about methodMissing

[??????]

hi,
     I am a fresh person for groovy. here is the code I have written:


       public class TestAction extends ActionBase {


        private static final long serialVersionUID = 8763503930833969428L;


        public void Test(){
                
                JSONObject params = requestParams != null? 
JSONObject.fromObject(requestParams): new JSONObject();
        
                try {
                        ClassLoader parent = getClass().getClassLoader();
                        
                        GroovyClassLoader loader = new 
GroovyClassLoader(parent); 
                        
                        String classText = "class DynamicGroovyClass { def 
methodMissing(String name, args) {println \"You called $name with $args\"  
args.size()  } }";
                        
                        @SuppressWarnings("rawtypes")
                        Class groovyClass = loader.parseClass(classText);
                        
                        GroovyObject groovyObject = (GroovyObject) 
groovyClass.newInstance(); 
                        
                        Object [] args = {"hello", "world", "feel", "happy"};
                        
                        Object result = groovyObject.invokeMethod("run", args);
                        
                        System.out.println("Received:" + result);
                        
                        sendJsonFormattedData(params.toString());
                } catch (Exception e) {
                        e.printStackTrace();
                }
        }
}




      when I run the code, it throws an exception:
        You called run with [hello, world, feel, happy]
        java.lang.NullPointerException: Cannot get property 'args' on null 
object
        at 
org.codehaus.groovy.runtime.NullObject.getProperty(NullObject.java:60)
        at 
org.codehaus.groovy.runtime.InvokerHelper.getProperty(InvokerHelper.java:172)
        at 
org.codehaus.groovy.runtime.callsite.NullCallSite.getProperty(NullCallSite.java:47)
        at 
org.codehaus.groovy.runtime.callsite.AbstractCallSite.callGetProperty(AbstractCallSite.java:296)
        at 
DynamicGroovyClass.methodMissing(script1452504513039443064905.groovy:1)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at 
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)


        at 
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at 
org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93)
        at groovy.lang.MetaClassImpl.invokeMissingMethod(MetaClassImpl.java:936)
        at 
groovy.lang.MetaClassImpl.invokePropertyOrMissing(MetaClassImpl.java:1259)
        at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1212)
        at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1019)
        at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:810)
        at DynamicGroovyClass.invokeMethod(script1452504513039443064905.groovy)
        at com.shanghe.action.TestAction.Test(TestAction.java:31)



      so. why groovy can't get property args but it can print args? how to 
resolve the exception?