Java Android报错：无法创建NoteViewModel类实例

开发Java语言实现的Android笔记应用时，持续触发运行时崩溃，核心错误提示为Cannot create an instance of class com.aashushaikh.notesjava.viewModel.NoteViewModel。已尝试StackOverflow同类问题的多种解决方案均未生效：

• 调用ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication())方法初始化ViewModel
• 自定义ViewModelFactory完成ViewModel实例化
• 相同业务逻辑用Kotlin实现可正常运行，仅Java版本触发崩溃

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.aashushaikh.notesjava, PID: 22110
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.aashushaikh.notesjava/com.aashushaikh.notesjava.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.aashushaikh.notesjava.viewModel.NoteViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.aashushaikh.notesjava.viewModel.NoteViewModel
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:269)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
        at com.aashushaikh.notesjava.MainActivity.onCreate(MainActivity.java:42)
        at android.app.Activity.performCreate(Activity.java:6679)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6119) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
     Caused by: java.lang.NoSuchMethodException: <init> [class android.app.Application]
        at java.lang.Class.getConstructor0(Class.java:2204)
        at java.lang.Class.getConstructor(Class.java:1683)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
        at com.aashushaikh.notesjava.MainActivity.onCreate(MainActivity.java:42) 
        at android.app.Activity.performCreate(Activity.java:6679) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6119) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

从堆栈最底层的报错java.lang.NoSuchMethodException: <init> [class android.app.Application]可以直接定位核心问题：
默认的AndroidViewModelFactory通过反射创建ViewModel实例时，只会查找仅接收单个Application类型参数的构造方法，但当前NoteViewModel的构造方法额外定义了一个无用的String类型参数mParam，工厂找不到匹配的构造方法，直接抛出实例化失败异常。
另外代码中还存在一个隐藏空指针风险：NoteRepository类中allNotes的初始化写在构造方法外，执行时noteDao还未完成赋值，会触发空指针。

1. 修正NoteViewModel构造方法，删除多余的无用String参数，保留符合AndroidViewModel实例化规则的单Application参数构造方法，修正后代码如下：

public class NoteViewModel extends AndroidViewModel {

    private NoteRepository noteRepository;
    public LiveData<List<Note>> allNotes;

    public NoteViewModel(@NonNull Application application) {
        super(application);

        NoteDao dao = NoteDatabase.getInstance(application).getNoteDao();
        noteRepository = new NoteRepository(dao);
        allNotes = noteRepository.getAllNotes();
    }

    public void deleteNote(Note note){
        noteRepository.delete(note);
    }
    public void insertNote(Note note){
        noteRepository.insert(note);
    }
    public LiveData<List<Note>> getAllNotes(){
        return allNotes;
    }
}

2. 修复NoteRepository的空指针隐患，将allNotes的初始化逻辑移到构造方法内部，保证noteDao完成赋值后再调用其方法，修正后代码如下：

public class NoteRepository {
    NoteDao noteDao;
    public LiveData<List<Note>> allNotes;
    public NoteRepository(NoteDao noteDao) {
        this.noteDao = noteDao;
        allNotes = noteDao.getAllNotes();
    }

    public void insert(Note note){
        new InsertNoteAsyncTask(noteDao).execute(note);
    }

    public void delete(Note note) {
        new DeleteNoteAsyncTask(noteDao).execute(note);
    }

    public LiveData<List<Note>> getAllNotes(){
        return allNotes;
    }

    private static class InsertNoteAsyncTask extends AsyncTask<Note, Void, Void> {

        private NoteDao noteDao;

        private InsertNoteAsyncTask(NoteDao noteDao) {
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Note...notes) {
            noteDao.insert(notes[0]);
            return null;
        }
    }

    private static class DeleteNoteAsyncTask extends AsyncTask<Note, Void, Void> {

        private NoteDao noteDao;

        private DeleteNoteAsyncTask(NoteDao noteDao) {
            this.noteDao = noteDao;
        }

        @Override
        protected Void doInBackground(Note...notes) {
            noteDao.delete(notes[0]);
            return null;
        }
    }

}

3. MainActivity中现有ViewModel初始化逻辑无需修改，保持原有写法即可正常运行：

viewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(getApplication())).get(NoteViewModel.class);

补充：只有需要给ViewModel传入自定义额外参数时，才需要实现自定义ViewModelFactory，当前场景无额外参数需求，使用默认工厂即可。

内容的提问来源于stack exchange，提问作者Aashu Shaikh