Room Dao空指针异常

Question

我在房间数据库类中创建的抽象 Dao 方法出现空指针异常。虽然我提到的几个示例项目做同样的事情，但它的工作原理。我不明白这个实例在其他项目中何时何地被初始化。下面是我的主要活动代码：这是我的应用程序数据库：package com.example.datta.roomtest.Database;import android.arch.persistence.room.Database;import android.arch.persistence.room.Room;import android.arch.persistence.room.RoomDatabase;import android.content.Context;import com.example.datta.roomtest.Dao.PlayerDao;import com.example.datta.roomtest.Entity.Player;@Database(entities = {Player.class},version = 1)public abstract class AppDatabase extends RoomDatabase {private static AppDatabase appDatabase;public static AppDatabase getInstance(Context context){    if(appDatabase!=null){        appDatabase = Room.databaseBuilder(context.getApplicationContext(),                AppDatabase.class, "database-name").build();        }    return appDatabase;}  public abstract PlayerDao getPlayerDao();}调用此 getPlayerDao() 方法时，我收到此空指针错误： Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.datta.roomtest.Dao.PlayerDao com.example.datta.roomtest.Database.AppDatabase.getPlayerDao()' on a null object reference        at com.example.datta.roomtest.MainActivity$DbAsync.doInBackground(MainActivity.java:97)        at com.example.datta.roomtest.MainActivity$DbAsync.doInBackground(MainActivity.java:71)        at android.os.AsyncTask$2.call(AsyncTask.java:316)

Answer 1

getInstance() 由于错误的测试总是返回 null：

public static AppDatabase getInstance(Context context){

    if(appDatabase!=null){ // <= ERROR HERE, should be == null

        appDatabase = Room.databaseBuilder(context.getApplicationContext(),

                AppDatabase.class, "database-name").build();

    }

    return appDatabase;

}