concept ContentProvider in category android
This is an excerpt from Manning's book Android in Action, Third Edition.
If an application manages data and needs to expose that data to other applications running in the Android environment, you should consider a ContentProvider. If an application component (Activity, Service, or BroadcastReceiver) needs to access data from another application, the component accesses the other application’s ContentProvider. The ContentProvider implements a standard set of methods to permit an application to access a data store. The access might be for read or write operations, or for both. A ContentProvider can provide data to an Activity or Service in the same containing application, as well as to an Activity or Service contained in other applications.
A ContentProvider can use any form of data-storage mechanism available on the Android platform, including files, SQLite databases, or even a memory-based hash map if data persistence isn’t required. The ContentProvider is a data layer that provides data abstraction for its clients and centralizing storage and retrieval routines in a single place.
Sharing files or databases directly is discouraged on the Android platform, and is enforced by the underlying Linux security system, which prevents ad hoc file access from one application space to another without explicitly granted permissions. Data stored in a ContentProvider can be traditional data types, such as integers and strings. Content providers can also manage binary data, such as image data. When binary data is retrieved, the suggested best practice is to return a string representing the filename that contains the binary data. If a filename is returned as part of a ContentProvider query, the application shouldn’t access the file directly; you should use the helper class, ContentResolver’s openInputStream method, to access the binary data. This approach navigates the Linux process and security hurdles, as well as keeps all data access normalized through the ContentProvider. Figure 1.5 outlines the relationship among ContentProviders, data stores, and their clients.
Figure 1.5. The content provider is the data tier for Android applications and is the prescribed manner in which data is accessed and shared on the device.
A ContentProvider’s data is accessed by an Android application through a Content URI. A ContentProvider defines this URI as a public static final String. For example, an application might have a data store managing material safety data sheets. The Content URI for this ContentProvider might look like this:
From this point, accessing a ContentProvider is similar to using Structured Query Language (SQL) in other platforms, though a complete SQL statement isn’t employed. A query is submitted to the ContentProvider, including the columns desired and optional Where and Order By clauses. Similar to parameterized queries in traditional SQL, parameter substitution is also supported when working with the ContentProvider class. Where do the results from the query go? In a Cursor class, naturally. We’ll provide a detailed ContentProvider example in chapter 5.
Beyond the basics, Android also allows applications to share data through a clever URI-based approach called a ContentProvider. This technique combines several other Android concepts, such as the URI-based style of intents and the Cursor result set seen in SQLite, to make data accessible across different applications. To demonstrate how this works, you’ll create another small sample application that uses built-in providers, then we’ll walk through the steps required to create your own ContentProvider.
In the following listing, as a prerequisite to extending the ContentProvider class for a custom provider, we define necessary constants for our Widget type.
This is an excerpt from Manning's book Android in Practice.
ContentProvider— Allows you to expose a data API to other applications
The media scanner is exposed to applications as an Android ContentProvider. We talked about ContentProviders extensively in chapters 7 and 8, so we won’t go into a lot of detail about how they work. Instead, let’s concentrate on the ContentProviders that we should use to find media on a device.