src/devices/tech/test_infra/tradefed/fundamentals/devices.jd

page.title=Working with Devices
@jd:body

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<p>Trade Federation uses an abstraction called
<code><a href="/reference/com/android/tradefed/device/ITestDevice.html">ITestDevice</a></code> to
run tests.  This abstraction objectifies the lowest-common-denominator Android device:</p>
<ul>
<li>It has a serial number</li>
<li>It has a state: Online, Available, Recovery, or Not Available</li>
<li>It has some notion of reliability.  For instance, if we run a command, we can differentiate
  between the case where the command hasn't finished yet, the case where the device doesn't support
  running commands, and the case where the device has become unresponsive while running the
  command.</li>
</ul>

<h2>Different Classes of Devices</h2>
<p>The three primary implementations of <code>ITestDevice</code> represent three common
usecases.</p>

<h3>Physical Device</h3>
<p>This is an actual piece of hardware, connected to the TF host machine either by USB, or by using
adb's TCP feature.  The <a href="/reference/com/android/tradefed/device/TestDevice.html"
>TestDevice</a> class sits atop the ddmlib library, which is a Java interface to adb.  So any
physical device listed in <code>adb devices</code> can be instantiated and used as a
<code>TestDevice</code>.
</p>

<h3>Emulator</h3>
<p>Emulators are handled specially by TF because they live in another process.  To interact with an
Emulator, specify the <code>--emulator</code> argument for the command.  See
<a href="/reference/com/android/tradefed/build/LocalSdkBuildProvider.html"
>LocalSdkBuildProvider</a> and
<a href="/reference/com/android/tradefed/targetprep/SdkAvdPreparer.html"
>SdkAvdPreparer</a> for more info.</p>

<h3>No Device</h3>
<p>Suppose you have a test that doesn't interact with a device at all.  For instance, it might just
download a file from some service and verify that the file itself is valid.  The
<a href="/reference/com/android/tradefed/device/NullDevice.html"
>NullDevice</a> is an <code>ITestDevice</code> that is just a stub.  It has a serial number like
<code>null-device-N</code>, and most attempted operations either no-op silently or throw.
</p>