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Multiple databases and content providers

In my app I have multiple sqlite databases which are all exported through a content provider for my other apps to be able to read the data.
Is it better to have a single content provider containing all my databases, or a content provider for each database? My other apps will only require access to one database at a time.
N.B. I have read similar posts, and my app MUST be able to have multiple databases.

As so often: It depends :-)
But if you must use multiple databases, chances are that those contain more or less unrelated data. In this case I would use a different ContentProvider for each database. That's a clear separation on concerns and makes the API easier to understand.
BTW: Keep security in mind. Use signature access level for the permissions if you only want your apps to use the data!

#WolframRittmeyer's answer is the way to go if the data in the databases are unrelated.
If you want to expose similar data from many sources you can try to embed multiple Cursors from your different databases into one MergeCursor.
Beware that the getColumns() and other related methods can potentially return different result according to the current MergeCursor position.

Should all data be exposed via ContentProviders?

So I come mostly from a web background and I'm trying to learn the architecture of android apps. Trying to grasp a proper understanding of content providers.
What I think I'm understanding is that content providers are pretty much your middle man between the activity/services and your data (DAO of sort). I also think that content provider are also to provide other applications access to your data (almost like a web service?)
What I'm not really getting is, what if you did not need this data exposed to other applications? Do I need to bother with content providers and all the uri defining etc? If not, is there a name for this ... pattern? (or lack of) Or am I better off just using ContentProviders and just accept the added benefit of exposing this data to other apps?
I know eventually I will want to sync data between the app and a external database. I saw a google IO presentation supporting the pattern of using a content provider for RESTful communication. But for the time being, I'm just trying to get comfortable with basic static data. Then hopefully swap it out to data from a REST service down the line once I get it.
Hope I'm not completely off here. Thanks.

Although it says that content providers are nice for sharing content they also give you a couple of other things free!
The best thing about content providers is that the system handles the threading issues for you :) when I ported my app over to Honeycomb i got lots of database errors (mind you ones that couldn't previously exist in gingerbread) I wasn't handling the access on different threads properly.
I quickly put a content provider on top of my database and well.. never looked back in that regards, it seems much easier once you've setup your content provider the code throughout the rest of the app is much nicer and you don't have to worry about concurrency

As written in the specs, and as you mentioned :
Content providers store and retrieve data and make it accessible to all applications. They're the only way to share data across applications; there's no common storage area that all Android packages can access.
So if the data you are storing should not be accessible by other applications there is no need to bother with ContentProviders. As you can see on Android Data Storage there are several ways in which you could store your data.
My opinion is that in your case, for usage of REST Services, and data that is not to be shared with other application you should use SQLiteDatabase. You can find a good example SQLiteDatabase example here.

problem to understand content provider

dear i am confused the use of content provider in android applications.i go through various sides and read it but i am still confused about content provider ..can any one explain it in some simple way ,how to use it?
Content providers store and retrieve data and make it accessible to all applications....whats that means?
can it means any package name or in same pakage name...
if i am making another app of diffrent package name then that c.p. is available also in that package?

OK, first of all, a content provider will be used if and only if you want to use the same data through several applications. You can still use it if you want to implement it in your application.
Now, by definition a ContentProvider "Content providers manage access to a structured set of data. They encapsulate the data, and provide mechanisms for defining data security." (According to Android Developer Guides)
So in other words, it's like a new layer of abstraction that you are adding to your data, assigning a new man-in-the-middle that will request everything you ask it to do (read, write, update your database, files, or webservices). And this man-in-the-middle is not only available for you, it's also available for other apps if you set the tag android:exported to true in the <provider> in your manifest file.
A ContentProvider needs you to implement:
A contract: Basically a class with public static variables with your tables names and the fields of info for those tables.
A data model: Normally a database in SQLite but you can implement it also with third party libraries like SugarORM, GreenDao, Retrofit, local files, etc).
Your custom ContentProvider: Extend ContentProvider and inside here set the calls to your data model.
This is a good tutorial on how to make your own ContentProvider:
http://www.grokkingandroid.com/android-tutorial-writing-your-own-content-provider/
There are also a few libraries that will help you create a ContentProvider automatically (I find it amazing because it takes you like 10 min to set up everything, instead of creating your own ContentProvider from scratch which could take you days).
My personal favorite is Schematic:
https://github.com/SimonVT/schematic
I hope it helps. Kind regards!

how to use it?
you actually don't use it directly, but via the content resolver (which will, according to the URI you give it, query the right ContentProvider) :
getContentResolver().query("content://com.myapp.myprovider/data/", ...);
will find your content provider if that one is registered to handle URIs that match "content://com.myapp.myprovider/data/"
if i am making another app of diffrent
package name then that c.p. is
available also in that package?
If you decide to publish the content provider, it is available outside of your application (this is a setting in the manifest).
what is the major benifit of using c.p.?
It is a common design pattern in Android to offer access to data. Major benefit is that you can abstract access to your data and decide whether to open it to other applications or not. For instance, without content providers, you could not access the media stored on the phone or the contacts of the phone.

