曾经在一个项目中使用表达式来判断处理一些条件，比如订单价格的折扣计算，库存数量的告警线设置等。我们引入了Jexl（一种表达式语言引擎）来处理这些表达式。这里根据几个例子来总结一下它的应用。

例一、算式表达式的处理：

public class jextTest {

    public static void main(String[] args) {

        //数学公式解析和计算

        String expression = "(a+b)/c*5";

        JexlContext jexlContext = new MapContext();

        jexlContext.set("a", 4);

        jexlContext.set("b", 2);

        jexlContext.set("c", 2);

        jexlContext.set("d", 5);

        Expression e = (Expression) new JexlEngine().createExpression(expression);

        Number num = (Number) e.evaluate(jexlContext);

        System.out.println(num);

        expression = "a || b && c || d";

        jexlContext = new MapContext();

        jexlContext.set("a", true);

        jexlContext.set("b", false);

        jexlContext.set("c", true);

        jexlContext.set("d", false);

        org.apache.commons.jexl2.Expression e1 = new JexlEngine().createExpression(expression);

        Boolean num1 = (Boolean) e1.evaluate(jexlContext);

        System.out.println(num1);

        JexlEngine jexl = new JexlEngine();

        jexl.setCache(512);//Sets a cache for expressions of the defined size

        jexl.setLenient(false);//Sets whether this engine considers unknown variables, methods and constructors as errors or evaluates them as null or zero.

        jexl.setSilent(false); //Sets whether this engine throws JexlException during evaluation when an error is triggered.

        jexl.setStrict(false); //Sets whether this engine behaves in strict or lenient mode.

        String calculate = "(a + b) > 10";

        e = jexl.createExpression(calculate);

        JexlContext context = new MapContext();

        context.set("a", "3");

        context.set("b", "5");

        Object res = e.evaluate(context);//silent为false时evaluate方法会抛异常，为true不抛异常但res为null

        System.out.println(res);

    }

}

运行结果如下：

15

true

false

从上例中可以看出，实现一个表达式的解析运算，共涉及到三个主要类：JexlContext、Expression和JexlEngine。

例二、数组取值

ublic class ArrayTest {

    public static void main(String[] args) {

        JexlEngine jexl = new JexlEngine();

        JexlContext jc = new MapContext();

        List l = new ArrayList();

        l.add("It is from location 0");

        Integer two = new Integer(2);

        l.add(two);

        jc.set("array", l);

        Expression e = jexl.createExpression("array[1]");//从array中取数据

        Object o = e.evaluate(jc);

        System.err.println("Object @ location 1 = " + o);

        e = jexl.createExpression("array[0]");//从array中取数据

        o = e.evaluate(jc);

        System.err.println("Object @ location 0 = " + o);

        e = jexl.createExpression("array[0].length()");

        o = e.evaluate(jc);

        System.err.println("The length of the string at location 0 is :" + o);

    }

}

运行结果如下：

Object @ location 1 = 2

Object @ location 0 = It is from location 0

The length of the string at location 0 is :21

例三、取方法运行

public class MethodPropertyTest {

    public static void main(String[] args) {

        JexlEngine jexl = new JexlEngine();

        JexlContext jc = new MapContext();

        MethodPropertyTest foo = new MethodPropertyTest();

        Integer number = new Integer(10);

        jc.set("foo", foo);

        jc.set("number", number);

        Expression e = jexl.createExpression("foo.getFoo()");

        Object o = e.evaluate(jc);

        System.err.println("value returned by the method getFoo() is : " + o);

        e = jexl.createExpression("foo.convert(1)");

        o = e.evaluate(jc);

        System.err.println("value of " + e.getExpression() + " is : " + o);

        e = jexl.createExpression("foo.convert(1+7)");

        o = e.evaluate(jc);

        System.err.println("value of " + e.getExpression() + " is : " + o);

        e = jexl.createExpression("foo.convert(1+number)");

        o = e.evaluate(jc);

        System.err.println("value of " + e.getExpression() + " is : " + o);

        e = jexl.createExpression("foo.bar");

        o = e.evaluate(jc);

        System.err.println("value returned for the property 'bar' is : " + o);

    }

    public String getFoo() {

        return "This is from getFoo()";

    }

    public String get(String arg) {

        return "This is the property " + arg;

    }

    public String convert(long i) {

        return "The value is : " + i;

    }

}

运行结果如下：

value returned by the method getFoo() is : This is from getFoo()

value of foo.convert(1) is : The value is : 1

value of foo.convert(1+7) is : The value is : 8

value of foo.convert(1+number) is : The value is : 11

value returned for the property 'bar' is : This is the property bar

表达式的处理也可以集成Groovy来处理。相比Jexl来讲，Groovy的功能更强大。