Question

如果我尝试在我的活动之外实例化并使用我的应用程序DBAdapter，则会抛出异常。我试图在我制作的一个简单的实用程序/辅助类中使用它：

public class MyClass
{
  Context context;
  DBAdapter db;

  public MyClass(Context c)
  {
    // Get the application context that is passed to the class
    context = c;

    // Instantiate the database using the application context
    db = new DBAdapter(context);

    // Open the database and do something
    db.open();  // <---- Exception thrown here
    db.dosomething();
    db.close();
  }
}

我像这样打电话给MyClass

public class MainActivity extends Activity
{
  @Override
  public void onCreate( Bundle savedInstanceState )
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    MyClass whatever = new MyClass(this);
  }
}

这是我的DBAdapter，非常简单和典型的任何Android SQLite教程：

public class DBAdapter
{
  private final Context context;
  private DatabaseHelper DBHelper;
  private SQLiteDatabase db;

  public DBAdapter(Context c)
  {
    context = c;
    DBHelper = new DatabaseHelper(context);
  }

  private static class DatabaseHelper extends SQLiteOpenHelper
  {
    public DatabaseHelper( Context context)
    {
      super(context, MY_DATABASE_NAME_HERE, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db)
    {
      try
      {
        db.execSQL(/*CREATE TABLE CODE HERE*/);
      }
      catch (Exception e)
      {
        e.printStackTrace();
      }
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
    {
      /* UPDATE CODE HERE */
    }
  }

  public DBAdapter open() throws SQLException
  {
    db = DBHelper.getWritableDatabase();
    return this;
  }

  public void close()
  {
    DBHelper.close();
  }

  public void dosomething()
  {
    // code here
  }

  // Other database abstraction layer methods down here...
}

最后，例外。在我看来，db对象是null或无效或我创建它时的东西。但似乎我做得对。我正在传递正确的应用程序/活动上下文。

FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.whatever.mysqltest/com.whatever.mysqltest.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:3683)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at com.whatever.whatever.DBAdapter.open(DBAdapter.java:71)
    at com.whatever.whatever.whatever.<init>(whatever.java:56)
    at com.whatever.mysqltest.MainActivity.onCreate(MainActivity.java:22)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
    ... 11 more

或者，如果我从Activity中调用db对象，一切都很好......

public class MainActivity extends Activity
{
  DBAdapter db;

  @Override
  public void onCreate( Bundle savedInstanceState )
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    db = new DBAdapter(this);
    db.open();  // <--- does NOT thrown an exception.
    db.dosomething();
    db.close();
  }
}

这里有什么问题？

Answer 1

你的“MyClass”期待一个上下文，但你传递的是一个活动。尝试将上下文传递给构造函数，如下所示：

MyClass whatever = new MyClass(getContext());

Android - DBAdapter外部Activity抛出异常

1 个答案: