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.oap.server.core.annotation;
  
  import com.google.common.collect.ImmutableSet;
  import com.google.common.reflect.ClassPath;
  import java.io.IOException;
  import java.lang.annotation.Annotation;
  import java.util.Comparator;
  import java.util.LinkedList;
  import java.util.List;
  import org.apache.skywalking.oap.server.core.storage.StorageException;
  
  /**
   * Scan the annotation, and notify the listener(s)
   */
  public class AnnotationScan {
  
      private final List<AnnotationListenerCache> listeners;
  
      public AnnotationScan() {
          this.listeners = new LinkedList<>();
      }
  
      /**
       * Register the callback listener
       *
       * @param listener to be called after class found w/ annotation
       */
      public void registerListener(AnnotationListener listener) {
          listeners.add(new AnnotationListenerCache(listener));
      }
  
      /**
       * Begin to scan classes.
       */
      public void scan() throws IOException, StorageException {
          ClassPath classpath = ClassPath.from(this.getClass().getClassLoader());
          ImmutableSet<ClassPath.ClassInfo> classes = classpath.getTopLevelClassesRecursive("org.apache.skywalking");
          for (ClassPath.ClassInfo classInfo : classes) {
              Class<?> aClass = classInfo.load();
  
              for (AnnotationListenerCache listener : listeners) {
                  if (aClass.isAnnotationPresent(listener.annotation())) {
                      listener.addMatch(aClass);
                  }
              }
          }
  
          for (AnnotationListenerCache listener : listeners) {
              listener.complete();
          }
      }
  
      private class AnnotationListenerCache {
          private AnnotationListener listener;
          private List<Class<?>> matchedClass;
  
          private AnnotationListenerCache(AnnotationListener listener) {
              this.listener = listener;
              matchedClass = new LinkedList<>();
          }
  
          private Class<? extends Annotation> annotation() {
              return this.listener.annotation();
          }
  
          private void addMatch(Class aClass) {
              matchedClass.add(aClass);
          }
  
          private void complete() throws StorageException {
              matchedClass.sort(Comparator.comparing(Class::getName));
              for (Class<?> aClass : matchedClass) {
                  listener.notify(aClass);
              }
          }
      }
  }
  