NullPointerException while trying to invoke methods from dynamic classes

Question

I am trying to dynamically load from a class file. The codes below fails to invoke the methods in the class.

    public static void main(String[] args) {
        try {
            File loadPath = new File("C:/loadtest/");
            URL url = loadPath.toURI().toURL();
            URL[] urls = new URL[]{url};
            ClassLoader cl = new URLClassLoader(urls);
            Class cls = cl.loadClass("TesterClass");
            System.out.println("Classname: " + cls.getName());
            Method[] m = cls.getMethods();
            for (Method m1 : m){
                try {
                    System.out.println("Method: " + m1.getName());
                    if (m1.getName().equals("getName")) {
                        Object o = m1.invoke(null, null);
                        System.out.println("o is : " + o);
                    }
                } catch (Exception ex) {
                    ex.printStackTrace();
                }
            }
        } catch (MalformedURLException | ClassNotFoundException e) {
        }
    }

The target Java class I am trying to invoke:

public class TesterClass {

    public hamster() {
    }

    public void getName() {
        System.out.println("TEST SUCCEED !!!");
    }

}

What I got was:

Classname: TesterClass
Method: getName
Method: getClass
Method: hashCode
Method: equals
Aug 07, 2013 4:06:44 PM Tester main
Method: toString
Method: notify
Method: notifyAll
Method: wait
SEVERE: null
Method: wait
Method: wait
java.lang.NullPointerException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at Tester.main(Tester.java:40)

Just a note, the line number 40 is inaccurate because I removed comments and unnecessary portions to compact the above codes.

Why do I get a NullPointerException ? I tried to modify it to calling only the "getName()" method and it crashed too. How do I resolve this ?

See https://stackoverflow.com/questions/18802277/getting-java-lang-nullpointerexception-when-calling-method-invoke — rogerdpack, Jan 14 '22 at 20:09

score 7 · Accepted Answer · answered Aug 07 '13 at 08:25

You're getting an NPE because you're passing null into invoke:

Object o = m1.invoke(null, null);

...for an instance method. You have to pass an instance of the class in to call instance methods.

Create an instance, probably just after getting the class:

Class cls = cl.loadClass("TesterClass");
Object inst = cls.newInstance(); // <== This is the new line

...and then when calling the method, pass in that instance. I also don't think you need the second null on versions of Java that support varargs (so, anything vaguely recent), so:

Object o = m1.invoke(inst);
// Instance here ----^

or on older versions:

Object o = m1.invoke(inst, null);
// Instance here ----^

NullPointerException while trying to invoke methods from dynamic classes

1 Answers1