java.lang.IllegalArgumentException: object is not an instance of declaring class when I use reflection

Question

I am new to Java reflection. I tried to call one method of my DAO Class using reflection, and I got the below mentioned error of illegal argument exception. Below is my code. My method contains two arguments: one is Dossier bean object and another is sessionfactory object. I got this error when I invoke my method. I searched lot on net but did not find the proper solution.

public String getDossierDetail(HttpSession session,DoerDAO doerDao,SessionFactory sessionFactory,String requestedUser) throws ClassNotFoundException{
    log.info("(getDossierDetail)Execution starts");
    ReviewerOne reviewer = new ReviewerOne();
    String message = "";        
    DoerDAO doerDaoInt = new DoerDAO();
    try{
        List<Dossier> dossierDetail = (List<Dossier>) session.getAttribute(ObjectConstant.dossierDetailBean);
        System.out.println("dossierDetail: "+dossierDetail.size()+"product nm: "+dossierDetail.get(0).getProductName()+"requested User: "+requestedUser);
        Method method =  DoerDAO.class.getDeclaredMethod(requestedUser,Dossier.class,SessionFactory.class);         
         method.invoke(dossierDetail.get(0), sessionFactory);           
    }catch(Exception e){
        e.printStackTrace();
        log.error("(getDossierDetail)Error is: ",e);            
        message = e.getLocalizedMessage();
    }
    return message;
}

my requestedUser value is :: getReviewerOneDetail.

/** DoerDao method ********/

public void getReviewerOneDetail(Dossier dossierBean,SessionFactory sessionFactory){
    log.info("(getReviewerOneDetail)Execution starts.");



    try{

        System.out.println("in reviewer one detail....");

    }catch(Exception e){
        e.printStackTrace();
        log.error("(getReviewerOneDetail)Error is: ",e);
    }


}

Answer 1

Short version: You are missing the first argument in your call to invoke.

Long version: You are calling

Method method = DoerDAO.class.getDeclaredMethod(requestedUser,
   Dossier.class,
   SessionFactory.class);       

Let's say that the value of requestedUser is getReviewerOneDetail, then you'd look up the method

getReviewerOneDetail(Dossier arg0, SessionFactory arg1)

Next you call

method.invoke(dossierDetail.get(0), sessionFactory);

The JavaDoc states that invoke gets as first parameter the instance(!) of the class to call the method on and as second, third, ... parameters the actual parameters for your invocation.

So, what you actually trying to call in your code, is

dossierDetail.get(0).getReviewerOneDetail(sessionFactory);

which does neither match the method signature (1 parameter vs. 2 parameters), nor the instance type on which the method is called (Dossier instead of DoerDAO).

Because you acquire the Method from the DoerDAO class, I guess what you intended to write there, was actually:

method.invoke(doerDao, dossierDetail.get(0), sessionFactory);

This would translate to

doerDao.getReviewerOneDetail(dossierDetail.get(0), sessionFactory);