Getting java.lang.NullPointerException when callin

问题:

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.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:616)
    at TestAnnotationParser.parse(Demo.java:24)
    at Demo.main(Demo.java:51)
Passed:0   Fail:1

Following is my code. Can someone point out what I have got wrong?

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Method;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@interface Test {
    Class expected();
}

class TestAnnotationParser {
    public void parse(Class<?> clazz) 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(null);
                    pass++;
                } catch (Exception e) {
                    if (Exception.class != expected) {
                        e.printStackTrace();
                        fail++;
                    } else {
                        pass++;
                    }
                }
            }
        }
        System.out.println("Passed:" + pass + "   Fail:" + fail);
    }
}

class MyTest {

    @Test(expected = RuntimeException.class)
    public void testBlah() {
    }
}

public class Demo {
    public static void main(String[] args) {
        TestAnnotationParser parser = new TestAnnotationParser();
        try {
            parser.parse(MyTest.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

回答1:

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.

回答2:

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.

回答3:

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.

回答4:

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.