java - 由于 Intent 中的 NullPointerException，Android 图库应用程序在安装后无法启动

Question

我是 java 和使用 Android Studio 的新手。我的应用程序安装后无法启动。检查源代码和XML，我似乎找不到任何错误。请帮我检查下面的代码

MainActivity.java

public class MainActivity extends AppCompatActivity {

    ArrayList<File> list;
    GridView gridView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        gridView = (GridView)findViewById(R.id.image_grid);

        list = imageReader(Environment.getExternalStorageDirectory());

        gridView.setAdapter(new gridAdapter());

        gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {

                Intent intent = new Intent(MainActivity.this, FullImageActivity.class);
                intent.putExtra("img", list.get(i).toString());

                startActivity(intent);
            }
        });



    }

    public class gridAdapter extends BaseAdapter{

        @Override
        public int getCount() {
            return list.size();
        }

        @Override
        public Object getItem(int i) {
            return list.get(i);
        }

        @Override
        public long getItemId(int i) {
            return 0;
        }

        @Override
        public View getView(int i, View view, ViewGroup viewGroup) {
            View convertView = null;

            if (convertView == null) {

                convertView = getLayoutInflater().inflate(R.layout.row_layout, viewGroup, false);
                ImageView myImage = (ImageView) convertView.findViewById(R.id.my_image);
                myImage.setImageURI(Uri.parse(list.get(i).toString()));
            }

            return convertView;
        }
    }

    private ArrayList<File> imageReader(File externalStorageDirectory) {

        ArrayList<File> b = new ArrayList<>();

        File[] files = externalStorageDirectory.listFiles();

        for (int i =0; i<files.length; i++){

            if (files[i].isDirectory()){

                b.addAll(imageReader(files[i]));

            }else {

                if (files[i].getName().endsWith(".jpg")) {

                    b.add(files[i]);
                }
            }
        }
        return b;
    }
}

这是activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <GridView
        android:id="@+id/image_grid"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:numColumns="auto_fit"
        android:columnWidth="100dp"
        android:verticalSpacing="100dp"
        android:horizontalSpacing="10dp"
        android:stretchMode="spacingWidthUniform" />

</RelativeLayout>

这是 FullImageActivity.java

public class FullImageActivity extends AppCompatActivity {

    ImageView fullImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_full_image);

        fullImage = (ImageView)findViewById(R.id.full_image);

        String data = getIntent().getExtras().getString("img");

        fullImage.setImageURI(Uri.parse(data));
    }
}

activity_full_image.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FullImageActivity">

    <ImageView
        android:id="@+id/full_image"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:src="@drawable/wp"
        />

</RelativeLayout>

row_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <ImageView
        android:id="@+id/my_image"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:src="@drawable/wp"
        />

</RelativeLayout>

如果我的问题得到关注，我会很高兴，因为它深深地困扰着我。

这是 LOGCAT

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
                                                                                   at com.jkjworks.galleryextra.FullImageActivity.onCreate(FullImageActivity.java:19)
                                                                                   at android.app.Activity.performCreate(Activity.java:7030

#2 更新 list 和 BUILD.GRADLE

这是 list

  <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.CAMERA"/>
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

    <application

        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity"/>
            <activity android:name=".FullImageActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>     

这是APP级别的Build.gradle

  android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.domain.galleryextra"
        minSdkVersion 16
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        multiDexEnabled true
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    //noinspection GradleCompatible

    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support:design:27.1.1'
    compile 'com.android.support:multidex:1.0.3'
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    implementation 'com.google.android.gms:play-services-ads:12.0.1'
    implementation 'com.android.support:support-vector-drawable:27.0.1'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'    

Answer 1

您确定 list.get(i) 不是 null 吗？我会放置一个断点并分析 list.get(i) 的返回值，以确保您传递的数据是有效的数据。

此外，使用 intent.getStringExtra("img") 而不是 getIntent().getExtras().getString("img")

您的问题是 getIntent() 正在接收 NULL，并且您试图访问它，从而导致发生 NullPointerException。

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.os.Bundle.getString(java.lang.String)' on a null object reference
                                                                                   at com.merizekworks.galleryextra.FullImageActivity.onCreate(FullImageActivity.java:19)

编辑(如何调试的快速分割)您能做的最好的事情就是通过调试器单步执行代码。如果您从未这样做过，那么实际上很容易做到，并且可以帮助您了解问题所在。

在MainActivity中更改此:

public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
    Intent intent = new Intent(MainActivity.this, FullImageActivity.class);
    intent.putExtra("img", list.get(i).toString());
    startActivity(intent);
}

为此，为了帮助我们更轻松地调试:

public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
    Intent intent = new Intent(MainActivity.this, FullImageActivity.class);
    File selectedFile = list.get(i); //PUT BREAKPOINT ON THIS LINE
    String filePath = selectedFile.toString();
    intent.putExtra("img", filePath );
    startActivity(intent);
}

注意:如果您不熟悉调试代码，那么我强烈建议您花几分钟阅读本文。在我看来，它是程序员拥有的最有值(value)的工具。 Click here to learn how to debug in Android Studio

左边的红圈是断点，代码会在这里停止执行，你可以分析每个变量，如上图所示。您甚至可以在应用程序运行且执行被断点暂停时“评估表达式”。这样做可以让您抓取内存中当前的任何对象并对其进行测试。

