调用 Method.invoke 时获取 java.lang.NullPointerException

Getting java.lang.NullPointerException when calling Method.invoke

提问by Can't Tell

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

我正在关注有关 Java annotaitons 的本教程并实现了 Test 注释，如图所示。但是在运行代码时，我得到以下输出。

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?

以下是我的代码。有人可以指出我有什么问题吗？

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();
        }
    }
}

采纳答案by dasblinkenlight

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

您传递给的参数invoke必须是调用该方法的对象，除非该方法是static. 你通过反射所做的相当于：

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.

当然，有一个 NPE。要解决此问题，请传递一个对象以在其上调用该方法，或创建该方法static。

Here is one way to make a fix:

这是一种修复方法：

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.

ideone 上的演示。

回答by Rob

This issue is here:

这个问题在这里：

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:

该方法的第一个参数是调用该方法的对象。这是动态（反射）等价物：

Object foo = null;
foo.toString();

Of course we would expect this code to give a NullPointerExceptionbecause foois null.

当然，我们希望这段代码给出一个NullPointerException因为foo是null。

回答by Gyanendra Dwivedi

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.

问题是您将空目标对象传递给method.invoke(object)方法。目标对象不应为空，否则会出现空指针异常。

The invoke method has below usages:

invoke 方法有以下用法：

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

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

回答by M. Abbas

The Method#invokehas the answer to your question:

该方法＃调用有回答你的问题：

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.

抛出： NullPointerException- 如果指定的对象为 null 并且该方法是一个实例方法。