How to retrieve custom attribute values of a part and set them through Api?

but i am getting this error can u help me out

ERROR [main] com.ptc.core.meta.type.runtime.server.impl.SecurityLabelConstraintFactory - wt.util.WTRemoteException: Unable to invoke remote method; nested exception is:

java.rmi.UnmarshalException: error unmarshalling return; nested exception is:

java.lang.ClassNotFoundException: wt.part.alternaterep.WTPartAlternateRep (no security manager: RMI class loader disabled)

Nested exception is: wt.util.WTRemoteException: Unable to invoke remote method

Nested exception is: java.rmi.UnmarshalException: error unmarshalling return; nested exception is:

java.lang.ClassNotFoundException: wt.part.alternaterep.WTPartAlternateRep (no security manager: RMI class loader disabled)

wt.util.WTException: wt.util.WTRemoteException: Unable to invoke remote method; nested exception is:

java.rmi.UnmarshalException: error unmarshalling return; nested exception is

the code is:

_WTPart ref1=part;

LWCNormalizedObject obj = new LWCNormalizedObject(ref1,null,null,null);

obj.load("CAGE","COMMODITY");

Object CAGEValue = obj.get("CAGE");

Object COMMODITYValue = obj.get("COMMODITY");

part is the retrieved WTPart from search