在我的应用中，我有两个片段：FragmentA和FragmentB，这些片段是在SlidingMenu的帮助下从我的MainActivity加载的。
感谢Crashlytics，我会在应用崩溃的时间和地点收到通知。在这种情况下，DatabaseHandler中的FragmentA有时会为空（对于某些用户而言），即使它已被初始化。
这是我的代码：
MainActivity
public class MainActivity extends AppCompatActivity implements MyFragment.OnListFragmentInteractionListener, AsyncResponse {
private FragmentA fragmentA = new FragmentA();
private DatabaseHandler databaseHandler = new DatabaseHandler(this);
private NavigationView navigationView;
private DrawerLayout drawer;
private Toolbar toolbar;
// index to identify current nav menu item
private static int navItemIndex = 0;
public static String CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
// toolbar titles respected to selected nav menu item
private String[] activityTitles;
// flag to load home fragment when user presses back key
private Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fragmentA.setDatabaseHandler(this.databaseHandler);
// Init UI
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mHandler = new Handler();
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
navigationView = (NavigationView) findViewById(R.id.nav_view);
fabSendButton = (FloatingActionButton) findViewById(R.id.fab);
// Navigation view header
navHeader = navigationView.getHeaderView(0);
// load toolbar titles from string resources
activityTitles = getResources().getStringArray(R.array.sliding_menu_item_activity_titles);
// initializing navigation menu
setUpNavigationView();
if (savedInstanceState == null) {
navItemIndex = 0;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
loadHomeFragment();
}
}
/***
* Returns respected fragment that user
* selected from navigation menu
*/
private void loadHomeFragment() {
// set toolbar title
setToolbarTitle();
// if user select the current navigation menu again, don't do anything
// just close the navigation drawer
if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
drawer.closeDrawers();
return;
}
// Sometimes, when fragment has huge data, screen seems hanging
// when switching between navigation menus
// So using runnable, the fragment is loaded with cross fade effect
// This effect can be seen in GMail app
Runnable mPendingRunnable = new Runnable() {
@Override
public void run() {
// update the activity_main_header_with_item content by replacing fragments
Fragment fragment = getFragment();
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.setCustomAnimations(android.R.anim.fade_in, android.R.anim.fade_out);
fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
fragmentTransaction.commit();
}
};
// If mPendingRunnable is not null, then add to the message queue
if (mPendingRunnable != null) {
mHandler.post(mPendingRunnable);
}
//Closing drawer on item click
drawer.closeDrawers();
// refresh toolbar menu
invalidateOptionsMenu();
}
private Fragment getFragment() {
switch (navItemIndex) {
case 0:
return this.fragmentA;
case 1:
Fragment B fragmentB = new FragmentB();
fragmentB.setDatabaseHandler(this.databaseHandler);
return fragmentB;
default:
return this.fragmentA;
}
}
private void setToolbarTitle() {
getSupportActionBar().setTitle(activityTitles[navItemIndex]);
}
private void setUpNavigationView() {
//Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
// This method will trigger on item Click of navigation menu
@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
//Check to see which item was being clicked and perform appropriate action
switch (menuItem.getItemId()) {
//Replacing the activity_main_header_with_item content with ContentFragment Which is our Inbox View;
case R.id.nav_A:
navItemIndex = 0;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_A;
break;
case R.id.nav_B:
navItemIndex = 1;
CURRENT_TAG = MyConstants.TAG_FRAGMENT_B;
break;
default:
navItemIndex = 0;
}
loadHomeFragment();
return true;
}
});
ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {
@Override
public void onDrawerClosed(View drawerView) {
// Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
super.onDrawerClosed(drawerView);
}
@Override
public void onDrawerOpened(View drawerView) {
// Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
super.onDrawerOpened(drawerView);
}
};
//Setting the actionbarToggle to drawer layout
drawer.setDrawerListener(actionBarDrawerToggle);
//calling sync state is necessary or else your hamburger icon wont show up
actionBarDrawerToggle.syncState();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main_header_with_item, menu);
return true;
}
}
FragmentA ：
public class FragmentA extends MyFragment implements AsyncResponse {
private View view;
private RecyclerView recyclerView;
public FragmentA() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
List<Song> songsList = databaseHandler.getAllSongs();
view = inflater.inflate(R.layout.home_list, container, false);
// Set the adapter
Context context = view.getContext();
recyclerView = (RecyclerView) view.findViewById(R.id.listinclude);
recyclerView.setLayoutManager(new LinearLayoutManager(context));
recyclerView.addItemDecoration(new DividerItemDecoration(getActivity(), R.drawable.divider));
setVisibilities(songsList);
this.recyclerViewAdapter = new RecyclerViewAdapter(songsList, mListener, false, true);
recyclerView.setAdapter(this.recyclerViewAdapter);
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
}
private void setVisibilities(List<Song> songsList) {
ViewFlipper viewFlipper = (ViewFlipper) view.findViewById(R.id.viewFlipper);
if (songsList.isEmpty() && viewFlipper.getDisplayedChild() == 0) {
viewFlipper.setDisplayedChild(1);
} else if (!songsList.isEmpty() && viewFlipper.getDisplayedChild() == 1) {
viewFlipper.setDisplayedChild(0);
}
}
@Override
public void processFinish(String output) {
// does something
}
}
MyFragment
public class MyFragment extends Fragment {
