使用MVEL计算表达式

Question

我需要在包含日期的MVEL中计算一个表达式。基本上，--我需要向给定的日期添加一定的天数并得到值.--当我试图在MVEL中计算表达式时，得到一些异常。

这是我的代码：

package Mvel;

import java.io.Serializable;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import org.mvel2.MVEL;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.integration.impl.MapVariableResolverFactory;

public class Mveldatetest {

    public static void main(String[] args) throws ParseException {
        // TODO Auto-generated method stub
        Map<String, Object> m1 = new HashMap<String, Object>();
        m1.put("name", "xyz");
        SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
        Date d1 = sdf.parse("02/10/2014");
        m1.put("doj", d1);  
        //Date d2=sdf.parse("05/10/2014");
        System.out.println("Given Date"+" "+d1);

        final Calendar c = Calendar.getInstance();
        c.setTime(d1);
        System.out.println(c.getTime());
         Date finaldate=(Date)  MVEL.eval("c.add(Calendar.DAY_OF_MONTH, 4)",m1);

         System.out.println(finaldate);


    }
}

我得到了以下例外：

Exception in thread "main" [Error: unresolvable property or identifier: c]
[Near : {... c.add(Calendar.DAY_OF_MONTH, 4 ....}]
             ^
[Line: 1, Column: 1]
    at org.mvel2.PropertyAccessor.getBeanProperty(PropertyAccessor.java:677)
    at org.mvel2.PropertyAccessor.getNormal(PropertyAccessor.java:179)
    at org.mvel2.PropertyAccessor.get(PropertyAccessor.java:146)
    at org.mvel2.PropertyAccessor.get(PropertyAccessor.java:126)
    at org.mvel2.ast.ASTNode.getReducedValue(ASTNode.java:187)
    at org.mvel2.MVELInterpretedRuntime.parseAndExecuteInterpreted(MVELInterpretedRuntime.java:106)
    at org.mvel2.MVELInterpretedRuntime.parse(MVELInterpretedRuntime.java:49)
    at org.mvel2.MVEL.eval(MVEL.java:165)
    at Mvel.Mveldatetest.main(Mveldatetest.java:31)

Answer 1

必须将c添加到上下文m1中。而且，Calender也是未知的，但是您可以只使用c (丑陋，但很有用)。最后，请注意，add返回void，即它在原地修改c。试试这个：

System.out.println(c.getTime());
m1.put("c", c);
MVEL.eval("c.add(c.DAY_OF_MONTH, 4)", m1);
System.out.println(c.getTime());

输出：

Thu Oct 02 00:00:00 CEST 2014
Mon Oct 06 00:00:00 CEST 2014