Getting java.lang.NullPointerException when calling Method.invoke

Question

I\'m following this tutorial on Java annotaitons and implemented the Test annotation as shown there. But when running the code I get the following output.

java.l         


        

Answer 1

The Method#invoke has the answer to your question:

public Object invoke(Object obj,
            Object... args)
              throws IllegalAccessException,
                     IllegalArgumentException,
                     InvocationTargetException

Throws: NullPointerException - if the specified object is null and the method is an instance method.

Answer 2

The problem is that you are passing a null target object to method.invoke(object) method. The target object should not be null, else a nullpointerexception is expected.

The invoke method has below usages:

Method.invoke(targetObject, args1, args2, args3...); where args1, args2, args3 etc are argument to the method being invoked.

Answer 3

This issue is here:

method.invoke(null);

This method's first parameter is the object to invoke the method on. This is the dynamic (reflection) equivalent of something like this:

Object foo = null;
foo.toString();

Of course we would expect this code to give a NullPointerException because foo is null.

Answer 4

The parameter that you pass to invoke must be an object on which the method is invoked, unless the method is static. What you did through reflection is equivalent to this:

MyTest obj = null;
obj.testBlah();

Naturally, there's an NPE. To fix this problem, pass an object on which to invoke the method, or make the method static.

Here is one way to make a fix:

public <T> void parse(Class<T> clazz, T obj) throws Exception {
    Method[] methods = clazz.getMethods();
    int pass = 0;
    int fail = 0;

    for (Method method : methods) {
        if (method.isAnnotationPresent(Test.class)) {
            Test test = method.getAnnotation(Test.class);
            Class expected = test.expected();
            try {
                method.invoke(obj);
                pass++;
            } catch (Exception e) {
                if (Exception.class != expected) {
                    e.printStackTrace();
                    fail++;
                } else {
                    pass++;
                }
            }
        }
    }
    System.out.println("Passed:" + pass + "   Fail:" + fail);
}

...

parser.parse(MyTest.class, new MyTest());

Demo on ideone.