单击按钮时无法将内部存储中的 mp3 文件显示为列表视图

Question

我一直在尝试在单击按钮时显示手机内部存储中的所有 mp3 文件。但是，一旦我单击该按钮，它就会要求存储权限，并在授予权限后显示Unfortunately, app has stopped。似乎我遇到了运行时异常，但我找不到解决此问题的方法。任何人都可以帮忙吗？ 这是按钮：

<Button
    android:id="@+id/button4"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/button3"
    android:layout_alignStart="@+id/button3"
    android:layout_below="@+id/button3"
    android:layout_marginTop="22dp"
    android:onClick="onClick3"
    android:text="@string/button_audio" />

这是课程：

   package com.example.dell_1.myapp3;  

   import android.content.Intent;
   import android.database.Cursor;
   import android.media.MediaPlayer;
   import android.net.Uri;
   import android.os.Bundle;
   import android.app.Activity;
   import android.os.Environment;
   import android.provider.MediaStore;
   import android.util.Log;
   import android.view.View;
   import android.widget.AdapterView;
   import android.widget.ArrayAdapter;
   import android.widget.ListView;
   import java.io.File;
   import java.io.IOException;


        public class Bacon1 extends Activity {

         private static int RESULT_LOAD_IMAGE = 1;
    private static int MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE = 1;

    private String[] mAudioPath;
    private MediaPlayer mMediaPlayer;
    private String[] mMusicList;;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_bacon1);

        final Button button4 = (Button) findViewById(R.id.button4);

        button4.setOnClickListener(
                new View.OnClickListener(){
                    public void onClick (View v){
                        buttonClicked(v); 

                        abc();

                    }
                });
    }


   private void buttonClicked(View view){

       if (ContextCompat.checkSelfPermission(this,
               Manifest.permission.READ_EXTERNAL_STORAGE)
               != PackageManager.PERMISSION_GRANTED) {

           Snackbar.make(view, "Permission not Granted, Requesting permission.", Snackbar.LENGTH_LONG).show();
           if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                   Manifest.permission.READ_EXTERNAL_STORAGE)) {

               Snackbar.make(view, "We need permission to internal storage for displaying songs", Snackbar.LENGTH_LONG).show();

           } else {

               Snackbar.make(view, "Allow myapp3 to access this device's internal storage", Snackbar.LENGTH_LONG).show();

               ActivityCompat.requestPermissions(this,
                       new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                       MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);

               // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
               // app-defined int constant. The callback method gets the
               // result of the request.
           }
       }
       }



    public void onClick2(View view) {

        Intent i = new Intent(
                Intent.ACTION_PICK,
                android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);

        startActivityForResult(i, RESULT_LOAD_IMAGE);
    }


    public void onClick4(View view) {

        Intent viewIntent1 = new Intent(Intent.ACTION_VIEW);
        File file = Environment.getExternalStorageDirectory();
        viewIntent1.setDataAndType(Uri.fromFile(file), "video/*");
        startActivity(Intent.createChooser(viewIntent1, null));
    }

    public void onClick5(View view) {

        Intent viewIntent1 = new Intent(Intent.ACTION_VIEW);
        File file = Environment.getExternalStorageDirectory();
        viewIntent1.setDataAndType(Uri.fromFile(file), "zip/*");
        startActivity(Intent.createChooser(viewIntent1, null));
    }
    public void onClick6(View view) {

        Intent viewIntent1 = new Intent(Intent.ACTION_VIEW);
        File file = Environment.getExternalStorageDirectory();
        viewIntent1.setDataAndType(Uri.fromFile(file), "text/*");
        startActivity(Intent.createChooser(viewIntent1, null));
    }

    public void onClick7(View view) {

        Intent viewIntent1 = new Intent(Intent.ACTION_VIEW);
        File file = Environment.getExternalStorageDirectory();
        viewIntent1.setDataAndType(Uri.fromFile(file), "application/*");
        startActivity(Intent.createChooser(viewIntent1, null));
    }


    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
            switch (requestCode) {
                case 1: {
                    // If request is cancelled, the result arrays are empty.
                    if (grantResults.length > 0
                            && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                        Toast.makeText(Bacon1.this, "WRITE_CONTACTS granted", Toast.LENGTH_SHORT)
                            .show();

                    } else {

                        Toast.makeText(Bacon1.this, "WRITE_CONTACTS Denied", Toast.LENGTH_SHORT)
                                .show();

                    }
                    return;
                }

                // other 'case' lines to check for other
                // permissions this app might request
            }
    }

    public void abc(){
        mMediaPlayer = new MediaPlayer();

