java.lang.NullPointerException：尝试调用虚拟方法“android.content.pm.ApplicationInfo”

问：

我正在制作一个音乐播放器，它工作正常，但经过一些调整后，它开始在 logcat 中出现错误：java.lang.RuntimeException：无法实例化活动 ComponentInfo{com.example.musicplayer/com.example.musicplayer.MainActivity}：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo（）” main_activity.java文件是：

package com.example.musicplayer;

import androidx.annotation.NonNull;
import androidx.annotation.RequiresApi;
import androidx.appcompat.app.AppCompatActivity;

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.ActivityManager;
import android.content.pm.PackageManager;
import android.media.MediaPlayer;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.SeekBar;
import android.widget.TextView;

import java.io.File;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Objects;


public class MainActivity extends AppCompatActivity {

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

    private static final String[] PERMISSIONS = {
            Manifest.permission.READ_EXTERNAL_STORAGE
    };

    private static final int REQUEST_PERMISSIONS=12345;
    private static final int permissions_count=1;
    @SuppressLint("NewApi")
    private boolean arepermissionsdenied(){
        for(int i=0;i<permissions_count;i++){
            if(checkSelfPermission(PERMISSIONS[i])!= PackageManager.PERMISSION_GRANTED){
                return true;
            }
        }
        return false;
    }
    @SuppressLint("NewApi")
    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults){
        super.onRequestPermissionsResult(requestCode,permissions,grantResults);
        if(arepermissionsdenied()){
            ((ActivityManager)(this.getSystemService(ACTIVITY_SERVICE))).clearApplicationUserData();
            recreate();
        }else{
            onResume();
        }
    }
    private boolean isMusicPLayerInit;
    private List<String> musicFilesList;
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    private void addMusicFilesFrom(String dirPath){
        final File musicDir=new File(dirPath);
        if(!musicDir.exists()){
           musicDir.mkdir();
           return;
        }
        if(musicDir==null){
            return;
        }
        final File[] files=musicDir.listFiles();
        for(File file: Objects.<File[]>requireNonNull(files)){
            final String path=file.getAbsolutePath();
            if(path.endsWith(".mp3")){
                musicFilesList.add(path);
            }
        }
    }
    @RequiresApi(api = Build.VERSION_CODES.KITKAT)
    private void fillMusicList(){
        musicFilesList.clear();
        addMusicFilesFrom(String.valueOf(Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_MUSIC)));
        addMusicFilesFrom(String.valueOf(Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_DOWNLOADS)));
    }
    private MediaPlayer mp;
    private int playMusicFile(String path){
        mp =new MediaPlayer();
        try{
            mp.setDataSource(path);
            mp.prepare();
            mp.start();
        }catch(Exception e){
            e.printStackTrace();
        }
        return mp.getDuration();
    }
    private int songPosition;
    private boolean isSongPlaying=false;
    final View playbackControls=findViewById(R.id.playbackButtons);

    @Override
    protected void onResume(){
        super.onResume();
        if(Build.VERSION.SDK_INT>=Build.VERSION_CODES.M && arepermissionsdenied()){
            requestPermissions(PERMISSIONS,REQUEST_PERMISSIONS);
            return;
        }
        if(!isMusicPLayerInit){
            final ListView listview=findViewById(R.id.listview);
            final TextAdapter textAdapter=new TextAdapter();
            musicFilesList=new ArrayList<>();
            fillMusicList();
            listview.setAdapter(textAdapter);
            if(musicFilesList.isEmpty()){
                textAdapter.setData(Collections.singletonList("please add files"));
                return;
            }else {
                textAdapter.setData(musicFilesList);
            }
            final SeekBar seekBar=findViewById(R.id.seekBar);
            seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
                int songProgress;
                @Override
                public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                    songProgress=progress;
                }

                @Override
                public void onStartTrackingTouch(SeekBar seekBar) {

                }

                @Override
                public void onStopTrackingTouch(SeekBar seekBar) {
                    songPosition=songProgress;
                    mp.seekTo(songPosition*1000);
                }
            });
            final TextView songPositionTextView=findViewById(R.id.currentPosition);
            final TextView songDurationTextView=findViewById(R.id.songDuration);
            final Button pauseButton=findViewById(R.id.pauseButton);
            pauseButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    if(isSongPlaying){
                        mp.pause();
                        pauseButton.setText("play");
                    }else{
                        mp.start();
                        pauseButton.setText("pause");
                    }
                    isSongPlaying=!isSongPlaying;
                }
            });

            listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                    final String musicFilePath=musicFilesList.get(position);
                    playMusicFile(musicFilePath);
                    final int songDuration=playMusicFile(musicFilePath)/1000;
                    seekBar.setMax(songDuration);
                    playbackControls.setVisibility(View.VISIBLE);
                    seekBar.setVisibility(View.VISIBLE);
                    songDurationTextView.setText(String.valueOf(songDuration/3600)+":"+String.valueOf((songDuration%3600)/60)+":"+String.valueOf((songDuration%3600)%60));
                    isSongPlaying=true;
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            songPosition = 0;
                            while (songPosition < songDuration) {
                                try {
                                    Thread.sleep(1000);
                                } catch (InterruptedException e) {
                                    e.printStackTrace();
                                }
                                if (isSongPlaying) {
                                    songPosition++;
                                    runOnUiThread(new Runnable() {
                                        @Override
                                        public void run() {
                                            seekBar.setProgress(songPosition);
                                            songPositionTextView.setText(String.valueOf(songPosition / 3600) + ":" + String.valueOf((songPosition % 3600) / 60) + ":" + String.valueOf((songPosition % 3600) % 60));
                                        }
                                    });
                                }
                            }

