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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.impl;

import org.mvel2.UnresolveablePropertyException;
import org.mvel2.integration.VariableResolver;
import org.mvel2.integration.VariableResolverFactory;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

/**
 * Use this class to extend you own VariableResolverFactories. It contains most of the baseline implementation needed
 * for the vast majority of integration needs.
 */
public abstract class BaseVariableResolverFactory implements VariableResolverFactory {
  protected Map variableResolvers = new HashMap();
  protected VariableResolverFactory nextFactory;

  protected int indexOffset = 0;
  protected String[] indexedVariableNames;
  protected VariableResolver[] indexedVariableResolvers;

  private boolean tiltFlag;

  private boolean breakFlag;

  public VariableResolverFactory getNextFactory() {
    return nextFactory;
  }

  public VariableResolverFactory setNextFactory(VariableResolverFactory resolverFactory) {
    if (resolverFactory != this) {
      nextFactory = resolverFactory;
    }
    return nextFactory;
  }

  public VariableResolver getVariableResolver(String name) {
    if (isResolveable(name)) {
      if (variableResolvers.containsKey(name)) {
        return variableResolvers.get(name);
      }
      else if (nextFactory != null) {
        return nextFactory.getVariableResolver(name);
      }
    }

    throw new UnresolveablePropertyException("unable to resolve variable '" + name + "'");
  }

  public boolean isNextResolveable(String name) {
    return nextFactory != null && nextFactory.isResolveable(name);
  }

  public void appendFactory(VariableResolverFactory resolverFactory) {
    if (nextFactory == null) {
      nextFactory = resolverFactory;
    }
    else {
      VariableResolverFactory vrf = nextFactory;
      while (vrf.getNextFactory() != null) {
        vrf = vrf.getNextFactory();
      }
      vrf.setNextFactory(nextFactory);
    }
  }

  public void insertFactory(VariableResolverFactory resolverFactory) {
    if (nextFactory == null) {
      nextFactory = resolverFactory;
    }
    else {
      resolverFactory.setNextFactory(nextFactory = resolverFactory);
    }
  }


  public Set getKnownVariables() {
    if (nextFactory == null) {
      return new HashSet(variableResolvers.keySet());
      //   return new HashSet(0);
    }
    else {
      HashSet vars = new HashSet(variableResolvers.keySet());
      vars.addAll(nextFactory.getKnownVariables());
      return vars;
    }
  }

  public VariableResolver createIndexedVariable(int index, String name, Object value) {
    if (nextFactory != null) {
      return nextFactory.createIndexedVariable(index - indexOffset, name, value);
    }
    else {
      throw new RuntimeException("cannot create indexed variable: " + name + "(" + index + "). operation not supported by resolver: " + this.getClass().getName());
    }
  }

  public VariableResolver getIndexedVariableResolver(int index) {
    if (nextFactory != null) {
      return nextFactory.getIndexedVariableResolver(index - indexOffset);
    }
    else {
      throw new RuntimeException("cannot access indexed variable: " + index + ".  operation not supported by resolver: " + this.getClass().getName());
    }
  }

  public VariableResolver createIndexedVariable(int index, String name, Object value, Class type) {
    if (nextFactory != null) {
      return nextFactory.createIndexedVariable(index - indexOffset, name, value, type);
    }
    else {
      throw new RuntimeException("cannot access indexed variable: " + name + "(" + index + ").  operation not supported by resolver.: " + this.getClass().getName());
    }
  }

  public Map getVariableResolvers() {
    return variableResolvers;
  }

  public void setVariableResolvers(Map variableResolvers) {
    this.variableResolvers = variableResolvers;
  }

  public String[] getIndexedVariableNames() {
    return indexedVariableNames;
  }

  public void setIndexedVariableNames(String[] indexedVariableNames) {
    this.indexedVariableNames = indexedVariableNames;
  }

  public int variableIndexOf(String name) {
    if (indexedVariableNames != null)
      for (int i = 0; i < indexedVariableNames.length; i++) {
        if (name.equals(indexedVariableNames[i])) {
          return i;
        }
      }
    return -1;
  }

  public VariableResolver setIndexedVariableResolver(int index, VariableResolver resolver) {
    if (indexedVariableResolvers == null) {
      return (indexedVariableResolvers = new VariableResolver[indexedVariableNames.length])[index - indexOffset] = resolver;
    }
    else {
      return indexedVariableResolvers[index - indexOffset] = resolver;
    }
  }

  public boolean isIndexedFactory() {
    return false;
  }

  public boolean tiltFlag() {
    return tiltFlag;
  }

  public void setTiltFlag(boolean tiltFlag) {
    this.tiltFlag = tiltFlag;
    if (nextFactory != null) nextFactory.setTiltFlag(tiltFlag);
  }
  public boolean breakFlag() {
    return breakFlag;
  }

  public void setBreakFlag(boolean breakFlag) {
    this.breakFlag = breakFlag;
    if (nextFactory != null) nextFactory.setBreakFlag(breakFlag);
  }
}