MvelEvaluator.java

package com.google.sitebricks;

import com.google.common.collect.MapMaker;
import com.google.sitebricks.compiler.Parsing;
import net.jcip.annotations.ThreadSafe;
import org.jetbrains.annotations.Nullable;
import org.mvel2.CompileException;
import org.mvel2.MVEL;
import org.mvel2.PropertyAccessException;

import java.io.Serializable;
import java.util.HashMap;
import java.util.concurrent.ConcurrentMap;

/**
 * @author Dhanji R. Prasanna (dhanji@gmail.com)
 */
@ThreadSafe
public class MvelEvaluator implements Evaluator {

  //lets do some caching of expressions to see if we cant go a bit faster
  private final ConcurrentMap<String, Serializable> compiledExpressions = new MapMaker().makeMap();

  @Nullable
  public Object evaluate(String expr, Object bean) {
    Serializable compiled = compiledExpressions.get(expr);

    //compile and store the expr (warms up the expression cache)
    if (null == compiled) {
      String preparedExpression = expr;

      //strip expression decorators as necessary
      if (Parsing.isExpression(expr)) {
        preparedExpression = Parsing.stripExpression(expr);
      }

      //compile expression
      compiled = MVEL.compileExpression(preparedExpression);

      //place into map under original key (i.e. as it came in)
      compiledExpressions.put(expr, compiled);
    }

    //lets use mvel to retrieve an expression value instead of a prop
    try {
      return MVEL.executeExpression(compiled, bean, new HashMap());
    } catch (PropertyAccessException e) {
      throw new IllegalArgumentException(
          String.format("Could not read property from expression %s (missing a getter?)", expr), e);
    } catch (NullPointerException npe) {
      throw new IllegalArgumentException(
          String.format("Evaluation of property expression [%s] resulted in a NullPointerException",
              expr), npe);
    } catch (CompileException e) {
      throw new IllegalArgumentException(
          String.format("Compile of property expression [%s] resulted in an error",
              expr), e);
    }
  }


  public void write(String expr, Object bean, Object value) {
    //lets use mvel to store an expression
    MVEL.setProperty(bean, expr, value);
  }

  public Object read(String property, Object contextObject) {
    return MVEL.getProperty(property, contextObject);
  }

}