A common source of confusion when implementing ContentProviders is that of thread-safety. We all know that any potentially expensive query should be asynchronous so as not to block the UI thread, but when, if ever, is it OK to make calls to the ContentProvider from multiple threads?
Threads and ContentProviders
The documentation on ContentProviders warns that its methods may be called from multiple threads and therefore must be thread-safe:
Data access methods (such as insert(Uri, ContentValues) and update(Uri, ContentValues, String, String)) may be called from many threads at once, and must be thread-safe.
In other words, Android does not synchronize access to the ContentProvider for you. If two calls to the same method are made simultaneously from separate threads, neither call will wait for the other. Requiring the client to deal with concurrency themselves makes sense from a framework developer's point of view. The abstract ContentProvider class cannot assume that its subclasses will require synchronization, as doing so would be horribly inefficient.
Ensuring Thread Safety
So now that we know that the ContentProvider is not thread safe, what do we need to do in order to eliminate potential race conditions? Just make every method synchronized, right?
Well... no, not necessarily. Consider a ContentProvider that uses a SQLiteDatabase as its backing data source. As per the documentation, access to the SQLiteDatabase is synchronized by default, thus guaranteeing that no two threads will ever touch it at the same time. In this case, synchronizing each of the ContentProvider's methods is both unnecessary and costly. Remember that a ContentProvider serves as a wrapper around the underlying data source; whether or not you must take extra measures to ensure thread safety often depends on the data source itself.
Although the ContentProvider lacks in thread-safety, often times you will find that no further action is required on your part with respect to preventing potential race conditions. The canonical example is when your ContentProvider is backed by a SQLiteDatabase; when two threads attempt to write to the database at the same time, the SQLiteDatabase will lock itself down, ensuring that one will wait until the other has completed. Each thread will be given mutually exclusive access to the data source, ensuring the thread safety is met.
This has been a rather short post, so don't hesitate to leave a comment if you have any clarifying questions. Don't forget to +1 this post below if you found it helpful! :)