具有内容提供程序竞争条件的 SQLite 数据库

Question

我实现了使用 SQLiteDatabase 作为其后备数据源的 Content Provider。

一个 activity 通过调用 getContentResolver().applyBatch(operations) 写入数据库，这应该是原子的。

protected void onPause() {
    new Thread(){
        @Override
        public void run() {
            ArrayList<ContentProviderOperation> ops = new ArrayList<>();
            ContentProviderOperation.Builder builder;
            for (Tag tag: mTopicAdapter.getTags()) {
                builder = ContentProviderOperation.newUpdate(QuizProvider.TAG_URI);
                builder.withValue(Tag.Table.SELECTED, tag.getSelectionStatus());
                builder.withSelection(Tag.Table._ID + " = " + tag.getId(), null);
                ops.add(builder.build());
            }
            try {
                ContentProviderResult[] res = getContentResolver().applyBatch(QuizProvider.AUTHORITY, ops);
                Timber.d("Update result: %d", res.length);
                getContentResolver().notifyChange(QuizProvider.TAG_URI, null);
                getContentResolver().notifyChange(QuizProvider.QUESTION_URI, null);
            } catch (RemoteException e) {
                e.printStackTrace();
            } catch (OperationApplicationException e) {
                e.printStackTrace();
            }
        }
    }.start();
    super.onPause();
}

第二次 activity 在 Cursor Loader 的帮助下从数据库读取，有时会得到旧数据（竞争条件）。

@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    Uri randQuestionUri = QuizProvider.QUESTION_URI
            .buildUpon()
            .appendPath("rand").appendPath(Integer.toString(QUIZ_SIZE))
            .build();
    return new CursorLoader(this, randQuestionUri, null, null, null, null);
}

@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    if (DEBUG) Timber.d("load finished: %d", data.hashCode());
    mPagerAdapter.swapCursor(data);
}

@Override
public void onLoaderReset(Loader<Cursor> loader) {
    mPagerAdapter.swapCursor(null);
}