        ListView mListView = (ListView) findViewById(R.id.list);

        mMusicList = getAudioList();

        ArrayAdapter<String> mAdapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, mMusicList);
        mListView.setAdapter(mAdapter);

        mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
                                    long arg3) {
                try {
                    playSong(mAudioPath[arg2]);
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalStateException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
    }

    private String[] getAudioList() {
        final Cursor mCursor = getContentResolver().query(
                MediaStore.Audio.Media.EXTERNAL_CONTENT_URI,
                new String[] { MediaStore.Audio.Media.DISPLAY_NAME, MediaStore.Audio.Media.DATA }, null, null,
                "LOWER(" + MediaStore.Audio.Media.TITLE + ") ASC");

        int count = mCursor.getCount();

        String[] songs = new String[count];
        String[] mAudioPath = new String[count];
        int i = 0;
        if (mCursor.moveToFirst()) {
            do {
                songs[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DISPLAY_NAME));
                mAudioPath[i] = mCursor.getString(mCursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DATA));
                i++;
            } while (mCursor.moveToNext());
        }

        mCursor.close();

        return songs;
    }

    private void playSong(String path) throws IllegalArgumentException,
            IllegalStateException, IOException {

        Log.d("ringtone", "playSong :: " + path);

        mMediaPlayer.reset();
        mMediaPlayer.setDataSource(path);
        mMediaPlayer.prepare();
        mMediaPlayer.start();
    }

    }

这是 Android 清单文件

 <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.dell_1.myapp3">

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


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

        <activity
            android:name=".apples"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".Bacon1"
            android:label="@string/title_activity_bacon1" />
        <activity
            android:name=".SDCard"
            android:label="@string/title_activity_sdcard" />
        <activity
            android:name=".Songmanager"
            android:label="@string/title_activity_songmanager"></activity>
    </application>

</manifest>

这里是 Logcat：

   07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/ExtMediaPlayer-JNI: env->IsInstanceOf fails
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/MediaPlayer-JNI: JNIMediaPlayerFactory: bIsQCMediaPlayerPresent 0
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/ExtMediaPlayer-JNI: env->IsInstanceOf fails
07-02 03:40:13.655 13229-13229/com.example.dell_1.myapp3 E/MediaPlayer-JNI: JNIMediaPlayerFactory: bIsQCMediaPlayerPresent 0
07-02 03:40:13.700 13229-13229/com.example.dell_1.myapp3 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                           Process: com.example.dell_1.myapp3, PID: 13229
                                                                           java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
                                                                               at com.example.dell_1.myapp3.Bacon1.abc(Bacon1.java:158)
                                                                               at com.example.dell_1.myapp3.Bacon1$1.onClick(Bacon1.java:48)
                                                                               at android.view.View.performClick(View.java:5609)
                                                                               at android.view.View$PerformClick.run(View.java:22263)
                                                                               at android.os.Handler.handleCallback(Handler.java:751)
                                                                               at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                               at android.os.Looper.loop(Looper.java:154)
                                                                               at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                                               at java.lang.reflect.Method.invoke(Native Method)
                                                                               at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                                                                               at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

Answer 1

您应该添加<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/> 到您的清单文件中。

您已在运行时请求许可：

if (ContextCompat.checkSelfPermission(thisActivity,Manifest.permission.READ_EXTERNAL_STORAGE)!= PackageManager.PERMISSION_GRANTED) {


if (ActivityCompat.shouldShowRequestPermissionRationale(thisActivity,
        Manifest.permission.READ_EXTERNAL_STORAGE)) {

    // Show an explanation to the user *asynchronously* -- don't block
    // this thread waiting for the user's response! After the user
    // sees the explanation, try again to request the permission.

} else {

    // No explanation needed, we can request the permission.

    ActivityCompat.requestPermissions(thisActivity,
            new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
            REQUEST_PERMISSION);

    // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
    // app-defined int constant. The callback method gets the
    // result of the request.
}

}

然后做任何你想做的事（如果用户授予权限）：

@Override
public void onRequestPermissionsResult(final int requestCode, @NonNull final String[] permissions, @NonNull final int[] grantResults) {
super.onRequestPermissionsResult(requestCode, permissions, grantResults);
if (requestCode == REQUEST_PERMISSION) {
    if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
        // Permission granted.
    } else {
        // User refused to grant permission.
    }
}
}

您可以在此处阅读更多内容： https://developer.android.com/training/permissions/requesting.html