FileProvider.getUriForFile空指针异常求助：旧Android相机拍照修复

时隔10年重新回到Android开发，尝试修复一款旧APP的相机拍照功能，执行以下代码时：

public void photoIntent() {
        Log.d("MainActivity", "photoIntent");
        Intent intent = new Intent("android.media.action.IMAGE_CAPTURE");
        File photo = null;
        try {
            File outputDir = context.getExternalFilesDir(Environment.DIRECTORY_PICTURES); // context being the Activity pointer
            photo = new File(outputDir+"picture.jpg");
            // place where to store camera taken picture
            //photo.delete();
        } catch (Exception e) {
            Log.d("MainActivity", "Can't create file to take picture!");
        }
        *Uri mImageUri = FileProvider.getUriForFile(this, BuildConfig.APPLICATION_ID+".fileprovider", photo);*
        intent.putExtra(MediaStore.EXTRA_OUTPUT, mImageUri);
        // start camera intent
        this.startActivityForResult(intent, 1);
    }

在Uri mImageUri = FileProvider.getUriForFile(this, BuildConfig.APPLICATION_ID+".fileprovider", photo);行出现空指针异常：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.XmlResourceParser android.content.pm.ProviderInfo.loadXmlMetaData(android.content.pm.PackageManager, java.lang.String)' on a null object reference
                                                                                                        at android.support.v4.content.FileProvider.parsePathStrategy(FileProvider.java:583)
                                                                                                        at android.support.v4.content.FileProvider.getPathStrategy(FileProvider.java:557)
                                                                                                        at android.support.v4.content.FileProvider.getUriForFile(FileProvider.java:399)
                                                                                                        at com.marzinp.mycelium.MainActivity.photoIntent(MainActivity.java:96)
                                                                                                        at com.marzinp.mycelium.MainActivity$1.onClick(MainActivity.java:69)
                                                                                                        at android.view.View.performClick(View.java:7892)
                                                                                                        at android.widget.TextView.performClick(TextView.java:16220)
                                                                                                        at android.view.View.performClickInternal(View.java:7869)
                                                                                                        at android.view.View.-$$Nest$mperformClickInternal(Unknown Source:0)
                                                                                                        at android.view.View$PerformClick.run(View.java:30891)
                                                                                                        at android.os.Handler.handleCallback(Handler.java:942)
                                                                                                        at android.os.Handler.dispatchMessage(Handler.java:99)
                                                                                                        at android.os.Looper.loopOnce(Looper.java:226)
                                                                                                        at android.os.Looper.loop(Looper.java:313)
                                                                                                        at android.app.ActivityThread.main(ActivityThread.java:8762)
                                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                                        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:604)
                                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1067)

当前配置 #

Manifest文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.marzinp.mycelium"
    android:versionCode="112"
    android:versionName="1.1.2" >

    <uses-sdk
        android:minSdkVersion="22"
        android:targetSdkVersion="33" />

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

    <application
        android:allowBackup="true"
        android:icon="@drawable/lombric"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <provider
            android:name="android.support.v4.content.FileProvider"
            android:authorities="com.marzinp.mycelium.provider"
            android:exported="false"
            android:grantUriPermissions="true">
            <meta-data
                android:name="android.support.FILE_PROVIDER_PATHS"
                android:resource="@xml/provider_paths"/>
        </provider>

        <activity
            android:name="com.marzinp.mycelium.MainActivity"
            android:configChanges="orientation|screenSize"         
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

res/xml/provider_paths文件：

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android"
    <external-path
        name="external_files"
        path="Pictures" />
</paths>

解决方案 #

• 匹配FileProvider的authorities参数：代码中使用的authorities是BuildConfig.APPLICATION_ID+".fileprovider"（即com.marzinp.mycelium.fileprovider），但Manifest里配置的android:authorities是com.marzinp.mycelium.provider，二者不匹配。将Manifest中的android:authorities修改为com.marzinp.mycelium.fileprovider，或者将代码中的authorities参数改为com.marzinp.mycelium.provider，保证两边一致。

• 修复provider_paths的XML语法错误：原文件中<paths>标签未正确闭合，缺少末尾的>，正确的内容应为：

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <external-path
        name="external_files"
        path="Pictures" />
</paths>

• 处理photo为空的情况：代码中try块如果创建文件失败，photo会为null，直接调用FileProvider.getUriForFile会导致崩溃。需要在调用前增加非空判断：

if (photo != null) {
    Uri mImageUri = FileProvider.getUriForFile(this, BuildConfig.APPLICATION_ID+".fileprovider", photo);
    intent.putExtra(MediaStore.EXTRA_OUTPUT, mImageUri);
    this.startActivityForResult(intent, 1);
} else {
    Log.e("MainActivity", "Failed to create photo file");
    // 可在此处添加用户提示，比如Toast
}

• 确认FileProvider依赖：由于使用的是android.support.v4.content.FileProvider，需确保项目中已正确引入support-v4库；如果项目已迁移到AndroidX，建议替换为androidx.core.content.FileProvider，并同步修改Manifest中的android:name属性及meta-data中的name为androidx.core.content.FileProvider。

内容的提问来源于stack exchange，提问作者Pierre Marzin