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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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package org.mvel2.optimizers.impl.refl.nodes;

import org.mvel2.ParserContext;
import org.mvel2.compiler.Accessor;
import org.mvel2.compiler.AccessorNode;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.optimizers.OptimizerFactory;

public class NullSafe implements AccessorNode {
  private AccessorNode nextNode;
  private char[] expr;
  private int start;
  private int offset;
  private ParserContext pCtx;

  public NullSafe(char[] expr, int start, int offset, ParserContext pCtx) {
    this.expr = expr;
    this.start = start;
    this.offset = offset;
    this.pCtx = pCtx;
  }

  public Object getValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory) {
    if (ctx == null) return null;
    if (nextNode == null) {
      final Accessor a = OptimizerFactory.getAccessorCompiler(OptimizerFactory.SAFE_REFLECTIVE)
          .optimizeAccessor(pCtx, expr, start, offset, ctx, elCtx, variableFactory, true, ctx.getClass());

      nextNode = new AccessorNode() {
        public AccessorNode getNextNode() {
          return null;
        }

        public AccessorNode setNextNode(AccessorNode accessorNode) {
          return null;
        }

        public Object getValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory) {
          return a.getValue(ctx, elCtx, variableFactory);
        }

        public Object setValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory, Object value) {
          return a.setValue(ctx, elCtx, variableFactory, value);
        }

        public Class getKnownEgressType() {
          return a.getKnownEgressType();
        }
      };


    }
    //   else {
    return nextNode.getValue(ctx, elCtx, variableFactory);
    //    }
  }

  public Object setValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory, Object value) {
    if (ctx == null) return null;
    return nextNode.setValue(ctx, elCtx, variableFactory, value);
  }

  public AccessorNode getNextNode() {
    return nextNode;
  }

  public AccessorNode setNextNode(AccessorNode accessorNode) {
    return this.nextNode = accessorNode;
  }

  public Class getKnownEgressType() {
    return Object.class;
  }
}