SQLite cursor.moveToNext()

cursor.moveToNext()會出異常，以下

E/AndroidRuntime( 2249): FATAL EXCEPTION: Thread-49
E/AndroidRuntime( 2249): java.lang.IllegalStateException: Cannot perform this operation because the connection pool has been closed.
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteConnectionPool.throwIfClosedLocked(SQLiteConnectionPool.java:962)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteConnectionPool.waitForConnection(SQLiteConnectionPool.java:599)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteConnectionPool.acquireConnection(SQLiteConnectionPool.java:348)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteSession.acquireConnection(SQLiteSession.java:894)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteSession.executeForCursorWindow(SQLiteSession.java:834)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteQuery.fillWindow(SQLiteQuery.java:62)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteCursor.fillWindow(SQLiteCursor.java:143)
E/AndroidRuntime( 2249): at android.database.sqlite.SQLiteCursor.getCount(SQLiteCursor.java:133)
E/AndroidRuntime( 2249): at android.database.AbstractCursor.moveToPosition(AbstractCursor.java:197)
E/AndroidRuntime( 2249): at android.database.AbstractCursor.moveToNext(AbstractCursor.java:245)

 

 

解決辦法，調用cursor.getCount().

緣由大概以下：

 

當咱們第一調用android.database.sqlite.SQLiteCursor的getCount()時，當前線程會鎖定數據庫，在該操做完成後才解鎖。

其調用關係以下：

at android.database.sqlite.SQLiteQuery.native_fill_window(Native Method) 

at android.database.sqlite.SQLiteQuery. fillWindow( SQLiteQuery.java:73) 

at android.database.sqlite.SQLiteCursor.fillWindow(SQLiteCursor.java:287) 

at android.database.sqlite.SQLiteCursor.getCount(SQLiteCursor.java:268) 

at android.widget.CursorAdapter. getCount (CursorAdapter.java:132) 

若是是第一次調用 SQLiteCursor 的 getCount () 的話，在getCount（）中，它會調用 fillWindow (),

在SQLiteCursor的 fillWindow() 中，它又會調用SQLiteQuery的 fillWindow()

android.database.sqlite.SQLiteCursor 的相關 源碼以下：

@Override

     public int getCount() {

         if (mCount == NO_COUNT) {

             fillWindow(0);

        }

         return mCount;

    }

 

     private void fillWindow ( int startPos) {

         if (mWindow == null) {

            // If there isn't a window set already it will only be accessed locally

            mWindow = new CursorWindow(true /* the window is local only */);

        }  else {

            mCursorState++;

                queryThreadLock();

                try {

                    mWindow.clear();

                } finally {

                    queryThreadUnlock();

                }

        }

        mWindow.setStartPosition(startPos);

        mCount =  mQuery.fillWindow(mWindow, mInitialRead, 0);

        // return -1 means not finished

         if (mCount == NO_COUNT){

            mCount = startPos + mInitialRead;

            Thread t = new Thread(new QueryThread(mCursorState), "query thread");

            t.start();

        } 

    }

在SQLiteQuery的 fillWindow() 中，它首先須要 lock數據庫 ,而後調用JNI層的 native_fill_window() 進行數據庫操做，在其操做完成以後才 unlock數據庫 。

android.database.sqlite.SQLiteQuery 的相關源碼以下：

/**

     * Reads rows into a buffer. This method acquires the database lock.

     *

     * @param window The window to fill into

     * @return number of total rows in the query

     */

    int fillWindow(CursorWindow window,

             int maxRead,  int lastPos) {

         long timeStart = SystemClock.uptimeMillis();

         mDatabase.lock();

        mDatabase.logTimeStat(mSql, timeStart, SQLiteDatabase.GET_LOCK_LOG_PREFIX);

         try {

            acquireReference();

             try {

                window.acquireReference();

                // if the start pos is not equal to 0, then most likely window is

                // too small for the data set, loading by another thread

                // is not safe in this situation. the native code will ignore maxRead

                 int numRows =  native_fill_window(window, window.getStartPosition(), mOffsetIndex,

                        maxRead, lastPos);

 

                // Logging

                 if (SQLiteDebug.DEBUG_SQL_STATEMENTS) {

                    Log.d(TAG, "fillWindow(): " + mSql);

                }

                mDatabase.logTimeStat(mSql, timeStart);

                return numRows;

            }  catch (IllegalStateException e){

                // simply ignore it

                return 0;

            }  catch (SQLiteDatabaseCorruptException e) {

                mDatabase.onCorruption();

                throw e;

            }  finally {

                window.releaseReference();

            }

        } finally {

            releaseReference();

             mDatabase.unlock();

        }

    }

 

結束！