android - 非法状态异常 : database already closed (using ViewPager)

Question

我对导致此错误的原因感到困惑，因为我已确定我正在正确关闭我的数据库适配器(至少我认为我是)。以下是 LogCat 的内容(它们的标签都是 AndroidRuntime):

FATAL EXCEPTION: main java.lang.IllegalStateException: database /data/data/com.acedit.assignamo/databases/data.db (conn# 0) already closed at android.database.sqlite.SQLiteDatabase.verifyDbIsOpen(SQLiteDatabase.java:2082) at android.database.sqlite.SQLiteDatabase.lock(SQLiteDatabase.java:413) at android.database.sqlite.SQLiteDatabase.lock(SQLiteDatabase.java:400) at android.database.sqlite.SQLiteQuery.fillWindow(SQLiteQuery.java:79) at android.database.sqlite.SQLiteCursor.fillWindow(SQLiteCursor.java:164) at android.database.sqlite.SQLiteCursor.onMove(SQLiteCursor.java:147) at android.database.AbstractCursor.moveToPosition(AbstractCursor.java:178) at android.support.v4.widget.CursorAdapter.getItemId(CursorAdapter.java:225) at android.widget.AbsListView.onSaveInstanceState(AbsListView.java:1569) at android.view.View.dispatchSaveInstanceState(View.java:9868) at android.view.ViewGroup.dispatchFreezeSelfOnly(ViewGroup.java:2310) at android.widget.AdapterView.dispatchSaveInstanceState(AdapterView.java:770) at android.view.ViewGroup.dispatchSaveInstanceState(ViewGroup.java:2296) at android.view.View.saveHierarchyState(View.java:9851) at android.support.v4.app.FragmentManagerImpl.saveFragmentViewState(FragmentManager.java:1561) at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:962) at android.support.v4.app.FragmentManagerImpl.detachFragment(FragmentManager.java:1233) at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:620) at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1431) at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:431) at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141) at android.support.v4.view.ViewPager.populate(ViewPager.java:895) at android.support.v4.view.ViewPager.populate(ViewPager.java:772) at android.support.v4.view.ViewPager.completeScroll(ViewPager.java:1539) at android.support.v4.view.ViewPager.computeScroll(ViewPager.java:1422) at android.view.View.getDisplayList(View.java:10406) at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2597) at android.view.View.getDisplayList(View.java:10380) at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2597) at android.view.View.getDisplayList(View.java:10380) at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2597) at android.view.View.getDisplayList(View.java:10380) at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:2597) at android.view.View.getDisplayList(View.java:10380) at android.view.HardwareRenderer$GlRenderer.draw(HardwareRenderer.java:875) at android.view.ViewRootImpl.draw(ViewRootImpl.java:1910) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1634) at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4575) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) at dalvik.system.NativeStart.main(Native Method)

我的代码:

public Cursor fetchIncompleteAssignments(Short course) {
    DbAdapter adapter = new DbAdapter(context, null, Values.ASSIGNMENT_TABLE);
    adapter.open();

    Cursor r;
    if (course == null) // Fetching from all courses
        r = adapter.fetchAllWhere(Values.ASSIGNMENT_LIST_FETCH, Values.ASSIGNMENT_KEY_STATUS + "=" + 0, Values.ASSIGNMENT_KEY_DUE_DATE);
    else
        r = adapter.fetchAllWhere(Values.ASSIGNMENT_LIST_FETCH, Values.ASSIGNMENT_KEY_COURSE + "=" + course
            + " AND " + Values.ASSIGNMENT_KEY_STATUS + "=" + 0, Values.ASSIGNMENT_KEY_DUE_DATE);
    adapter.close();
    return r;
}

// Part of DbAdapter:
public DbAdapter open() throws SQLException {
    dbHelper = new DatabaseHelper(context);
    db = dbHelper.getWritableDatabase();
    return this;
}

public void close() {
    if (db != null) {
        try {
            db.close();
            dbHelper.close();
        } catch (NullPointerException e) {
            Log.e("Close", "Error: " + e + " " + e.getMessage());
        }
    } else
        Log.e("Close", "Error! db \"" + DATABASE_TABLE + "\" is null.");
}

从 fetchAllAssignments() 返回的 Cursor 用于通过自定义 CursorAdapter 填充 ListView。 ListView 是一个 ListFragment 的内容，这个 ListFragment 的多个实例显示在 ViewPager 的每个页面中(一个用于显示所有类(class)的作业，然后一个每个单独类(class)的页面)。

有什么问题吗？据我所知，我正确地关闭了我的数据库。如果您需要更多代码，请告诉我。提前致谢!

编辑: 数据是从数据库中提取的，它也填充了 ListView。我可以轻扫各个页面，但只要 Fragment 停止，它似乎就会崩溃。当我启动另一个 Activity 时，它经常会崩溃，但并非总是如此。当我滑动页面时，它偶尔也会崩溃，但这种情况很少见。这是我的 fragment 的 onPause() 方法(我没有 onStop() 或 onDestroy()):

public void onPause() {
    super.onPause();
    if (!assignmentsCursor.isClosed()) {
        assignmentsCursor.close();
        assignmentsCursor = null;
    }
}

在我的 onResume() 中，我只是从数据库重新加载列表并通过 getActivity() 获取上下文的新副本。也许某些东西正在被清除(出于内存目的)而我没有重新加载它？我没有收到任何类型的 NullPointerException，所以这对我来说似乎不太可能......

Answer 1

这个异常实际上是因为你两次关闭你的数据库而引发的，而不是因为你没有关闭它。

所以..

在您的close() 方法中，替换代码中的以下行:

if (db != null) {

与:

if (db != null && db.isOpen()) {