Helper for Camunda OptimisticLockingException from erroneous updates when only reading variables

SuppressAutoUpdateContext.java

package de.bfs.camunda;

import java.lang.reflect.Field;
import java.util.LinkedList;

import org.camunda.bpm.engine.impl.context.Context;
import org.camunda.bpm.engine.impl.interceptor.CommandContext;
import org.camunda.bpm.engine.impl.interceptor.CommandContextListener;

/**
 * This Class provides a method to prevent the auto-update mechanism
 * of Camunda when reading variables from a task or execution.
 * <p>
 * Normally when getVariable() (or analogous methods) are called, the
 * underlying Class will register a listener with the CommandContext
 * and when the current Transaction is closed, all variables will check
 * if their value has been changed and send an update to the database.
 * <p>
 * Under certain circumstances* this can trigger an update in the database
 * even when the variable is only read and not changed. You can use
 * this class to read Variables without them being registered for automatic
 * update checking, if you need the variables only read-only.
 * <p>
 * <h4>Example:</h4>
 * <pre>
 * // This will read variables without registering them for auto-update
 * SuppressAutoUpdateContext.executeWithoutVariableUpdates(() -> {
 *   vars = execution.getVariables();
 * });
 * </pre>
 * <p>
 * <h4>*certain circumstances:</h4>
 * The TypedVariable class checks serialized variables for changes by
 * serializing the java object again and comparing the resulting byte-array
 * with the byte-array saved in the database. Depending on the serializer
 * the resulting bytes can be different depending on available type-information.
 * This often happens for external variables which are provided without
 * generic type information (e.g. HashMap instead of HashMap<String,String>)
 * 
 * @author [email protected]
 * @license MIT (https://opensource.org/license/mit/)
 */
public class SuppressAutoUpdateContext {

  /**
   * This method will run the function f without registering any Listeners
   * in the CommandContext (where variables are registered for updating)
   * 
   * Use this method only for reading variables, since it will prevent any
   * CommandContext-Listeners from being added while running {@code f}
   * 
   * @param f A function which is run without registering CommandContextListeners
   */
  @SuppressWarnings("unchecked")
  public static void executeWithoutVariableUpdates(Runnable f) {
    CommandContext ctx = Context.getCommandContext();
    Field ctxListenerField = null;
    LinkedList<CommandContextListener> originalListeners = null;
    try {
      ctxListenerField = ctx.getClass().getDeclaredField("commandContextListeners");
      ctxListenerField.setAccessible(true);
      originalListeners = (LinkedList<CommandContextListener>) ctxListenerField.get(ctx);
      ctxListenerField.set(ctx, new LinkedList<>());

      f.run();

    } catch (Exception e) {
      throw new RuntimeException(e);

    } finally {
      try {
        if (ctxListenerField != null) {
          if (originalListeners != null) {
            ctxListenerField.set(ctx, originalListeners);
          }
          ctxListenerField.setAccessible(false);
        }
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
  }
}