1page.title=Working with Devices
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19
20<p>Trade Federation uses an abstraction called
21<code><a href="/reference/com/android/tradefed/device/ITestDevice.html">ITestDevice</a></code> to
22run tests.  This abstraction objectifies the lowest-common-denominator Android device:</p>
23<ul>
24<li>It has a serial number</li>
25<li>It has a state: Online, Available, Recovery, or Not Available</li>
26<li>It has some notion of reliability.  For instance, if we run a command, we can differentiate
27  between the case where the command hasn't finished yet, the case where the device doesn't support
28  running commands, and the case where the device has become unresponsive while running the
29  command.</li>
30</ul>
31
32<h2>Different Classes of Devices</h2>
33<p>The three primary implementations of <code>ITestDevice</code> represent three common
34usecases.</p>
35
36<h3>Physical Device</h3>
37<p>This is an actual piece of hardware, connected to the TF host machine either by USB, or by using
38adb's TCP feature.  The <a href="/reference/com/android/tradefed/device/TestDevice.html"
39>TestDevice</a> class sits atop the ddmlib library, which is a Java interface to adb.  So any
40physical device listed in <code>adb devices</code> can be instantiated and used as a
41<code>TestDevice</code>.
42</p>
43
44<h3>Emulator</h3>
45<p>Emulators are handled specially by TF because they live in another process.  To interact with an
46Emulator, specify the <code>--emulator</code> argument for the command.  See
47<a href="/reference/com/android/tradefed/build/LocalSdkBuildProvider.html"
48>LocalSdkBuildProvider</a> and
49<a href="/reference/com/android/tradefed/targetprep/SdkAvdPreparer.html"
50>SdkAvdPreparer</a> for more info.</p>
51
52<h3>No Device</h3>
53<p>Suppose you have a test that doesn't interact with a device at all.  For instance, it might just
54download a file from some service and verify that the file itself is valid.  The
55<a href="/reference/com/android/tradefed/device/NullDevice.html"
56>NullDevice</a> is an <code>ITestDevice</code> that is just a stub.  It has a serial number like
57<code>null-device-N</code>, and most attempted operations either no-op silently or throw.
58</p>
59