调用在 Android 中具有 getContentResolver().query 的方法时的 NPE

Question

从 Android 中的另一个活动调用 getPlaylist() 方法时出现

NullPointerException。 该方法应该获取音频信息，然后使用 Cursor cursor = getContentResolver().query(MediaStore...)

返回它

这里是详细信息。

我正在使用两个活动（AudioPlayerActivity、FileManager）开发自定义音频播放器。 AudioPlayerActivity 使用 FileManager** 获取音频文件列表，然后播放它们。

这里是它们的sn-ps。以下是AudioPlayerActivity.java

ArrayList<HashMap<String, String>> audioList = new ArrayList<HashMap<String, String>>();

mp = new MediaPlayer();
fileManager = new FileManager();

audioList = fileManager.getPlaylist(); // NPE when calling this method!

playAudio(0);

接下来是FileManager.java，它使用MediaStore API读取所有音频文件并将详细信息（如MP3 ID标签）存储在ArrayList中。

public class FileManager extends Activity {

    @Override
    public void onCreate(Bundle SavedInstanceState) {
        super.onCreate(savedInstanceState);
        // no view. just getting audio files
    }

    public ArrayList<HashMap<String, String> getPlaylist() {
        String[] proj = { MediaStore.Audio.Media._ID };

        Cursor cursor = getContentResolver().query(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, proj, null, null, null}

        Log.d("debug", "is cursor?"); // NPE here!

        return list;
    }

LogCat

09-30 11:55:35.265: D/AndroidRuntime(13516): Shutting down VM
09-30 11:55:35.265: W/dalvikvm(13516): threadid=1: thread exiting with uncaught exception (group=0x41d37c08)
09-30 11:55:35.270: E/AndroidRuntime(13516): FATAL EXCEPTION: main
09-30 11:55:35.270: E/AndroidRuntime(13516): Process: project.sample.audioplayer, PID: 13516
09-30 11:55:35.270: E/AndroidRuntime(13516): java.lang.RuntimeException: Unable to start activity ComponentInfo{project.sample.audioplayer/project.sample.audioplayer.AudioPlayerActivity}: java.lang.NullPointerException
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread.access$900(ActivityThread.java:161)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.os.Handler.dispatchMessage(Handler.java:102)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.os.Looper.loop(Looper.java:157)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread.main(ActivityThread.java:5356)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at java.lang.reflect.Method.invokeNative(Native Method)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at java.lang.reflect.Method.invoke(Method.java:515)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at dalvik.system.NativeStart.main(Native Method)
09-30 11:55:35.270: E/AndroidRuntime(13516): Caused by: java.lang.NullPointerException
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.content.ContextWrapper.getContentResolver(ContextWrapper.java:104)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at project.sample.audioplayer.FileManager.getPlaylist(FileManager.java:35)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at project.sample.audioplayer.AudioPlayerActivity.onCreate(AudioPlayerActivity.java:70)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.Activity.performCreate(Activity.java:5426)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
09-30 11:55:35.270: E/AndroidRuntime(13516):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269)
09-30 11:55:35.270: E/AndroidRuntime(13516):    ... 11 more

它在具有 getContentResolver().query() 的行给出 NullPointerException。

我错过了什么？还是不能在方法中使用 Cursor cursor = getContentResolver().query(MediaStore...) 而不是 onCreate() 方法？

我很确定 AudioPlayerAcitivity.java 不是问题，因为它在 FileManger.java 中使用 File 类而不是 MediaStore 时有效。

Answer 1

getContentResolver() 需要在适当的上下文中调用。

不用从Activity扩展FileManager，只需从调用Activity（AudioPlayerActivity.java）传入上下​​文，然后就可以使用Cursor cursor = context.getContentResolver()...中的上下文