例如，通过上面设置的断点，我们可以使用“调试”选项卡中的“评估表达式”按钮:

以下是有关如何评估表达式的快速屏幕截图，摘自有关如何使用 AndroidStudio 调试器 ( Full guide here ) 的文章:

<小时/>

更新#2

请注意，如果您使用的是 API 23 或更高版本，那么您必须确保拥有从 Android 设备读取/写入的权限。

在 AndroidManifest.xml 内的 Manifest 标记中包含以下内容:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

这是我在项目中使用的解决方案，它对我来说非常有效，特别是如果您计划请求多个权限。

public class MainActivity extends AppCompatActivity {
    private final String TAG = "DEBUG";
    private Activity activity;
    private ArrayList<File> list;
    private GridView gridView;

    private final int REQUEST_PERMISSIONS_CODE = 1;
    private String[] PERMISSIONS = {
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE,
    };

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        int totalGrantedCount = 0;
        for(int x=0; x < grantResults.length; x++){
            if (grantResults[x] == PackageManager.PERMISSION_GRANTED){
                totalGrantedCount++;
            }
        }

        if(totalGrantedCount == grantResults.length){
            startApp();
        }else{
            boolean somePermissionsForeverDenied = false;
            for(String permission: permissions){
                if(ActivityCompat.shouldShowRequestPermissionRationale(this, permission)){
                    //denied
                    Log.d(TAG, "PERMISSIONS: DENIED " + permission);
                }else{
                    if(ActivityCompat.checkSelfPermission(this, permission) == PackageManager.PERMISSION_GRANTED){
                        //allowed
                        Log.d(TAG, "PERMISSIONS: ALLOWED " + permission);
                    } else{
                        //set to never ask again
                        somePermissionsForeverDenied = true;
                    }
                }
            }
            if(somePermissionsForeverDenied){
                final AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(this);
                alertDialogBuilder.setTitle("Permissions Required")
                        .setMessage("You have forcefully denied some of the required permissions " +
                                "for this action. Please open settings, go to permissions and allow them manually.")
                        .setPositiveButton("Settings", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                                Intent intent = new Intent(Settings.ACTION_APPLICATION_DETAILS_SETTINGS,
                                        Uri.fromParts("package", getPackageName(), null));
                                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                                startActivity(intent);
                            }
                        })
                        .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialog, int which) {
                            }
                        })
                        .setCancelable(false)
                        .create()
                        .show();
            }else{
                new AlertDialog.Builder(this)
                        .setTitle("PERMISSIONS DENIED")
                        .setMessage("Unable to proceed, this application requires that permissions be accepted to operate correctly. \r\n\r\n YOU MUST APPROVE ALL PERMISSIONS BEFORE CONTINUING.")
                        .setPositiveButton("TAP HERE TO CLOSE AND TRY AGAIN.", new DialogInterface.OnClickListener(){
                            public void onClick(DialogInterface dialog, int which){
                                activity.finish();
                                System.exit(0);
                            }
                        })
                        .setIcon(android.R.drawable.ic_dialog_alert)
                        .setCancelable(false)
                        .show();
            }
        }
    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        this.activity = this;

        //REQUIRED FOR API 23+ - REQUEST FOR PERMISSIONS - ONLY REQUIRED TO BE DONE ONCE PER APP INSTALL.
        int permission = ActivityCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE);

        if (permission != PackageManager.PERMISSION_GRANTED) {
            // We don't have permission so prompt the user
            ActivityCompat.requestPermissions(
                    this,
                    PERMISSIONS,
                    REQUEST_PERMISSIONS_CODE);
        }else{
            startApp();
        }

    }

    public void startApp(){
        gridView = (GridView)findViewById(R.id.image_grid);
        list = imageReader(Environment.getExternalStorageDirectory());
        gridView.setAdapter(new gridAdapter());
        gridView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {

                Intent intent = new Intent(MainActivity.this, FullImageActivity.class);
                intent.putExtra("img", list.get(i).toString());

                startActivity(intent);
            }
        });
    }

    public class gridAdapter extends BaseAdapter {

        @Override
        public int getCount() {
            return list.size();
        }

        @Override
        public Object getItem(int i) {
            return list.get(i);
        }

        @Override
        public long getItemId(int i) {
            return 0;
        }

        @Override
        public View getView(int i, View view, ViewGroup viewGroup) {
            View convertView = null;

            if (convertView == null) {

                convertView = getLayoutInflater().inflate(R.layout.row_layout, viewGroup, false);
                ImageView myImage = (ImageView) convertView.findViewById(R.id.my_image);
                myImage.setImageURI(Uri.parse(list.get(i).toString()));
            }

            return convertView;
        }
    }

    private ArrayList<File> imageReader(File externalStorageDirectory) {

        ArrayList<File> b = new ArrayList<>();

        File[] files = externalStorageDirectory.listFiles();

        for (int i =0; i<files.length; i++){

            if (files[i].isDirectory()){

                b.addAll(imageReader(files[i]));

            }else {

                if (files[i].getName().endsWith(".jpg")) {

                    b.add(files[i]);
                }
            }
        }
        return b;
    }
}