表达式处理

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。