Since a package is meant as a unique identifier for an application, how can you create an app with the same package name? If this was your question, I think I've answered it. If you don't understand something else - feel free to ask. Good luck!
P.S. Consider accepting some answers, your accept ratio is low.

When to use a Content Provider

I understand that Content Providers are made to allow publicly sharing data between applications. However, I'm wondering if anyone has thoughts about making a Content Provider to use just within your own app. Would there be any advantages to doing this? Any disadvantages?
In the past I've just implemented the SQliteOpenHelper to access data from my database, but I'm considering creating a Content Provider. I feel like the URI approach to requesting data is clear and concise. On the other hand, will using a Content Provider just for my application be redundant ( since within it I will have a SQliteOpenHelper class ) and more work than I need?

I would argue it is definitely a good idea to use a ContentProvider even if you don't intend to make it public.
It's good practice to provide the extra level of abstraction over your data to make it easier to change internally. What if you decide to change the underlying database structure at a later time? If you use a ContentProvider you can contain all the structural changes within it, where as if you don't use one, you are forced to change all areas of the code that are affected by the structural changes. Besides, it's nice to be able to re-use the same standard API for accessing data rather than littering your code with low-level access to the database.
Also, there is always the chance that you might want to expose your data in the future. If you don't use a ContentProvider up front, it will be much harder to retrofit it in at a later date.
Then, there's the other parts of the Android where ContentProvider's are required/recommended such as when using SyncAdapters and if you want an App Widget that involves data access for instance.
In summary, there is very little overhead involved in writing a ContentProvider up front (once you have learned the API which is a good idea anyway) so it makes sense to do so, even for private data.

If you are not planning to share data, don't think about Content Providers. They are powerful but hard to write and it will be just silly to implement them if you are going to use them internally.
However, I'm wondering if anyone has thoughts about making a Content Provider to use just within your own app.
Of course... for instance, for an old TODO list app I wrote, I had to write a content provider to allow other apps retrieve and access the tasks states. It was part of the requirements, but more than that it made sense and made the app nicer.

Take a look at the MOTODEV Studio for Eclipse. It is a development environment that extends Eclipse. They have a tool where you can automatically generate a content provider for a database. If a content provider makes it easier to access your data and it doesn't have a significant impact on performance go ahead and use it. In most scenarios this will be the case.

In short,Content Providers helps in managing your data effectively. I would suggest to use them for the following reasons.
It acts as an abstraction layer between your UI and database. You can implement data validation in ContentProviders to validate the data entered by the user. It also lets you to modify the structure of the database without touching the UI and other parts.
They play along nicely with other android framework classes like SyncAdapter. For eg., you can automatically refresh a list, when a value in a database changes using ContentProviders along with CursorLoader. Without ContentProviders you have to implement a lot of functionalities like these on your own.
We can safely expose our private data to other apps. Using ContentProviders will allow us to share our data easily and safely with other apps.
So even if you don't need any of these functionalities now, you might need them in future and its good to go the extra mile and implement them right now.

I agree ContentProviders are a little difficult to grasp but they are definitely helpful, even if you want to use them internally for you own app. The best thing about it is that you can customize the contentproviders for suitable URIs.
Here's a scenario where you may have 5 tables in your database, but you need to join a few of them in certain orders before using them. And make a content URI for each of these joins. You could then each use these URIs as a table :)
I suggest you go ahead with Content Provider, you'll be amazed to see how powerful it is.

In my view point, the content-provider comes with plenty of advantages leave alone just sharing data with other apps. If you need to synchronize with the server using a Sync-Adapter, use google cloud messaging, auto update the UI when the underlying data in the DB changes using Loaders, implement search, use widgets... then the content provider is for you.
I prefer you follow the guideline on because one day you may need to implement some of the above features attached to the content-provider
By the way, you can quickly build you database and CP in less than 5 minutes using content provider generator

As said in documentation:
Creating a Content provider
You don't need a provider to use an SQLite database if the use is
entirely within your own application.
So why bother developing this overhead? You want easier and faster development, right? So one layer of abstraction (SQLiteOpenHelper descendent) is enough.
See Occam's Razor
Do not make an entities without very good reason.

Using a Content Provider can help in an additional level of abstraction - Putting it within your own application make add a significant development time to your project. However if you are using it to share data, application settings or configurations across multiple applications then the Content Provider is your choice.
Watch your security levels and I would recommend using SQLcipher to encrypt data-at-reset (DAR) if your Content Provider is writing to SQLite. (I've used a content provider in a few solutions and provided the ability to take a live "snap shot" of the operational values for debugging and testing.)

Do not use content provider if do not wish to share data with other apps. Use simple sqlitedatabase to perform database operations. Be careful while using content providers for storing confidential data because your confidential information may be accessed by other apps

Exact Difference between "Content-Provider" and "SQLite Database"

i have done SQLite database programming for Android, but i dont know anything about
Content-Provider except this: "As i have referred Android Developer page , Android SDK explained about "Content-provider" as it is used to store and retrieve data."
But then,
What is the exact difference between "Content-Provider" and "SQLite Database"?
Which is best to store data, when ?
Any example or helps !!

