jexl表达式解析、计算工具类.md 根据表达式可以动态反向解析出变量；适合动态表达式，参数未知场景 如 (A0.2+B0.8)/C 解析出 A\B\C，把ABC参数值代入计算 初始化引擎

private static final JexlEngine ENGINE = new JexlEngine();

static {

ENGINE.setCache(512);

ENGINE.setLenient(false);

ENGINE.setSilent(false);

}

解析出表达式中的变量 ExpressionUtils

package com.its.cloud.utils;

import com.google.common.base.Joiner;

import com.google.common.collect.Lists;

import org.apache.commons.jexl2.Expression;

import org.apache.commons.jexl2.ExpressionImpl;

import org.apache.commons.jexl2.JexlEngine;

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

import java.util.Collections;

import java.util.List;

import java.util.Set;

/**

* Created by hongliangpan@gmail.com on 2015/1/19.

*/

public class ExpressionUtils {

private static final Logger LOGGER = LoggerFactory.getLogger(ExpressionUtils.class);

private static final JexlEngine JEXL = new JexlEngine();

static {

JEXL.setCache(512);

JEXL.setLenient(false);

JEXL.setSilent(false);

}

public static JexlEngine getJexlEngine() {

return JEXL;

}

public static boolean isValidExpression(String expression) {

try {

JEXL.createExpression(expression);

return true;

} catch (Throwable t) {

LOGGER.error("表达式{}配置错误{}。", expression, t.getMessage(), t);

return false;

}

}

/**

* 获取表达式中的变量参数

*

* @param expression 表达式，如 ping.max>0 and ping.min>0 and pin.lost==0

*/

public static List getVariables(String expression) {

Expression exp;

try {

exp = JEXL.createExpression(expression);

} catch (Throwable t) {

LOGGER.error("表达式{}配置错误{}。", expression, t.getMessage(), t);

return Collections.emptyList();

}

return getVariables(exp);

}

public static List getVariables(Expression exp) {

List metricDefIds = Lists.newArrayList();

Set> variables = JEXL.getVariables((ExpressionImpl) exp);

for (List var : variables) {

metricDefIds.add(Joiner.on(".").join(var));

}

return metricDefIds;

}

}

解析变量，调用示例

jexl反向解析表达式中的变量

JexlEngine jexl = new JexlEngine();

String threshold = "(value==3)and (cpu>90 and mem <70)";

Expression exp = jexl.createExpression(threshold);

List variables = jexl.getVariables(((ExpressionImpl)exp));

表达式示例

String threshold = "value>90";

Expression exp = getJexlEngine().createExpression(threshold);

JexlContext jc = new MapContext();

jc.set("value", data.getValue());

Object evaluate = exp.evaluate(jc);

if (evaluate instanceof Boolean) {

return (boolean) evaluate;

} else {

LOGGER.error("表达式错误{}，不是boolean返回值;{}", exp, rule);

return false;

}