guide/basics/appmodel.jd

page.title=Application Model
@jd:body
<h1>Android Application Model: Applications, Tasks, Processes, and Threads</h1>

<p>In most operating systems, there is a strong 1-to-1 correlation between
the executable image (such as the .exe on Windows) that an application lives in,
the process it runs in, and the icon and application the user interacts with.
In Android these associations are much more fluid, and it is important to
understand how the various pieces can be put together.</p>

<p>Because of the flexible nature of Android applications, there is some
basic terminology that needs to be understood when implementing the
various pieces of an application:</p>

<ul>
<li><p>An <strong>android package</strong> (or <strong>.apk</strong> for short)
is the file containing an application's code and its resources.  This is the
file that an application is distributed in and downloaded by the user when
installing that application on their device.</p></li>

<li><p>A <strong>task</strong> is generally what the user perceives as
an "application" that can be launched: usually a task has an icon in the
home screen through which it is accessed, and it is available as a top-level
item that can be brought to the foreground in front of other
tasks.</p></li>

<li><p>A <strong>process</strong> is a low-level kernel process in which
an application's code is running.  Normally all of the code in a
.apk is run in one, dedicated process for that .apk; however, the
{@link android.R.styleable#AndroidManifestApplication_process process} tag
can be used to modify where that code is run, either for
{@link android.R.styleable#AndroidManifestApplication the entire .apk}
or for individual
{@link android.R.styleable#AndroidManifestActivity activity},
{@link android.R.styleable#AndroidManifestReceiver receiver},
{@link android.R.styleable#AndroidManifestService service}, or
{@link android.R.styleable#AndroidManifestProvider provider}, components.</p></li>
</ul>

<h2 id="Tasks">Tasks</h2>

<p>A key point here is: <em>when the user sees as an "application," what
they are actually dealing with is a task</em>.  If you just create a .apk
with a number of activities, one of which is a top-level entry point (via
an {@link android.R.styleable#AndroidManifestIntentFilter intent-filter} for
the action <code>android.intent.action.MAIN</code> and
category <code>android.intent.category.LAUNCHER</code>), then there will indeed
be one task created for your .apk, and any activities you start from there
will also run as part of that task.</p>

<p>A task, then, from the user's perspective your application; and from the
application developer's perspective it is one or more activities the user
has traversed through in that task and not yet closed, or an activity stack.
A new task is created by
starting an activity Intent with the {@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK
Intent.FLAG_ACTIVITY_NEW_TASK} flag; this Intent will be used as the root Intent of
the task, defining what task it is.  Any activity started without this flag
will run in the same task as the activity that is starting it (unless that
activity has requested a special launch mode, as discussed later).  Tasks can
be re-ordered: if you use FLAG_ACTIVITY_NEW_TASK but there is already a task
running for that Intent, the current task's activity stack will be brought
to the foreground instead of starting a new task.</p>

<p>FLAG_ACTIVITY_NEW_TASK must only be used with care: using it says that,
from the user's perspective, a new application starts at this point.  If this
is not the behavior you desire, you should not be creating a new task.  In
addition, you should only use the new task flag if it is possible for the user
to navigate from home back to where they are and launch the same Intent as a
new task.  Otherwise, if the user presses HOME instead of BACK from the task
you have launched, your task and its activities will be ordered behind the
home screen without a way to return to them.</p>

<h3>Task Affinities</h3>

<p>In some cases Android needs to know which task an activity belongs to even when
it is not being launched in to a specific task.  This is accomplished through
task affinities, which provide a unique static name for the task that one or more
activities are intended to run in.  The default task affinity for an activity
is the name of the .apk package name the activity is implemented in.  This
provides the normally expected behavior, where all of the activities in a
particular .apk are part of a single application to the user.</p>

