计算字符串

`--- theme: github

private double strToCal(String expressionString) {
	JexlEngine jexlEngine = new JexlBuilder().create();
	JexlExpression jexlExpression = jexlEngine.createExpression(expressionString);
	Object evaluate = jexlExpression.evaluate(null);
	double douNumber = Double.parseDouble(evaluate.toString());
	return douNumber;
}

如： "2.00*2.00" --> 4.00d