Using java reflection

Question

I am trying to invoke a private method belonging to one class from another class using java reflection. Both these classes belong to different packages. Code sample is as below. But everytime I run the getDeclaredMethod it returns with NoSuchMethodException. How do I invoke the getCacheKey method from my class?

Thanks,

Class A

package com.abc;

public class TicketHelper
{
    static String getCacheKey(String ticketString, Ticket ticket) throws TicketException, UnsupportedEncodingException, NoSuchAlgorithmException {
      ...
    }
}

Class B

package com.def;

...

private Method method = null;

public class TicketHelper
{
    ...

    try {
        method = TicketHelper.class.getDeclaredMethod("getCacheKey", new Class[] {String.class, Ticket.class});
        } catch (SecurityException e1) {
            setTrace("Security exception2 " + e1.getMessage());
        } catch (NoSuchMethodException e1) {
            setTrace("No such method exception2 " + e1.getMessage());
    }
    method.setAccessible(true);
    m_cacheKey = method.invoke(null, new Object[] {ticketString, ticket});
}

What does this have to do with asp.net and java-ee?

Miserable Variable
– Miserable Variable

2012-03-01 21:25:29 +00:00
Commented Mar 1, 2012 at 21:25 — Miserable Variable
– Miserable Variable, Commented Mar 1, 2012 at 21:25

Miserable Variable · Accepted Answer · 2012-03-01 21:36:51Z

3

Is the class in com.def also called TicketHelper? In that case you need to qualify as com.abc.TicketHelper

EDIT

There are several compilation errors in the code you posted. Always try to come up with a short example that reproduces the problem; in most cases you will see you error in that process. The following works for me. It is same package, but that should not matter:

public class TicketHelperUser
{
    public static void main(String[] args) throws Exception
    {
    for (java.lang.reflect.Method m : TicketHelper.class.getDeclaredMethods())
    {
        System.out.println(m);
    }
    java.lang.reflect.Method method = TicketHelper.class.getDeclaredMethod("getCacheKey", String.class, Ticket.class);
    method.setAccessible(true);
    method.invoke(null, new Object[] {"", new Ticket()});
    }
}

public class TicketHelper
{
    static String getCacheKey(String ticketString, Ticket ticket) 
    {
    return "cacheKey";
    }

}

public class Ticket {}

edited Mar 1, 2012 at 21:36

answered Mar 1, 2012 at 21:28

Miserable Variable

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3 Comments

jpm Over a year ago

Pretty sure this is correct. As is, TicketHelper.class is returning a reference to the com.def.TicketHelper Class object, which will obviously not contain the getCacheKey method.

Contact Rao Over a year ago

I was able to list the methods of the class using this piece of code. Do you have any idea about the downside of using java reflection? forum posts suggest to avoid this as much as possible.

Miserable Variable Over a year ago

Reflection should not be used if it can be avoided. It postpones error checking to run-time and has performance overhead. If it has to be used, get the reference to the Method object is most expensive, not the invoke call; so better to keep that cached.

Andreas Dolk · Accepted Answer · 2012-03-01 21:35:04Z

0

Not a solution but a test to identify the problem: get all declared methods and verify, that the getCacheKey is visible/present:

Method[] methods = TicketHelper.class.getDeclaredMethods();
for (Method method:methods) {
  if (method.getName().equals("getCacheKey")) {
    System.out.println(method);  // <-- breakpoint and inspect the method object
  }
}

answered Mar 1, 2012 at 21:35

Andreas Dolk

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Comments

Peter.Ladanyi · Accepted Answer · 2012-03-01 21:38:32Z

0

Take a look at: http://www.wikijava.org/wiki/Class_and_static_Method_Reflection_example

they are using getMethod instead getDeclaredMethod

answered Mar 1, 2012 at 21:38

Peter.Ladanyi

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1 Comment

jpm Over a year ago

afaik, the differences between those two methods are: 1) getDeclaredMethod finds private methods, while getMethod only finds public methods (so not very helpful here), 2) getMethod will also search methods declared in supertypes. While this has it's uses, it's not very important here.

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