<p>When starting a new activity without the
{@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK
Intent.FLAG_ACTIVITY_NEW_TASK} flag, task affinities have no impact on the
task the new activity will run in: it will always run in the task of the
activity that is starting it.  However, if the NEW_TASK flag is being used,
then the affinity will be used to determine if a task already exists with
the same affinity.  If so, that task will be brought to the front and the
new activity launched at the top of that task.</p>

<p>This behavior is most useful for situations where you must use the
NEW_TASK flag, in particular launching activities from status bar notifications
or home screen shortcuts.  The result is that, when the user launches your
application this way, its current task state will be brought to the foreground,
and the activity they now want to look at placed on top of it.</p>

<p>You can assign your own task affinities in your manifest's
{@link android.R.styleable#AndroidManifestApplication application} tag for
all activities in the .apk, or the
{@link android.R.styleable#AndroidManifestActivity activity} tag of
individual activities.  Some examples of how this can be used are:</p>

<ul>
<li>If your .apk contains multiple top-level applications that the user can
launch, then you will probably want to assign different affinities to each
of the activities that the users sees for your .apk.  A good convention for
coming up with distinct names is to append your .apk's package name with
a colon separated string.  For example, the "com.android.contacts" .apk
may have the affinities "com.android.contacts:Dialer" and
"com.android.contacts:ContactsList".</ul>
<li>If you are replacing a notification, shortcut, or other such "inner"
activity of an application that can be launched from outside of it, you may
need to explicitly set the taskAffinity of your replacement activity to be
the same as the application you are replacing.  For example, if you are
replacing the contacts details view (which the user can make and invoke
shortcuts to), you would want to set the taskAffinity to
"com.android.contacts".</li>
</ul>

<h3>Launch Modes and Launch Flags</h3>

<p>The main way you control how activities interact with tasks is through
the activity's
{@link android.R.styleable#AndroidManifestActivity_launchMode launchMode}
attribute and the {@link android.content.Intent#setFlags flags} associated
with an Intent.  These two parameters can work together in various ways
to control the outcome of the activity launch, as described in their
associated documentation.  Here we will look at some common use cases and
combinations of these parameters.</p>

<p>The most common launch mode you will use (besides the default
<code>standard</code> mode) is <code>singleTop</code>.  This does not have
an impact on tasks; it just avoids starting the same activity multiple times
on the top of a stack.

<p>The <code>singleTask</code> launch mode has a major
impact on tasks: it causes the activity to always be started in
a new task (or its existing task to be brought to the foreground).  Using
this mode requires a lot of care in how you interact with the rest of the
system, as it impacts every path in to the activity.  It should only be used
with activities that are front doors to the application (that is, which
support the MAIN action and LAUNCHER category).</p>

<p>The <code>singleInstance</code> launch mode is even more specialized, and
should only be used in applications that are implemented entirely as one
activity.</p>

<p>A situation you will often run in to is when another entity (such as the
{@link android.app.SearchManager} or {@link android.app.NotificationManager})
starts one of your activities.  In this case, the
{@link android.content.Intent#FLAG_ACTIVITY_NEW_TASK
Intent.FLAG_ACTIVITY_NEW_TASK} flag must be used, because the activity is
being started outside of a task (and the application/task may not even
exist).  As described previously, the standard behavior in this situation
is to bring to the foreground the current task matching the new activity's
affinity and start the new activity at the top of it.  There are, however,
other types of behavior that you can implement.</p>

<p>One common approach is to also use the
{@link android.content.Intent#FLAG_ACTIVITY_CLEAR_TOP
Intent.FLAG_ACTIVITY_CLEAR_TOP} flag in conjunction with NEW_TASK.  By doing so,
if your task is already running, then it will be brought to the foreground,
all of the activities on its stack cleared except the root activity, and the
root activity's {@link android.app.Activity#onNewIntent} called with the
Intent being started.  Note that the activity often also use the <code>singleTop</code>
or <code>singleTask</code> launch mode when using this approach, so that
the current instance is given the new intent instead of requiring that it
be destroyed and a new instance started.</p>

