执行query
执行SQLiteDatabase类中query系列函数时，只会构造查询信息，不会执行查询。
（query的源码追踪路径）
执行move（里面的fillwindow是真正打开文件句柄并分配内存的地方）
当执行Cursor的move系列函数时，第一次执行，会为查询结果集创建一块共享内存，即cursorwindow
moveToPosition源码路径
fillWindow----真正耗时的地方
然后会执行sql语句，向共享内存中填入数据，
fillWindow源码路径
在SQLiteCursor.java中可以看到
1 @Override
2 public boolean onMove(int oldPosition, int newPosition) { 3 // Make sure the row at newPosition is present in the window 4 if (mWindow == null || newPosition < mWindow.getStartPosition() || 5 newPosition >= (mWindow.getStartPosition() + mWindow.getNumRows())) { 6 fillWindow(newPosition); 7 } 8 9 return true; 10 }
如果请求查询的位置在cursorWindow的范围内，不会执行fillWindow，
而超出cursorwindow的范围，会调用fillWindow，
而在nativeExecuteForCursorWindow中，
获取记录时，如果要请求的位置超出窗口范围，会发生CursorWindow的清空：
1 CopyRowResult cpr = copyRow(env, window, statement, numColumns, startPos, addedRows);
2 if (cpr == CPR_FULL && addedRows && startPos + addedRows < requiredPos) {
3 // We filled the window before we got to the one row that we really wanted. 4 // Clear the window and start filling it again from here. 5 // TODO: Would be nicer if we could progressively replace earlier rows. 6 window->clear(); 7 window->setNumColumns(numColumns); 8 startPos += addedRows; 9 addedRows = 0; 10 cpr = copyRow(env, window, statement, numColumns, startPos, addedRows); 11 }
CursorWindow的清空机制会影响到多线程读（通常认为不可以并发读写，sqlite的并发实际上是串行执行的，但可以并发读，这里要强调的是多线程读也可能有问题），具体见稍后一篇文章“listview并发读写数据库”。
Cursor关闭（显式调用close()的理由）
追踪源码看关闭
1 //SQLiteCursor
2
3 super.close(); 4 synchronized (this) { 5 mQuery.close(); 6 mDriver.cursorClosed(); 7 } 8 9 10 //AbstractCursor 11 12 public void close() { 13 mClosed = true; 14 mContentObservable.unregisterAll(); 15 onDeactivateOrClose(); 16 } 17 18 protected void onDeactivateOrClose() { 19 if (mSelfObserver != null) { 20 mContentResolver.unregisterContentObserver(mSelfObserver); 21 mSelfObserverRegistered = false; 22 } 23 mDataSetObservable.notifyInvalidated(); 24 } 25 26 27 //AbstractWindowedCursor 28 29 /** @hide */ 30 @Override 31 protected void onDeactivateOrClose() { 32 super.onDeactivateOrClose(); 33 closeWindow(); 34 } 35 36 protected void closeWindow() { 37 if (mWindow != null) { 38 mWindow.close(); 39 mWindow = null; 40 } 41 } 42 43 44 45 //SQLiteClosable 46 47 public void close() { 48 releaseReference(); 49 } 50 51 public void releaseReference() { 52 boolean refCountIsZero = false; 53 synchronized(this) { 54 refCountIsZero = --mReferenceCount == 0; 55 } 56 if (refCountIsZero) { 57 onAllReferencesReleased(); 58 } 59 } 60 61 //CursorWindow 62 63 @Override 64 protected void onAllReferencesReleased() { 65 dispose(); 66 } 67 68 private void dispose() { 69 if (mCloseGuard != null) { 70 mCloseGuard.close(); 71 } 72 if (mWindowPtr != 0) { 73 recordClosingOfWindow(mWindowPtr); 74 nativeDispose(mWindowPtr); 75 mWindowPtr = 0; 76 } 77 }
跟CursorWindow有关的路径里，最终调用nativeDispose()清空cursorWindow;
当Cursor被GC回收时，会调用finalize：
1 @Override
2 protected void finalize() { 3 try { 4 // if the cursor hasn't been closed yet, close it first 5 if (mWindow != null) { 6 if (mStackTrace != null) { 7 String sql = mQuery.getSql(); 8 int len = sql.length(); 9 StrictMode.onSqliteObjectLeaked( 10 "Finalizing a Cursor that has not been deactivated or closed. " + 11 "database = " + mQuery.getDatabase().getLabel() + 12 ", table = " + mEditTable + 13 ", query = " + sql.substring(0, (len > 1000) ? 1000 : len), 14 mStackTrace); 15 } 16 close(); 17 } 18 } finally { 19 super.finalize(); 20 } 21 }
然而finalize()并没有释放CursorWindow，而super.finalize();里也只是解绑了观察者，没有去释放cursorwindow
所以不调用cursor.close(),最终会导致cursorWindow所在的共享内存(1M或2M)泄露。
http://www.cnblogs.com/hellocwh/p/4924732.html
