java - 琐碎、简短且简单的 android 单元测试抛出 NullPointerException

Question

我有一个很小的 ​​Android 项目，是我拼凑而成的，用于了解 Android 单元和仪器测试。我使用dog.ceo API 来获取狗的随机图像。程序本身按其应有的方式运行。但是，每当我运行或调试单元测试时，我都会在所有测试上得到 NullPointerExceptions，每个测试都有以下堆栈跟踪:

java.lang.NullPointerException
    at com.jmarkman.dog.DogAPI.getDogURL(DogAPI.java:27)
    at com.jmarkman.dog.DogAPITest.getJSON(DogAPITest.java:37)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)

这是我的主要(唯一) Activity :https://gist.github.com/jmarkman/733b0a1cbeb41a1c5733c0c715e4606b

这是我与dog.ceo API 交互的类。它是一个常规的旧 Java 类，没有应用任何类型的设计模式: https://gist.github.com/jmarkman/c38546d99b0ca06099175a8da68d76b6

package com.jmarkman.dog;

import android.net.Uri;
import android.util.Log;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Random;
import java.util.Scanner;

public class DogAPI
{
    private final String DOG_API_URL = "https://dog.ceo/api";

    public DogAPI() { }

    public URL getDogURL()
    {
        URL url = null;

        Uri uri = Uri.parse(DOG_API_URL).buildUpon()
                .appendPath("breeds")
                .appendPath("image")
                .appendPath("random")
                .build();

        try
        {
            url = new URL(uri.toString());
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }

        return url;
    }

    public URL getDogURL(String breed, boolean random)
    {
        URL url = null;
        Uri uri;

        if (random)
        {
            uri = Uri.parse(DOG_API_URL).buildUpon()
                    .appendPath("breed")
                    .appendPath(breed)
                    .appendPath("images")
                    .appendPath("random")
                    .build();
        }
        else
        {
            uri = Uri.parse(DOG_API_URL).buildUpon()
                    .appendPath("breed")
                    .appendPath(breed)
                    .appendPath("images")
                    .build();
        }

        try
        {
            url = new URL(uri.toString());
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }

        return url;
    }

    public String getJSON(URL url) throws IOException
    {
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream stream = connection.getInputStream();
        Scanner scanner = new Scanner(stream);
        scanner.useDelimiter("\\A");

        boolean hasData = scanner.hasNext();

        try
        {
            if (hasData)
            {
                return scanner.next();
            }
            else
            {
                return null;
            }
        }
        catch (Exception e)
        {
            Log.d("Error", e.toString());
            return null;
        }
        finally
        {
            connection.disconnect();
        }
    }

    public Uri getImage(String json)
    {
        final String MESSAGE = "message";
        Random random = new Random();
        Uri imageURL = null;

        try
        {
            JSONObject imageObj = new JSONObject(json);
            Object imageMsg = imageObj.get(MESSAGE);
            if (imageMsg instanceof String)
                imageURL = Uri.parse(imageMsg.toString());
            else if (imageMsg instanceof JSONArray)
            {
                int urlIndex = random.nextInt(((JSONArray) imageMsg).length() + 1);
                imageURL = Uri.parse(((JSONArray) imageMsg).getString(urlIndex));
            }
        }
        catch (JSONException jse)
        {
            jse.printStackTrace();
        }

        return imageURL;
    }
}

仪器测试运行没有问题。我的单元测试并不太复杂；它们甚至比我在 Pluralsight 类(class)中观看的有关 Android 测试的内容还要短。以下是我的测试文件中的每一行逐字记录:

package com.jmarkman.dog;

import android.net.Uri;
import org.junit.Test;
import java.net.URL;
import static org.junit.Assert.*;

public class DogAPITest {
    @Test
    public void getDogURL() throws Exception
    {
        DogAPI dog = new DogAPI();
        String expectedURL = "https://dog.ceo/api/breeds/image/random";
        String actualURL = dog.getDogURL().toString();

        assertEquals(expectedURL, actualURL);
    }

    @Test
    public void getDogURLForBreed() throws Exception
    {
        DogAPI dog = new DogAPI();
        String url = dog.getDogURL("corgi", true).toString();

        assertEquals("https://dog.ceo/api/breed/corgi/images/random", url);

    }

    @Test
    public void getJSON() throws Exception
    {
        DogAPI dog = new DogAPI();
        URL url = dog.getDogURL();
        String returnJSON = dog.getJSON(url);

        assertNotNull(returnJSON);
    }

    @Test
    public void getImage() throws Exception
    {
        DogAPI dog = new DogAPI();
        URL url = dog.getDogURL();
        String returnJSON = dog.getJSON(url);

        Uri uri = dog.getImage(returnJSON);

        assertNotNull(uri);
    }
}

我的 build.gradle 文件中有 unitTests.returnDefaultValues = true 。我已经进入构建菜单并清理并重建了项目。是的，我已经阅读了“什么是 NullPointerException”问题。我错过了什么？

Answer 1

您的 DogAPI 类(特别是 getDogURL() 方法)使用 android.net.Uri.parse()，它是在特定于 Android 框架的类中。 Android 特定的类在本地 JUnit 测试中不可用，如果启用 unitTests.returnDefaultValues = true，它们仅作为 stub 存在。来自 the documentation :

If you run a test that calls an API from the Android SDK that you do not mock, you'll receive an error that says this method is not mocked. That's because the android.jar file used to run unit tests does not contain any actual code (those APIs are provided only by the Android system image on a device).

你有两个选择。首先是使用Robolectric测试库，它提供了用于在本地 JUnit 测试中运行的 Android 类的功能版本。只需在测试顶部添加 @RunWith(RobolectricTestRunner.class) 即可。

或者，使用不同的 Uri 解析器，例如 java.net.URL ，巧合的是也有一个 toURI()方法。