<p>Another approach you can take is to set the notification activity's
<code>android:taskAffinity</code> to the empty string "" (indicating no affinity)
and setting the
<code>{@link android.R.styleable#AndroidManifestActivity_noHistory
android:noHistory}</code> and
<code>{@link android.R.styleable#AndroidManifestActivity_excludeFromRecents
android:excludeFromRecents}</code> attributes.
This approach is useful if you would like the notification
to take the user to a separate activity describing it, rather than return
to the application's task.  By specifying these attributes, the activity will
be finished whether the user leaves it with BACK or HOME and it will not
show up in the recent tasks list; if the <code>noHistory</code> attribute
isn't specified, pressing HOME will result in the activity and its task
remaining in the system, possibly with no way to return to it.</p>

<p>Be sure to read the documentation on the
{@link android.R.styleable#AndroidManifestActivity_launchMode launchMode attribute}
and the {@link android.content.Intent#setFlags Intent flags} for the details
on these options.</p>

<h2 id="Processes">Processes</h2>

<p>In Android, processes are entirely an implementation detail of applications
and not something the user is normally aware of.  Their main uses are simply:</p>

<ul>
<li> Improving stability or security by putting untrusted or unstable code
into another process.
<li> Reducing overhead by running the code of multiple .apks in the same
process.
<li> Helping the system manage resources by putting heavy-weight code in
a separate process that can be killed independently of other parts of the
application.
</ul>

<p>As described previously, the
{@link android.R.styleable#AndroidManifestApplication_process process} attribute
is used to control the process that particular application components run in.
Note that this attribute can not be used to violate security of the system: if
two .apks that are not sharing the same user ID try to run in the same process,
this will not be allowed and different distinct processes will be created for
each of them.</p>

<p>See the <a href="{@docRoot}devel/security.html">security</a> document for
more information on these security restrictions.</p>

<h2 id="Threads">Threads</h2>

<p>Every process has one or more threads running in it.  In most situations, Android
avoids creating additional threads in a process, keeping an application
single-threaded unless it creates its own threads.  An important repercussion
of this is that all calls to {@link android.app.Activity},
{@link android.content.BroadcastReceiver}, and {@link android.app.Service}
instances are made only from the main thread of the process they are running in.</p>

<p>Note that a new thread is <strong>not</strong> created for each
Activity, BroadcastReceiver, Service, or ContentProvider instance:
these application components are instantiated in the desired process (all in the
same process unless otherwise specified), in the main thread of that process.
This means that none of these components (including services) should perform
long or blocking operations (such as networking calls or computation loops)
when called by the system, since this will block
all other components in the process.  You can use the standard library
{@link java.lang.Thread} class or Android's {@link android.os.HandlerThread}
convenience class to perform long operations on another thread.</p>

<p>There are a few important exceptions to this threading rule:</p>

<ul>
<li><p>Calls on to an {@link android.os.IBinder} or interface implemented on
an IBinder are dispatched from the thread calling them or a thread pool in the
local process if coming from another process, <em>not</em>
from the main thread of their process.  In particular, calls on to the IBinder
of a {@link android.app.Service} will be called this way.  (Though
calls to methods on Service itself are done from the main thread.)
This means that <em>implementations of IBinder interfaces must always be
written in a thread-safe way, since they can be called from any number of
arbitrary threads at the same time</em>.</p></li>

<li><p>Calls to the main methods of {@link android.content.ContentProvider}
are dispatched from the calling thread or main thread as with IBinder.  The
specific methods are documented in the ContentProvider class.
This means that <em>implementations of these methods must always be
written in a thread-safe way, since they can be called from any number of
arbitrary threads at the same time</em>.</p></li>

<li><p>Calls on {@link android.view.View} and its subclasses are made from the
thread that the view's window is running in.  Normally this will be the main
thread of the process, however if you create a thread and show a window from
there then the window's view hierarchy will be called from that thread.</p></li>
</ul>