@Override
  public Object convert(
      Object existingDestinationFieldValue,
      Object sourceFieldValue,
      Class<?> destinationClass,
      Class<?> sourceClass) {
    try {
      if (currentExchange.get() == null) {
        throw new IllegalStateException("Current exchange has not been set for ExpressionMapper");
      }

      Expression exp;

      // Resolve the language being used for this expression and evaluate
      Exchange exchange = currentExchange.get();
      Language expLang = exchange.getContext().resolveLanguage(getLanguagePart());
      String scheme = getSchemePart();
      if (scheme != null
          && (scheme.equalsIgnoreCase("classpath")
              || scheme.equalsIgnoreCase("file")
              || scheme.equalsIgnoreCase("http"))) {
        String path = getPathPart();
        try {
          exp = expLang.createExpression(resolveScript(scheme + ":" + path));
        } catch (IOException e) {
          throw new IllegalStateException("Expression script specified but not found", e);
        }
      } else {
        exp = expLang.createExpression(getExpressionPart());
      }
      return exp.evaluate(exchange, destinationClass);
    } finally {
      done();
    }
  }
 private Expression createFileLanguageExpression(String expression) {
   Language language;
   // only use file language if the name is complex (eg. using $)
   if (expression.contains("$")) {
     language = getCamelContext().resolveLanguage("file");
   } else {
     language = getCamelContext().resolveLanguage("constant");
   }
   return language.createExpression(expression);
 }
  /**
   * Asserts that the given language name and expression evaluates to the given value on a specific
   * exchange
   */
  protected void assertExpression(
      Exchange exchange, String languageName, String expressionText, Object expectedValue) {
    Language language = assertResolveLanguage(languageName);

    Expression expression = language.createExpression(expressionText);
    assertNotNull(
        "No Expression could be created for text: " + expressionText + " language: " + language,
        expression);

    assertExpression(expression, exchange, expectedValue);
  }