Java Reflection - Object is not an instance of declaring class

Question

This question is being asked everywhere on Google but I'm still having trouble with it. Here is what I'm trying to do. So like my title states, I'm getting an 'object is not an instance of declaring class' error. Any ideas? Thanks!

Main.java

Class<?> base = Class.forName("server.functions.TestFunction");
Method serverMethod = base.getMethod("execute", HashMap.class);
serverMethod.invoke(base, new HashMap<String, String>());

TestFunction.java

package server.functions;

import java.util.HashMap;
import java.util.Map;

import server.*;

public class TestFunction extends ServerBase {

    public String execute(HashMap<String, String> params)
    {
        return "Test function successfully called";
    }
}

raven1981 · Accepted Answer · 2012-11-11 22:23:18Z

84

You're invoking the method with the class, but you need an instance of it. Try this:

serverMethod.invoke(base.newInstance(), new HashMap<String, String>());

answered Nov 11, 2012 at 22:23

raven1981

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Comments

DNA · Accepted Answer · 2020-03-02 10:47:44Z

22

You are trying to invoke the execute method on the object base, which is actually a Class object returned by your Class.forName() call.

This would work for a static (class) method - but execute is a non-static (instance) method.

(It would also work for calling an instance method of an object of type Class - but that's not what you are trying to achieve here!)

You need an actual instance of TestFunction to invoke the method on, or you need to make the method static.

When invoking a static method by reflection, the first argument to invoke() is ignored, so it is conventional to set it to null, which clarifies the fact that there's no instance involved.

Although your current example method would do the same thing for any TestFunction object, in general an instance method could produce a different result for each object - so the .invoke() reflection method needs to know which object to run the method on.

edited Mar 2, 2020 at 10:47

answered Nov 11, 2012 at 22:22

DNA

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6 Comments

DNA Over a year ago

Yes indeed, as described in the JavaDoc.

tier1 Over a year ago

Thanks for the detailed explanation. I wish I could select multiple correct answers for this question.

Stephen C Over a year ago

@tkcsam - the idea is you select the BEST correct answer ... in your opinion.

Stephen C Over a year ago

@tkcsam - If you mean ... for rewarding other answers than the one that you accepted, then ... yes, that it true.

vadipp Over a year ago

"This would only work for a static (class) method" - that's not true, it would work only for a method of class java.lang.Class. Conversely, to invoke static methods one has to pass obj = null to invoke.

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