当我们第一调用android.database.sqlite.SQLiteCursor的getCount()时，当前线程会锁定数据库，在该操作完成后才解锁。
其调用关系如下：
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)
如果是第一次调用SQLiteCursor的getCount()的话，在getCount（）中，它会调用fillWindow(),
在SQLiteCursor的fillWindow()中，它又会调用SQLiteQuery的fillWindow()
@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，它首先需要lock数据库,然后调用JNI层的native_fill_window()进行数据库操作，在其操作完成之后才unlock数据库。
/**
* 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();
}
}
结束！
