docs/html/guide/platform/index.jd

page.title=Platform Architecture
page.keywords=platform,architecture,framework
@jd:body


<div id="qv-wrapper">
  <div id="qv">
    <h2>
      In this document
    </h2>

    <ol>
      <li>
        <a href="#linux-kernel">The Linux Kernel</a>
      </li>

      <li>
        <a href="#hal">Hardware Abstraction Layer (HAL)</a>
      </li>

      <li>
        <a href="#art">Android Runtime</a>
      </li>

      <li>
        <a href="#native-libs">Native C/C++ Libraries</a>
      </li>

      <li>
        <a href="#api-framework">Java API Framework</a>
      </li>

      <li>
        <a href="#system-apps">System Apps</a>
      </li>
    </ol>
  </div>
</div>

<p>
  Android is an open source, Linux-based software stack created for a wide
  array of devices and form factors. The following diagram shows the major
  components of the Android platform.
</p>

<img src="/guide/platform/images/android-stack_2x.png" alt="" width=
"519">
<p class="img-caption">
  <strong>Figure 1.</strong> The Android software stack.
</p>

<h2 id="linux-kernel">
The Linux Kernel
</h2>

<p>
  The foundation of the Android platform is the Linux kernel. For example,
  <a href="#art">the Android Runtime (ART)</a> relies on the Linux kernel for
  underlying functionalities such as threading and low-level memory management.
</p>

<p>
  Using a Linux kernel allows Android to take advantage of <a href=
  "https://source.android.com/security/overview/kernel-security.html">key
  security features</a> and allows device manufacturers to develop hardware
  drivers for a well-known kernel.
</p>

<h2 id="hal">
Hardware Abstraction Layer (HAL)
</h2>

<p>
  The <a href=
  "https://source.android.com/devices/index.html#Hardware%20Abstraction%20Layer">
  hardware abstraction layer (HAL)</a> provides standard interfaces that expose
  device hardware capabilities to the higher-level <a href=
  "#api-framework">Java API framework</a>. The HAL consists of multiple library
  modules, each of which implements an interface for a specific type of hardware
  component, such as the <a href=
  "https://source.android.com/devices/camera/index.html">camera</a> or <a href=
  "https://source.android.com/devices/bluetooth.html">bluetooth</a> module.
  When a framework API makes a call to access device hardware, the Android
  system loads the library module for that hardware component.
</p>

<h2 id="art">
Android Runtime
</h2>

<p>
  For devices running Android version 5.0 (API level 21) or higher, each app
  runs in its own process and with its own instance of the <a href=
  "http://source.android.com/devices/tech/dalvik/index.html">Android Runtime
  (ART)</a>. ART is written to run multiple virtual machines on low-memory
  devices by executing DEX files, a bytecode format designed specially for
  Android that's optimized for minimal memory footprint. Build toolchains, such
  as <a href="https://source.android.com/source/jack.html">Jack</a>, compile
  Java sources into DEX bytecode, which can run on the Android platform.
</p>

<p>
  Some of the major features of ART include the following:
</p>

<ul>
  <li>Ahead-of-time (AOT) and just-in-time (JIT) compilation
  </li>

  <li>Optimized garbage collection (GC)
  </li>

  <li>Better debugging support, including a dedicated sampling profiler,
  detailed diagnostic exceptions and crash reporting, and the ability to set
  watchpoints to monitor specific fields
  </li>
</ul>

<p>
  Prior to Android version 5.0 (API level 21), Dalvik was the Android runtime.
  If your app runs well on ART, then it should work on Dalvik as well, but
  <a href="/guide/platform/verifying-apps-art.html">the reverse may not be
  true</a>.
</p>

<p>
  Android also includes a set of core runtime libraries that provide most of
  the functionality of the Java programming language, including some <a href=
  "/guide/platform/j8-jack.html">Java 8 language features</a>, that the Java
  API framework uses.
</p>

<h2 id="native-libs">
  Native C/C++ Libraries
</h2>

<p>
  Many core Android system components and services, such as ART and HAL, are
  built from native code that require native libraries written in C and C++.
  The Android platform provides Java framework APIs to expose the functionality
  of some of these native libraries to apps. For example, you can access
  <a href="/guide/topics/graphics/opengl.html">OpenGL ES</a> through the
  Android framework’s <a href=
  "/reference/android/opengl/package-summary.html">Java OpenGL API</a> to add
  support for drawing and manipulating 2D and 3D graphics in your app.
</p>

<p>
  If you are developing an app that requires C or C++ code, you can use the
  <a href="/ndk/index.html">Android NDK</a> to access some of these <a href=
  "/ndk/guides/stable_apis.html">native platform libraries</a> directly from
  your native code.
</p>

<h2 id="api-framework">
Java API Framework
</h2>

<p>
  The entire feature-set of the Android OS is available to you through APIs
  written in the Java language. These APIs form the building blocks you need to
  create Android apps by simplifying the reuse of core, modular system
  components and services, which include the following:
</p>

<ul>
  <li>A rich and extensible <a href="/guide/topics/ui/overview.html">View
  System</a> you can use to build an app’s UI, including lists, grids, text
  boxes, buttons, and even an embeddable web browser
  </li>

  <li>A <a href="/guide/topics/resources/overview.html">Resource Manager</a>,
  providing access to non-code resources such as localized strings, graphics,
  and layout files
  </li>

  <li>A <a href="/guide/topics/ui/notifiers/notifications.html">Notification
  Manager</a> that enables all apps to display custom alerts in the status bar
  </li>

  <li>An <a href="/guide/components/activities.html">Activity Manager</a> that
  manages the lifecycle of apps and provides a common <a href=
  "/guide/components/tasks-and-back-stack.html">navigation back stack</a>
  </li>

  <li>
    <a href="/guide/topics/providers/content-providers.html">Content
    Providers</a> that enable apps to access data from other apps, such as
    the Contacts app, or to share their own data
  </li>
</ul>

<p>
  Developers have full access to the same <a href=
  "/reference/packages.html">framework APIs</a> that Android system apps use.
</p>

<h2 id="system-apps">
System Apps
</h2>

<p>
  Android comes with a set of core apps for email, SMS messaging, calendars,
  internet browsing, contacts, and more. Apps included with the platform have
  no special status among the apps the user chooses to install. So a
  third-party app can become the user's default web browser, SMS messenger, or
  even the default keyboard (some exceptions apply, such as the system's
  Settings app).
</p>

<p>
  The system apps function both as apps for users and to provide key
  capabilities that developers can access from their own app. For example, if
  your app would like to deliver an SMS message, you don't need to build that
  functionality yourself—you can instead invoke whichever SMS app is already
  installed to deliver a message to the recipient you specify.
</p>