完整项目here。

日志输出：

NSA:QuizProvider:138: update db: selected=false _id = 4
NSA:QuizProvider:138: update db: selected=false _id = 5
NSA:QuizProvider:138: update db: selected=false _id = 26
NSA:QuizProvider:138: update db: selected=false _id = 19
NSA:QuizProvider:138: update db: selected=false _id = 28
NSA:QuizProvider:138: update db: selected=false _id = 10
NSA:QuizProvider:138: update db: selected=false _id = 12
NSA:QuizProvider:138: update db: selected=false _id = 15
NSA:QuizProvider:138: update db: selected=false _id = 18
NSA:QuizProvider:138: update db: selected=false _id = 25
NSA:QuizProvider:138: update db: selected=false _id = 16
NSA:QuizProvider:138: update db: selected=false _id = 17
NSA:QuizProvider:138: update db: selected=false _id = 8
NSA:QuizProvider:138: update db: selected=false _id = 3
NSA:QuizProvider:138: update db: selected=false _id = 20
NSA:QuizProvider:138: update db: selected=false _id = 29
NSA:QuizProvider:138: update db: selected=false _id = 24
NSA:QuizProvider:138: update db: selected=false _id = 23
NSA:QuizProvider:138: update db: selected=false _id = 30
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuizProvider:103: query db: content://doit.study.droi    question/ran    280 null null
NSA:QuizProvider:138: update db: selected=false _id = 6
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuizProvider:138: update db: selected=false _id = 1
NSA:QuizProvider:138: update db: selected=false _id = 14
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuizProvider:138: update db: selected=false _id = 7
NSA:QuizProvider:138: update db: selected=false _id = 27
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuestionsActivity:84: load finished: 154982045
NSA:QuestionsPagerAdapter:66: swap cursor, cnt: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:35: instantiateItem, pos=0
NSA:QuestionsPagerAdapter:25: getItem, pos=0
NSA:QuestionsPagerAdapter:35: instantiateItem, pos=1
NSA:QuestionsPagerAdapter:25: getItem, pos=1
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuizProvider:138: update db: selected=false _id = 2
NSA:QuizProvider:138: update db: selected=false _id = 11
NSA:QuizProvider:138: update db: selected=false _id = 22
NSA:QuizProvider:138: update db: selected=false _id = 9
NSA:QuizProvider:138: update db: selected=false _id = 31
NSA:QuizProvider:138: update db: selected=false _id = 21
NSA:QuizProvider:138: update db: selected=false _id = 32
NSA:QuizProvider:138: update db: selected=false _id = 13
NSA:QuizProvider:138: update db: selected=false _id = 4
W/FragmentManager: moveToState: Fragment state for QuestionFragment{5c25e01 #0 id=0x7f0f00e5} not updated inline; expected state 3 found 2
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:52: title pos: 0, questions: tags: [User Interfaces]
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:52: title pos: 1, questions: tags: [User Interfaces]
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuizProvider:103: query db: content://doit.study.droi    tag null null
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuestionsPagerAdapter:42: counter: 104
NSA:QuizProvider:138: update db: selected=false _id = 5
NSA:QuizProvider:138: update db: selected=false _id = 26
NSA:QuizProvider:103: query db: content://doit.study.droi    tag null null
NSA:QuizProvider:138: update db: selected=false _id = 19
NSA:QuizProvider:138: update db: selected=false _id = 28
NSA:QuizProvider:138: update db: selected=false _id = 10
NSA:QuizProvider:138: update db: selected=false _id = 12
NSA:QuizProvider:138: update db: selected=false _id = 15
NSA:QuizProvider:138: update db: selected=false _id = 18
NSA:QuizProvider:138: update db: selected=false _id = 25
NSA:QuizProvider:138: update db: selected=false _id = 16
NSA:QuizProvider:138: update db: selected=false _id = 17
NSA:QuizProvider:138: update db: selected=false _id = 8
NSA:QuizProvider:138: update db: selected=false _id = 3
NSA:QuizProvider:138: update db: selected=false _id = 20
NSA:QuizProvider:138: update db: selected=false _id = 29
NSA:QuizProvider:138: update db: selected=false _id = 24
NSA:QuizProvider:138: update db: selected=false _id = 23
NSA:QuizProvider:138: update db: selected=false _id = 30
NSA:QuizProvider:138: update db: selected=false _id = 6
NSA:QuizProvider:138: update db: selected=false _id = 1
NSA:QuizProvider:138: update db: selected=false _id = 14
NSA:QuizProvider:138: update db: selected=false _id = 7
NSA:QuizProvider:138: update db: selected=false _id = 27
NSA:QuizProvider:138: update db: selected=false _id = 2
NSA:QuizProvider:138: update db: selected=false _id = 11
NSA:QuizProvider:138: update db: selected=false _id = 22
NSA:QuizProvider:138: update db: selected=false _id = 9
NSA:QuizProvider:138: update db: selected=false _id = 31
NSA:QuizProvider:138: update db: selected=false _id = 21
NSA:QuizProvider:138: update db: selected=false _id = 32
NSA:QuizProvider:138: update db: selected=false _id = 13
NSA:QuizProvider:103: query db: content://doit.study.droi    question/ran    280 null null
NSA:QuestionsActivity:84: load finished: 19434496
NSA:QuestionsPagerAdapter:66: swap cursor, cnt: 0
NSA:QuestionsPagerAdapter:42: counter: 0
NSA:QuestionsPagerAdapter:42: counter: 0

日志显示applyBatch未完成，Cursor Loader获取部分修改数据（cursor counter=104，应为0或280）。

一些资源（抱歉，不能添加超过两个链接）：

_http://developer.android.com/guide/topics/providers/content-provider-basics.html#Batch

_http://www.androiddesignpatterns.com/2012/10/sqlite-contentprovider-thread-safety.html

_

_http://www.grokkingandroid.com/better-performance-with-contentprovideroperation/

你知道哪里出了问题吗？

Answer 1

这里基本上要考虑三件事：

将那些 notifyChange() 调用移至您的 ContentProvider。这样您就可以确保在您进行更改时调用它们。或者换句话说：您可能会忘记在其他地方这样做。客户不应该对此负责 - 它不属于这里。
确保您的 applyBatch() 方法实际使用事务。只有这样您才能获得所需的性能优势，并且只有这样才能以您需要的方式使用锁。
确保当 applyBatch() 为运行时，不会发出任何通知。否则你的 Loader 会被调用得太频繁。你绝对想避免这种情况。

您可以查看我的 cpsample project 内容提供商的内容。

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