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TBEL is a powerful expression language for ThingsBoard platform user-defined functions. Original implementation is based on MVEL.

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/**
 * MVEL 2.0
 * Copyright (C) 2007 The Codehaus
 * Mike Brock, Dhanji Prasanna, John Graham, Mark Proctor
 *
 * Licensed 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.mvel2.integration;

import java.util.HashMap;
import java.util.Map;

public class PropertyHandlerFactory {
  protected static Map propertyHandlerClass =
      new HashMap();

  protected static PropertyHandler nullPropertyHandler;
  protected static PropertyHandler nullMethodHandler;

  public static PropertyHandler getPropertyHandler(Class clazz) {
    return propertyHandlerClass.get(clazz);
  }

  public static boolean hasPropertyHandler(Class clazz) {
    if (clazz == null) return false;
    if (!propertyHandlerClass.containsKey(clazz)) {
      Class clazzWalk = clazz;
      do {
        if (clazz != clazzWalk && propertyHandlerClass.containsKey(clazzWalk)) {
          propertyHandlerClass.put(clazz, propertyHandlerClass.get(clazzWalk));
          return true;
        }
        for (Class c : clazzWalk.getInterfaces()) {
          if (propertyHandlerClass.containsKey(c)) {
            propertyHandlerClass.put(clazz, propertyHandlerClass.get(c));
            return true;
          }
        }
      }
      while ((clazzWalk = clazzWalk.getSuperclass()) != null && clazzWalk != Object.class);
      return false;
    }
    else {
      return true;
    }
  }

  public static void registerPropertyHandler(Class clazz, PropertyHandler propertyHandler) {
    propertyHandlerClass.put(clazz, propertyHandler);
  }

  public static void setNullPropertyHandler(PropertyHandler handler) {
    nullPropertyHandler = handler;
  }

  public static boolean hasNullPropertyHandler() {
    return nullPropertyHandler != null;
  }

  public static PropertyHandler getNullPropertyHandler() {
    return nullPropertyHandler;
  }

  public static void setNullMethodHandler(PropertyHandler handler) {
    nullMethodHandler = handler;
  }

  public static boolean hasNullMethodHandler() {
    return nullMethodHandler != null;
  }

  public static PropertyHandler getNullMethodHandler() {
    return nullMethodHandler;
  }

  public static void unregisterPropertyHandler(Class clazz) {
    propertyHandlerClass.remove(clazz);
  }

  public static void disposeAll() {
    nullMethodHandler = null;
    nullPropertyHandler = null;
    propertyHandlerClass.clear();
  }
}