我正在使用MVEL评估算术和逻辑表达式或两者的组合。事实是，我事先不知道所有变量类型，而没有为表达式本身（通过设置文件传递）创建一个非常复杂的解析方法。我只有在浏览数据并更新上下文时才知道它们的类型。
例如，考虑表达式(a && b) && (c == 10) && (d < 5)我将变量与运算符分开并初始化我的上下文，但是我不知道哪个是布尔值，哪个是整数。我试图用null或new Object()初始化上下文中的所有变量，但是它没有按预期工作。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，我得到以下消息：
Exception in thread "main" org.mvel2.ScriptRuntimeException: expected Boolean; but found: java.lang.Object如果我用null初始化，则会崩溃并说..but found: null我的猜测是它在MVEL.eval（）方法中指出了它应该接收布尔值作为第一个变量，但我发现行为不一致。第二个例子使我更加困惑。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!");
}
}
在第二个示例中，崩溃的错误消息为：
Exception in thread "main" [Error: failed to subEval expression][Near : {... ( a > b + 2 ) && ( c < a - 5 ) ....}] ^我的上下文变量没有默认的初始化可能吗？我可以用
new Boolean(false)初始化它们，但这会影响表达式。我必须使用严格输入还是强类型输入？顺便说一句，我找不到这些类的任何体面的文档..任何建议，不胜感激。谢谢。 最佳答案
首先，您可以要求MVEL自动获取输入变量。不幸的是，MVEL不会告诉您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，当使用值设置HashMap时，可以只放入字符串值，而MVEL可以“自动转换”它。