org.apache.skywalking.oap.server.core.analysis.metrics.annotation.MetricsFunction;
  
  @SuppressWarnings("UnstableApiUsage")
  public class MetricsHolder {
      private static final Map<String, Class<? extends Metrics>> REGISTER = new HashMap<>();
      private static volatile boolean INITIALIZED = false;
  
      private static void init() throws IOException {
          ClassPath classpath = ClassPath.from(MetricsHolder.class.getClassLoader());
          ImmutableSet<ClassPath.ClassInfo> classes = classpath.getTopLevelClassesRecursive("org.apache.skywalking");
          for (ClassPath.ClassInfo classInfo : classes) {
              Class<?> aClass = classInfo.load();
  
              if (aClass.isAnnotationPresent(MetricsFunction.class)) {
                  MetricsFunction metricsFunction = aClass.getAnnotation(MetricsFunction.class);
                  REGISTER.put(
                      metricsFunction.functionName(),
                      (Class<? extends Metrics>) aClass
                  );
              }
          }
      }
  
      @SneakyThrows
      public static Class<? extends Metrics> find(String functionName) {
          if (!INITIALIZED) {
              init();
              INITIALIZED = true;
          }
  
          Class<? extends Metrics> metricsClass = REGISTER.get(functionName);
          if (metricsClass == null) {
              throw new IllegalArgumentException("Can't find metrics, " + functionName);
          }
          return metricsClass;
      }
  }
  