I found one major difference, as follows:
Storing your data in a database is one good way to persist your data, but there's a caveat in Android-databases created in Android are visible only to the application that created them. That is to say, a SQLite database created on Android by one application is usable only by that application, not by other applications.
So, if you need to share data between applications, you need to use the content provider model as recommended in Android. This article presents the basics of content providers and how you can implement one.
I found this article at this link
Really nice information provided.

What is the exact difference between
"Content-Provider" and "SQLite
Database"?
ContentProvider is a facade -- an API you can implement that exposes databases to other processes. It can be implemented in a way where the data is stored in a SQLite database, but it does not have to be.
Which is best to store data, when ?
That is impossible to answer in the abstract. Generally speaking, unless something is requiring you to use a ContentProvider, just use a database.

I have made many good apps with thousands of users using them which simply used SQLite methods. But that was a while ago and I had to manually write lots of code which now can easily be taken care of by ContentProvider. Back then I was not in favour of using Content Providers because it seemed to only add complexity in the code.
However for last couple of years, as Android has evolved, I have moved to ContentProvider as it saves time and allows you do to more. I now use it extensively. Once you have a Content Provider class written, your life becomes much easier. With ContentProvider I can much easily deal with Cursor Loaders, Loader Callbacks and Bulk Inserts for which I had to write everything manually in the past and still it didn't work as efficiently. Especially when updating the list view, which is now automatically updated thanks to just one notifychange() method. This means now I don't have to type my own listeners and manually updating the content in list views and adapters. Plus, I don't need to worry about opening and closing of databases or worry about memory leaks. That's all handled by the Content Provider. The only problem which once in a while I face is that that you cannot do some complex queries in ContentProviders. In this case you can still use raw queries and use the old fashioned manual interaction with sqlite.
If you have previously written your own DbAdapter, Helper and Observer, you can safely carry them on to your new apps without spending time to convert everything to ContentProvider. But based on my experience, I would highly recommend to move to ContentProvider. It'll take some time to get used to it, but once you have got experience with it, you'll stay with it.
UPDATE 2017
I have now switched to Realm, a much better way to use databases on any platform. Spend a few hours learning it, and save countless hours in your app development career.

1. Content Providers are not Thread Safe
By default content providers are not thread safe. If you have multiple threads using a content provider you can see many different exceptions being thrown and other data inconsistencies. The easiest way to fix this is to use the synchronized keyword on each of the public methods exposed by the content provider.
In this way only one thread at a time can access these methods.
2. Play nice when doing lots of writes
I have the need in the new Serval Maps application to import data from binary files into the database used internally by the application. In order to do this and play nice with the rest of the application it is best to:
Spawn a new thread to undertake the import so other threads are not adversely impacted, in particularly the thread in charge of updating the UI; and
Pause briefly at the end of the each import to give other threads which need to use the synchronized methods more of a chance.
3. Content providers force you to think laterally sometimes
The way that content providers in Android work is to provide a layer of abstraction between the rest of your code and the underlying database. This is mainly due to the fact, as far as I can tell, that content providers can access data from places other than databases.
This means that you can’t execute raw SQL queries on the underlying database and you need to specify the various components of a SQL query using variables passed to the various methods such as the query method. If you have a task that doesn’t fit into the way that SQL is handled by a content provider you have two options:
Think laterally about the query, maybe you can get the data that you need by alternative queries and accessing the results from the cursor; and
Use a URI for accessing the data normally and a special URI that is matched to a specific query for those tasks that don’t have alternatives.

Content Providers are used when you want to share your data across applications.
If you have a database attached with an application and you want another application to use some data, you can implement a content provider that exposes the data

The main difference is: when your app needs to share information to another apps, use Content-Provider. SQLite only storage data for the app who creates it

I read this answer while looking for same doubt, so thought of sharing it.
it states -
It's good practice to provide the extra level of abstraction over your data to make it easier to change internally. What if you decide to change the underlying database structure at a later time? If you use a ContentProvider you can contain all the structural changes within it, where as if you don't use one, you are forced to change all areas of the code that are affected by the structural changes. Besides, it's nice to be able to re-use the same standard API for accessing data rather than littering your code with low-level access to the database.
So, using a content provider would be a good idea.

Think of advanced Content Management Systems. Each object (page, image, news article, event item, etc.) has a content, an address, user permissions, and ways to interact with it from different parts of the system. Content Providers do that for Android. You can now share files or images you may have stored in your application. You can also create custom sharable objects, like bussiness contacts, editable notes, etc. And specify security and the default application to deal with such object when you open them from any other application.

One difference is that Content Providers have platform support for Content Observers. Your going to need to implement your own Observable pattern for a SQLite database.
How to automatically re-query with LoaderManager
ContentObserver for SQLite?