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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.optimizers;

import org.mvel2.optimizers.dynamic.DynamicOptimizer;
import org.mvel2.optimizers.impl.asm.ASMAccessorOptimizer;
import org.mvel2.optimizers.impl.refl.ReflectiveAccessorOptimizer;

import java.util.HashMap;
import java.util.Map;
import java.util.logging.Level;
import java.util.logging.Logger;

public class OptimizerFactory {
  public static String DYNAMIC = "dynamic";
  public static String SAFE_REFLECTIVE = "reflective";

  private static final Logger LOG = Logger.getLogger(OptimizerFactory.class.getName());
  private static String defaultOptimizer;
  private static final Map accessorCompilers = new HashMap();

  private static ThreadLocal> threadOptimizer
      = new ThreadLocal>();

  static {
    accessorCompilers.put(SAFE_REFLECTIVE, new ReflectiveAccessorOptimizer());
    accessorCompilers.put(DYNAMIC, new DynamicOptimizer());
    /**
     * By default, activate the JIT if ASM is present in the classpath
     */
    try {
      if (OptimizerFactory.class.getClassLoader() != null) {
          OptimizerFactory.class.getClassLoader().loadClass("org.mvel2.asm.ClassWriter");
      } else {
          ClassLoader.getSystemClassLoader().loadClass("org.mvel2.asm.ClassWriter");
      }
      accessorCompilers.put("ASM", new ASMAccessorOptimizer());
    }
    catch (ClassNotFoundException e) {
      defaultOptimizer = SAFE_REFLECTIVE;
    }
    catch (Throwable e) {
      LOG.log(Level.WARNING, "[MVEL] Notice: Possible incorrect version of ASM present (3.0 required).  " +
          "Disabling JIT compiler.  Reflective Optimizer will be used.", e);
      defaultOptimizer = SAFE_REFLECTIVE;
    }

    if (Boolean.getBoolean("mvel2.disable.jit"))
      setDefaultOptimizer(SAFE_REFLECTIVE);
    else
      setDefaultOptimizer(DYNAMIC);
  }

  public static AccessorOptimizer getDefaultAccessorCompiler() {
    try {
      return accessorCompilers.get(defaultOptimizer).getClass().newInstance();
    }
    catch (Exception e) {
      throw new RuntimeException("unable to instantiate accessor compiler", e);
    }
  }

  public static AccessorOptimizer getAccessorCompiler(String name) {
    try {
      return accessorCompilers.get(name).getClass().newInstance();
    }
    catch (Exception e) {
      throw new RuntimeException("unable to instantiate accessor compiler", e);
    }
  }

  public static AccessorOptimizer getThreadAccessorOptimizer() {
    if (threadOptimizer.get() == null) {
      threadOptimizer.set(getDefaultAccessorCompiler().getClass());
    }
    try {
      return threadOptimizer.get().newInstance();
    }
    catch (Exception e) {
      throw new RuntimeException("unable to instantiate accessor compiler", e);
    }
  }

  public static void setThreadAccessorOptimizer(Class optimizer) {
    if (optimizer == null) throw new RuntimeException("null optimizer");
    threadOptimizer.set(optimizer);
  }

  public static void setDefaultOptimizer(String name) {
    try {
      //noinspection unchecked
      AccessorOptimizer ao = accessorCompilers.get(defaultOptimizer = name);
      ao.init();
      //clear optimizer so next call to getThreadAccessorOptimizer uses the default again, don't set thread optimizer
      //or else static initializers setting the default will unintentionally set up ThreadLocals
      threadOptimizer.set(null);
    }
    catch (Exception e) {
      throw new RuntimeException("unable to instantiate accessor compiler", e);
    }
  }

  public static void clearThreadAccessorOptimizer() {
    threadOptimizer.set(null);
    threadOptimizer.remove();
  }

  public static boolean isThreadAccessorOptimizerInitialized() {
    return threadOptimizer.get() != null;
  }
}