我在房间数据库类中创建的抽象 Dao 方法上收到空指针异常。虽然我提到的几个示例项目做同样的事情，但它有效。我不明白这个实例是何时何地在其他项目中初始化的。这是我的主要活动代码如下：
主要活动：
package com.example.datta.roomtest;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.webkit.WebView;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import com.example.datta.roomtest.Dao.PlayerDao;
import com.example.datta.roomtest.Database.AppDatabase;
import com.example.datta.roomtest.Entity.Player;
import java.util.List;
public class MainActivity extends AppCompatActivity {
AppDatabase appDatabase;
TextView tvName,tvRole;
EditText etName,etRole;
Button btnGet,btnPut;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
appDatabase= AppDatabase.getInstance(this);
init();
setListeners();
}
private void init() {
tvName=(TextView)findViewById(R.id.tvPlayername);
tvRole=(TextView)findViewById(R.id.tvPlayerRole);
etName=(EditText)findViewById(R.id.etName);
etRole=(EditText)findViewById(R.id.etRole);
btnGet=(Button)findViewById(R.id.btnGetData);
btnPut=(Button)findViewById(R.id.btnPutData);
}
private void setListeners() {
btnGet.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new DbAsync().execute("get");
}
});
btnPut.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new DbAsync().execute("put");
}
});
}
class DbAsync extends AsyncTask<String,Void,Void>{
@Override
protected Void doInBackground(String... strings) {
Log.i("MainTag",strings[0]);
if(strings[0].equals("get")){
PlayerDao playerDao = appDatabase.getPlayerDao();
List<Player>playerList=playerDao.getAllPlayers();
for (Player player:playerList){
tvName.setText(player.getName());
tvRole.setText(player.getRole());
}
}
if(strings[0].equals("put")){
PlayerDao playerDao=appDatabase.getPlayerDao();
playerDao.insertAll(new Player("Ronaldo","Striker"));
playerDao.insertAll(new Player("Messi","Winger"));
}
return null;
}
@Override
protected void onPostExecute(Void aVoid) {
super.onPostExecute(aVoid);
}
}
}
这是我的应用数据库：
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)
由于错误的测试，getInstance()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;
}