关于OTA升级从高版本到低版本升级后，数据库无法降级引起的异常分析

1.问题描述：
OTA升级从高版本升级到低版本成功，开机后桌面闹钟图标不见了，且桌面闹钟报错，无法打开应用。

参考log如下：

01-14 20:29:22.290: E/AndroidRuntime(2722): FATAL EXCEPTION: AsyncHandler

01-14 20:29:22.290: E/AndroidRuntime(2722): Process: com.android.deskclock, PID: 2722

01-14 20:29:22.290: E/AndroidRuntime(2722): android.database.sqlite.SQLiteException: Can't downgrade database from version 10 to 7

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.database.sqlite.SQLiteOpenHelper.onDowngrade(SQLiteOpenHelper.java:361)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:255)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)

01-14 20:29:22.290: E/AndroidRuntime(2722): at com.android.deskclock.provider.ClockProvider.query(ClockProvider.java:96)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.content.ContentProvider.query(ContentProvider.java:922)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.content.ContentProvider$Transport.query(ContentProvider.java:213)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.content.ContentResolver.query(ContentResolver.java:462)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.content.ContentResolver.query(ContentResolver.java:405)

01-14 20:29:22.290: E/AndroidRuntime(2722): at com.android.deskclock.provider.AlarmInstance.getInstances(AlarmInstance.java:188)

01-14 20:29:22.290: E/AndroidRuntime(2722): at com.android.deskclock.alarms.AlarmStateManager.FixAlarmInstancesForReboot(AlarmStateManager.java:684)

01-14 20:29:22.290: E/AndroidRuntime(2722): at com.android.deskclock.AlarmInitReceiver$1.run(AlarmInitReceiver.java:102)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.os.Handler.handleCallback(Handler.java:808)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.os.Handler.dispatchMessage(Handler.java:103)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.os.Looper.loop(Looper.java:193)

01-14 20:29:22.290: E/AndroidRuntime(2722): at android.os.HandlerThread.run(HandlerThread.java:61)

2.分析结果：
    从上面的log中可以看到，此类异常由数据库无法将降级导致，此时恢复出厂设置可以恢复正常。

3.解决方案

      可通过在DatabaseHelper.java重写父类SQLiteOpenHelper.java的onDowngrade（）方法来规避该问题，或者直接在SQLiteOpenHelper.java中对该方法就行修改。

 public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {

     throw new SQLiteException("Can't downgrade database from version " +

             oldVersion + " to " + newVersion);

 }

注：允许数据库降级，前提是对数据库版本号毫不关心。因此建议保持平台默认设计，不应该去规避该问题，因为：
You should care about this "Database Version thing" if ever your database schema is ever going to change