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我正在使用 MVEL 来评估算术和逻辑表达式或两者的组合。问题是我事先并不知道所有变量类型，而没有为表达式本身创建一个非常复杂的解析方法（通过设置文件传递）。只有当我浏览我的数据并更新上下文时，我才知道它们的类型。例如，考虑将(a && b) && (c == 10) && (d < 5)变量与运算符分开并初始化上下文的表达式，但我不知道哪些是布尔值，哪些是整数。我试图用nullor初始化上下文中的所有变量，new Object()但它没有按预期工作。请参见下面的示例代码：

import java.util.HashMap;
import java.util.Map;

import org.mvel2.MVEL;

public class Test {

    private static Map<String, Object> context = new HashMap<String, Object>();

    public static void main(String[] args){

        String expression = "(a && b) && (c == 10) && (d < 5)";

        context.put("a", new Object());
        context.put("b", new Object());
        context.put("c", new Object());
        context.put("d", new Object());

        //do some processing and get the right value, replacing it in the context with the right type

        evaluate(expression,context); //just to try to evaluate with nothing set, crashes

        context.put("a", new Boolean(true));
        evaluate(expression,context);//crashes same as above

        context.put("b", new Boolean(true));
        evaluate(expression,context); //works. the expression is not yet true but it does not crash

        context.put("c", new Integer(10));
        context.put("d", new Integer(1));
        evaluate(expression,context); //works. expression is true

}

    private static void evaluate(String expression, Map<String,Object> context){
        if((Boolean)MVEL.eval(expression, context))
            System.out.println("Hooray");
        else
            System.out.println("Boo!"); 
    }

}

当它崩溃时，我收到这条消息：Exception in thread "main" org.mvel2.ScriptRuntimeException: expected Boolean; but found: java.lang.Object如果我用它初始化null会崩溃说..but found: null我的猜测是它在MVEL.eval()方法中计算出它应该接收布尔值作为第一个变量，但我发现行为不一致。第二个例子让我更加困惑。请参见下面的示例代码：

import java.util.HashMap;
import java.util.Map;

import org.mvel2.MVEL;

public class Test {

    private static Map<String, Object> context = new HashMap<String, Object>();

    public static void main(String[] args){

        context.put("a", null);
        context.put("b", null);
        context.put("c", null);

        String expression = "( a > b + 2 ) && ( c < a - 5 )";

        evaluate(expression,context); //this time it does not crash. it evaluates correctly as false

         //do some processing and get the right value, and replace it in the context

        context.put("a",new Integer(25));
        evaluate(expression,context); //crashes. error below
        context.put("b", new Integer(20));
        context.put("c", new Integer(10));

        evaluate(expression,context); //evaluates correctly to true.
}

    private static void evaluate(String expression, Map<String,Object> context){
        if((Boolean)MVEL.eval(expression, context))
            System.out.println("Hooray");
        else
            System.out.println("Boo!"); 
    }

}

第二个示例中的崩溃错误消息是：Exception in thread "main" [Error: failed to subEval expression] [Near : {... ( a > b + 2 ) && ( c < a - 5 ) ....}] ^ 我的上下文变量是否有默认初始化？我可以用初始化它们，new Boolean(false)但这会影响表达式。我必须使用严格类型或强类型吗？顺便说一句，我没有找到任何像样的类文档..任何建议都值得赞赏。谢谢你。

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1 回答 1

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首先，您可以让 MVEL 自动获取输入变量。不幸的是，MVEL 并没有告诉你它认为对象类型是什么。因此，例如：

Public class MvelVarTest {

public static void main(String[] args) {
    String expression = "( a > b + 2 ) && ( c < a - 5 )";
    ParserContext context = new ParserContext();
    Serializable compiledExpression = MVEL.compileExpression(expression, context);

    //Now the context will have a list of all the inputs.  Unfortunatly it does not tell you what type of Object the input is.
    for (Map.Entry<String,Class> entry : context.getInputs().entrySet()) {
        System.out.println("Variable name : "+entry.getKey()+", Data Type = "+entry.getValue().toString());
    }

    //Now, you can assign values to the data and run the expression.
    Map values = new HashMap();
    values.put("a",25);
    values.put("b",20);
    values.put("c",10);

    //And we can get a boolean answer
    System.out.println("Result of running formula with (a=25, b=20, c=10) = "+MVEL.executeExpression(compiledExpression,values,Boolean.class));
}

}

但是，当您使用您的值设置 HashMap 时，您可以只放入字符串值，而 MVEL 会“自动转换”它。

于 2015-01-28T17:35:26.040 回答