protected DatabaseHandler databaseHandler;
protected static final String ARG_COLUMN_COUNT = "column-count";
protected int mColumnCount = 1;
protected MyFragment.OnListFragmentInteractionListener mListener;
protected RecyclerViewAdapter recyclerViewAdapter;
public void setDatabaseHandler(DatabaseHandler databaseHandler) {
this.databaseHandler = databaseHandler;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Timber.i( "onCreate");
if (getArguments() != null) {
mColumnCount = getArguments().getInt(ARG_COLUMN_COUNT);
}
}
@Override
public void onPause() {
Timber.i( "onPause");
super.onPause();
}
@Override
public void onAttach(Context context) {
Timber.i( "onAttach");
super.onAttach(context);
if (context instanceof OnListFragmentInteractionListener) {
mListener = (FragmentA.OnListFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString() + " must implement OnListFragmentInteractionListener");
}
}
@Override
public void onDetach() {
Timber.i( "onDetach");
super.onDetach();
mListener = null;
}
public interface OnListFragmentInteractionListener {
// TODO: Update argument type and name
void onListFragmentInteraction(Song item);
}
}
数据库处理器：
public class DatabaseHandler extends SQLiteOpenHelper {
// All Static variables
// Database Version
private static final int DATABASE_VERSION = 1;
// Database Name
private static final String DATABASE_NAME = "appName";
// Songs table name
private static final String TABLE_SONGS = "songs";
@Override
public void onCreate(SQLiteDatabase db) {
// create Tables
}
// Upgrading database
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
switch(oldVersion) {
case 1:
//upgrade logic from version 1 to 2
case 2:
//upgrade logic from version 2 to 3
case 3:
//upgrade logic from version 3 to 4
break;
default:
throw new IllegalStateException(
"onUpgrade() with unknown oldVersion " + oldVersion);
}
}
// Getting All Songs
public List<Song> getAllSongs() {
// Select All Query
String selectQuery = "SELECT * FROM " + TABLE_SONGS + " ORDER BY ID DESC";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
List<Song> songList = new ArrayList<>();
songList.addAll(getSongsFromCursor(cursor));
cursor.close();
// return title list
return songList;
}
}
问题：
Fatal Exception: java.lang.RuntimeException
Unable to start activity ComponentInfo{com.myapp.myappname/com.myapp.myappname.ui.activity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
详情：
Fatal Exception: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.myapp.myappname/com.myapp.myappname.ui.activity.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2984)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.List com.myapp.myappname.database.DatabaseHandler.getAllSongs()' on a null object reference
at com.myapp.myappname.ui.fragment.FragmentA.onCreateView(FragmentA.java:51)
at android.support.v4.app.Fragment.performCreateView(Fragment.java:2192)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1299)
at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1528)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1595)
at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:2900)
at android.support.v4.app.FragmentController.dispatchActivityCreated(FragmentController.java:201)
at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:603)
at android.support.v7.app.AppCompatActivity.onStart(AppCompatActivity.java:178)
at com.myapp.myappname.ui.activity.MainActivity.onStart(MainActivity.java:590)
at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1256)
at android.app.Activity.performStart(Activity.java:6972)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2937)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
答案 0 :(得分：0)
尝试这个
databaseHandler=new DataBaseHandler(getActivity());
List<Song> songsList = databaseHandler.getAllSongs();
在DataBaseHandler类中添加构造函数
public DataBaseHandler(Context context){
super(context, DB_NAME, null, 1);
}
答案 1 :(得分：0)
在FragmentA上试试这个。
databaseHandler=new DataBaseHandler(getContext());
您正在使用mainActivity的上下文来初始化databaseHandler但是在片段中使用。
答案 2 :(得分：0)
Found a solution based on Serg's answer:
FragmentA:
public class FragmentA extends MyFragment implements AsyncResponse {
private View view;
private RecyclerView recyclerView;
public FragmentA() {
}
public static FragmentA newInstance(DatabaseHandler DatabaseHandler) {
FragmentA fragment = new FragmentA();
Bundle bundle = new Bundle();
bundle.putSerializable(MyConstants.FRAGMENT_ARGUMENT_KEY, databaseHandler);
fragment.setArguments(bundle);
return fragment;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
databaseHandler = (DatabaseHandler) getArguments().getSerializable(MyConstants.FRAGMENT_ARGUMENT_KEY);
List<Song> songsList = databaseHandler.getAllSongs();
view = inflater.inflate(R.layout.home_list, container, false);
// Set the adapter
Context context = view.getContext();
recyclerView = (RecyclerView) view.findViewById(R.id.listinclude);
recyclerView.setLayoutManager(new LinearLayoutManager(context));
recyclerView.addItemDecoration(new DividerItemDecoration(getActivity(), R.drawable.divider));
setVisibilities(songsList);
this.recyclerViewAdapter = new RecyclerViewAdapter(songsList, mListener, false, true);
recyclerView.setAdapter(this.recyclerViewAdapter);
return view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
}
private void setVisibilities(List<Song> songsList) {
ViewFlipper viewFlipper = (ViewFlipper) view.findViewById(R.id.viewFlipper);
if (songsList.isEmpty() && viewFlipper.getDisplayedChild() == 0) {
viewFlipper.setDisplayedChild(1);
} else if (!songsList.isEmpty() && viewFlipper.getDisplayedChild() == 1) {
viewFlipper.setDisplayedChild(0);
}
}
@Override
public void processFinish(String output) {
// does something
}
}
MainActivity:
fragmentA = FragmentA.newInstance(this.databaseHandler);