                            mp.pause();
                            songPosition = 0;
                            mp.seekTo(songPosition);
                            songPositionTextView.setText("0:0:0");
                            pauseButton.setText("play");
                            isSongPlaying = false;
                            seekBar.setProgress(songPosition);
                        }
                    }
                    );

                }
            });
            isMusicPLayerInit=true;
        }
    }
    class TextAdapter extends BaseAdapter{
        private List<String> data=new ArrayList<>();
        void setData(List<String> mdata){
            data.clear();
            data.addAll(mdata);
            notifyDataSetChanged();
        }
        @Override
        public int getCount(){
            return data.size();
        }
        @Override
        public String getItem(int position){
            return null;
        }
        @Override
        public long getItemId(int position){
            return 0;
        }
        @Override
        public View getView(int position, View convertView, ViewGroup parent){
            if(convertView==null){
                convertView= LayoutInflater.from(parent.getContext()).inflate(R.layout.item,parent,false);
                convertView.setTag(new ViewHolder((TextView) convertView.findViewById(R.id.myItem)));
            }
            ViewHolder holder=(ViewHolder) convertView.getTag();
            final String item=data.get(position);
            holder.info.setText(item.substring(item.lastIndexOf('/')+1));
            return convertView;
        }
    }
    class ViewHolder{
        TextView info;
        ViewHolder(TextView minfo){
            info=minfo;
        }
    }
}

logcat 错误是：

2020-10-28 20:23:40.437 7927-7927/? I/ple.musicplaye: Not late-enabling -Xcheck:jni (already on)
2020-10-28 20:23:40.454 7927-7927/? E/ple.musicplaye: Unknown bits set in runtime_flags: 0x8000
2020-10-28 20:23:40.454 7927-7927/? W/ple.musicplaye: Unexpected CPU variant for X86 using defaults: x86
2020-10-28 20:23:40.530 7927-7927/com.example.musicplayer W/RenderThread: type=1400 audit(0.0:47): avc: denied { write } for name="property_service" dev="tmpfs" ino=6871 scontext=u:r:untrusted_app:s0:c144,c256,c512,c768 tcontext=u:object_r:property_socket:s0 tclass=sock_file permissive=0
2020-10-28 20:23:40.545 7927-7961/com.example.musicplayer D/libEGL: Emulator has host GPU support, qemu.gles is set to 1.
2020-10-28 20:23:40.545 7927-7961/com.example.musicplayer W/libc: Unable to set property "qemu.gles" to "1": connection failed; errno=13 (Permission denied)
2020-10-28 20:23:40.550 7927-7961/com.example.musicplayer D/libEGL: loaded /vendor/lib/egl/libEGL_emulation.so
2020-10-28 20:23:40.552 7927-7961/com.example.musicplayer D/libEGL: loaded /vendor/lib/egl/libGLESv1_CM_emulation.so
2020-10-28 20:23:40.556 7927-7961/com.example.musicplayer D/libEGL: loaded /vendor/lib/egl/libGLESv2_emulation.so
2020-10-28 20:23:40.592 7927-7927/com.example.musicplayer D/AndroidRuntime: Shutting down VM
2020-10-28 20:23:40.593 7927-7927/com.example.musicplayer E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.musicplayer, PID: 7927
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.musicplayer/com.example.musicplayer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:163)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:174)
        at android.content.Context.obtainStyledAttributes(Context.java:738)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
        at com.example.musicplayer.MainActivity.<init>(MainActivity.java:108)
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:41)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1243)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

需要一些帮助 Item.xml 文件为：

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/myItem"
    android:layout_width="match_parent"
    android:layout_height="150dp"
    android:gravity="center_vertical"
    android:textSize="20sp"
    android:paddingStart="16dp"
    android:paddingLeft="16dp"
    android:paddingEnd="16dp"
    android:paddingRight="16dp"
    android:textColor="#00ffff">

</TextView>

activity_main.xml文件是：

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

    <ListView
        android:id="@+id/listview"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

    <LinearLayout
        android:id="@+id/playbackButtons"
        android:visibility="gone"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal" >
        <TextView
            android:paddingStart="16dp"
            android:id="@+id/currentPosition"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:paddingLeft="16dp" />
        <Button
            android:id="@+id/pauseButton"
            android:text="pause"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"/>
        <TextView
            android:gravity="end"
            android:paddingEnd="16dp"
            android:paddingRight="16dp"
            android:id="@+id/songDuration"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"/>
    </LinearLayout>
    <SeekBar
        android:visibility="gone"
        android:id="@+id/seekBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>
</LinearLayout>