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<html devsite>
<head>
<title>Architecture</title>
<meta name="project_path" value="/_project.yaml" />
<meta name="book_path" value="/_book.yaml" />
</head>
<body>
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<p>
Android system architecture contains the following components:
</p>
<img src="../images/ape_fwk_all.png">
<figcaption><strong>Figure 1.</strong> Android system architecture</figcaption>
<ul>
<li>
<strong>Application framework</strong>. The application framework is used
most often by application developers. As a hardware developer, you should be
aware of developer APIs as many map directly to the underlying HAL
interfaces and can provide helpful information about implementing drivers.
</li>
<li>
<strong>Binder IPC</strong>. The Binder Inter-Process Communication (IPC)
mechanism allows the application framework to cross process boundaries and
call into the Android system services code. This enables high level
framework APIs to interact with Android system services. At the application
framework level, this communication is hidden from the developer and things
appear to "just work".
</li>
<li>
<strong>System services</strong>. System services are modular, focused
components such as Window Manager, Search Service, or Notification Manager.
Functionality exposed by application framework APIs communicates with system
services to access the underlying hardware. Android includes two groups of
services: <em>system</em> (such as Window Manager and Notification Manager)
and <em>media</em> (services involved in playing and recording media).
</li>
<li>
<strong>Hardware abstraction layer (HAL)</strong>. A HAL defines a standard
interface for hardware vendors to implement, which enables Android to be
agnostic about lower-level driver implementations. Using a HAL allows you to
implement functionality without affecting or modifying the higher level
system. HAL implementations are packaged into modules and loaded by the
Android system at the appropriate time. For details, see
<a href="/devices/architecture/hal.html">Hardware Abstraction Layer
(HAL)</a>.
</li>
<li>
<strong>Linux kernel</strong>. Developing your device drivers is similar to
developing a typical Linux device driver. Android uses a version of the
Linux kernel with a few special additions such as Low Memory Killer (a
memory management system that is more aggressive in preserving memory), wake
locks (a
<a href="https://developer.android.com/reference/android/os/PowerManager.html" class="external"><code>PowerManager</code></a>
system service), the Binder IPC driver, and other features important for a
mobile embedded platform. These additions are primarily for system
functionality and do not affect driver development. You can use any version
of the kernel as long as it supports the required features (such as the
binder driver). However, we recommend using the latest version of the
Android kernel. For details, see
<a href="/setup/building-kernels.html">Building Kernels</a>.
</li>
</ul>
<h2 id="hidl">HAL interface definition language (HIDL)</h2>
<p>
Android 8.0 re-architected the Android OS framework (in a project known as
<em>Treble</em>) to make it easier, faster, and less costly for
manufacturers to update devices to a new version of Android. In this new
architecture, the HAL interface definition language (HIDL, pronounced
"hide-l") specifies the interface between a HAL and its users, enabling the
Android framework to be replaced without rebuilding the HALs.
</p>
<aside class="note">
<strong>Note:</strong> For more details on Project Treble, refer to the
developer blog posts
<a href="https://android-developers.googleblog.com/2017/05/here-comes-treble-modular-base-for.html" class="external">Here
comes Treble: A modular base for Android</a> and
<a href="https://android-developers.googleblog.com/2018/05/faster-adoption-with-project-treble.html" class="external">Faster
Adoption with Project Treble</a>.
</aside>
<p>
HIDL separates the vendor implementation (device-specific, lower-level
software written by silicon manufacturers) from the Android OS framework via
a new vendor interface. Vendors or SOC makers build HALs once and place them
in a <code>/vendor</code> partition on the device; the framework, in its own
partition, can then be replaced with an
<a href="/devices/tech/ota/">over-the-air (OTA) update</a> without
recompiling the HALs.
</p>
<p>
The difference between the legacy Android architecture and the current,
HIDL-based architecture is in the use of the vendor interface:
</p>
<ul>
<li>
In Android 7.x and earlier, no formal vendor interface exists, so device
makers must update large portions of the Android code to move a device to a
newer version of Android:<br><br>
<img src="images/treble_blog_before.png">
<figcaption><strong>Figure 2.</strong> Legacy Android update
environment</figcaption>
</li>
<li>
In Android 8.0 and higher, a new stable vendor interface provides access to
the hardware-specific parts of Android, so device makers can deliver
new Android releases simply by updating the Android OS
framework—without additional work required from the silicon
manufacturers:<br><br>
<img src="images/treble_blog_after.png">
<figcaption><strong>Figure 3.</strong> Current Android update
environment</figcaption>
</li>
</ul>
<p>
All new devices launching with Android 8.0 and higher can take advantage of
the new architecture. To ensure forward compatibility of vendor
implementations, the vendor interface is validated by the
<a href="/devices/tech/vts/index.html">Vendor Test Suite (VTS)</a>, which is
analogous to the <a href="/compatibility/cts/">Compatibility Test Suite
(CTS)</a>. You can use VTS to automate HAL and OS kernel testing in both
legacy and current Android architectures.
</p>
<h2 id="resources">Architecture resources</h2>
<p>
For details on the Android architecture, see the following sections:
</p>
<ul>
<li>
<a href="/devices/architecture/hal-types.html">HAL Types</a>. Describes
binderized, passthrough, Same-Process (SP), and legacy HALs.
</li>
<li>
<a href="/devices/architecture/hidl/index.html">HIDL (General)</a>. Contains
general information about the interface between a HAL and its users.
</li>
<li>
<a href="/devices/architecture/hidl-cpp/index.html">HIDL (C++)</a>. Contains
details for creating C++ implementations of HIDL interfaces.
</li>
<li>
<a href="/devices/architecture/hidl-java/index.html">HIDL (Java)</a>.
Contains details about the Java frontend for HIDL interfaces.
</li>
<li>
<a href="/devices/architecture/configstore/index.html">ConfigStore HAL</a>.
Describes APIs for accessing read-only configuration items used to configure
the Android framework.
</li>
<li>
<a href="/devices/architecture/dto/index.html">Device Tree Overlays</a>.
Provides details on using device tree overlays (DTOs) in Android.
</li>
<li>
<a href="/devices/architecture/vndk/index.html">Vendor Native Development
Kit (VNDK)</a>. Describes the set of vendor-exclusive libraries for
implementing vendor HALs.
</li>
<li>
<a href="/devices/architecture/vintf/index.html">Vendor Interface Object
(VINTF)</a>. Describes the objects that aggregate relevant information about
a device and make that information available through a queryable API.
</li>
<li>
<a href="/security/selinux/images/SELinux_Treble.pdf">SELinux for Android
8.0</a>. Details SELinux changes and customizations.
</li>
</ul>
</body>
</html>
|
