FileProvider.getUriForFile 上下文抛出空指针异常

Question

我从FileProvider.getUriForFile 收到以下错误

java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“android.content.res.Resources android.content.Context.getResource()”

但是，当我显示 context.toString() 时，我得到了这个（我认为）证明该对象不为空：

uk.co.letsdelight.letsdelight.MainActivity@8eec139

代码将Completed download of update file 写入logcat，但不会在API 24 设备上写入URI successfully set。它适用于 API 23。这是代码的重要部分。 Update 类从 MainActivity 和 new Update(status, message).check(this); 调用

public class Update extends Activity {

    private File outputFile;
    private TextView status;
    private TextView message;
    private String TAG = "LETSDELIGHT";

    public Update(TextView status){
        this.status = status;
    }
    public Update(TextView status, TextView message){
        this.status  = status;
        this.message = message;
    }

    public void check(final Context c) {
        status.setText("Checking for updates");

        // Check for updates and give user option to update

            // UPDATE button pressed
            UpdateApp updateApp = new UpdateApp();
            updateApp.setContext(c);
            updateApp.execute(c.getString(R.string.apk_uri));
            status.setText("Downloading update");
    }

    public class UpdateApp extends AsyncTask<String,Void,Void> {
        private Context context;
        private Throwable thrown = null;
        private String statusMessage;
        public void setContext(Context contextf){
            context = contextf;
        }

        @Override
        protected Void doInBackground(String... arg0) {
            try {
                URL url = new URL(arg0[0]);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                conn.setDoOutput(true);
                conn.connect();

                Log.i(TAG,"Starting download of update file");
                statusMessage = "Starting download of update file";

                File file = context.getCacheDir();
                file.mkdirs();
                outputFile = new File(file, "update.apk");
                if(outputFile.exists()){
                    outputFile.delete();
                }
                FileOutputStream fos = new FileOutputStream(outputFile);

                InputStream is = conn.getInputStream();

                byte[] buffer = new byte[1024];
                int len1 = 0;
                while ((len1 = is.read(buffer)) != -1) {
                    fos.write(buffer, 0, len1);
                }
                fos.close();
                is.close();
                outputFile.setReadable(true, false);

                Log.i(TAG, "Completed download of update file");
                statusMessage = "Completed download of update file";

                Uri uri;
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                    uri = FileProvider.getUriForFile(context, getString(R.string.file_provider_authority), outputFile);
                } else {
                    uri = Uri.fromFile(outputFile);
                }

                Log.i(TAG, "URI successfully set");
                statusMessage = "URI successfully set";

                Intent intent = new Intent(Intent.ACTION_VIEW);
                intent.setDataAndType(uri, "application/vnd.android.package-archive");
                intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                context.startActivity(intent);

            } catch (Throwable ex) {
                Log.e(TAG, ex.toString());
                thrown = ex;
            }
            return null;
        }

        @Override
        protected void onPostExecute(Void v) {
            status.setText("");
            if (statusMessage != null) {
                status.setText(statusMessage);
            }
            if (thrown != null) {
                status.setText(context.toString());
                message.setText(thrown.toString());
            }
            outputFile.delete();
            super.onPostExecute(v);
        }
    }
}

AndroidManifest.xml 文件包含以下条目：

    <provider
        android:name="android.support.v4.content.FileProvider"
        android:authorities="@strings/file_provider_authority"
        android:exported="false"
        android:grantUriPermissions="true">
        <meta-data
            android:name="android.support.FILE_PROVIDER_PATHS"
            android:resource="@xml/file_provider_paths" />
    </provider>

file_provider_paths.xml 看起来像这样：

<?xml version="1.0" encoding="utf-8"?>
<paths>
    <cache-path name="cache" path="."/>
</paths>

我尝试将context 更改为getApplication() 和getApplicationContext()，这两者都会导致NullPointerException，但资源措辞略有不同。

我还能尝试什么来解决这个问题？

编辑：

我已经修改了代码，试图找出问题的根源：

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
                statusMessage = "Context is: " + context.getString(R.string.file_provider_authority);
                uri = FileProvider.getUriForFile(context, context.getString(R.string.file_provider_authority), outputFile);
                statusMessage = "uri set";
            } else {
                uri = Uri.fromFile(outputFile);
            }

statusMessage 设置为显示正确的提供者权限字符串。它没有设置为“uri set”。因此，肯定是 FileProvider.getUriForFile() 失败了。

鉴于context 不为空且Authority 是正确的字符串，还有什么可能在这里抛出错误？

Answer 1

您正在尝试在非活动类中调用 getString 尝试替换

getString(R.string.file_provider_authority)

与

context.getString(R.string.file_provider_authority)

编辑：我看到您正在扩展您的 Update 类中的活动，您是否在清单中声明了它？如果您已经在传递上下文，为什么还要扩展活动？