docs/html/topic/performance/launch-time.jd

page.title=Launch-Time Performance
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
<li><a href="#internals">Launch Internals</a>
  <ol>
    <li><a href="#cold">Cold start</a></li>
    <li><a href="#warm">Warm start</a></li>
    <li><a href="#lukewarm">Lukewarm start</a></li>
  </ol>
</li>
<li><a href="#profiling">Profiling Launch Performance</a>
  <ol>
    <li><a href="#time-initial">Time to initial display</a></li>
    <li><a href="#time-full">Time to full display</a></li>
  </ol>
</li>
<li><a href="#common">Common Issues</a>
   <ol>
      <li><a href="#heavy-app">Heavy app initialization</a></li>
      <li><a href="#heavy-act">Heavy activity initialization</a></li>
      <li><a href="#themed">Themed launch screens</a></li>
   </ol>
      </li>
</ol>
</div>
</div>

<p>
Users expect apps to be responsive and fast to load. An app with a slow startup
time doesn’t meet this expectation, and can be disappointing to users. This
sort of poor experience may cause a user to rate your app poorly on the Play
store, or even abandon your app altogether.
</p>

<p>
This document provides information to help you optimize your app’s launch time.
It begins by explaining the internals of the launch process. Next, it discusses
how to profile startup performance. Last, it describes some common startup-time
issues, and gives some hints on how to address them.
</p>

<h2 id="internals">Launch Internals</h2>

<p>
App launch can take place in one of three states, each affecting how
long it takes for your app to become visible to the user: cold start,
warm start, and lukewarm start. In a cold start, your app starts from scratch.
In the other states, the system needs to bring the app from the background to
the foreground. We recommend that you always optimize based on an assumption of
a cold start. Doing so can improve the performance of warm and lukewarm starts,
as well.
</p>

<p>
To optimize your app for fast startup, it’s useful to understand what’s
happening at the system and app levels, and how they interact, in each of
these states.
</p>

<h3 id="cold">Cold start</h3>

<p>
A cold start refers to an app’s starting from scratch: the system’s process
has not, until this start, created the app’s process. Cold starts happen in
cases such as your app’s being launched for the first time since the device
booted, or since the system killed the app. This type of start presents the
greatest challenge in terms of minimizing startup time, because the system
and app have more work to do than in the other launch states.
</p>

<p>
At the beginning of a cold start, the system has three tasks. These tasks are:
</p>

<ol style="1">
   <li>Loading and launching the app.</li>
   <li>Displaying a blank starting window for the app immediately after launch.
   </li>
   <li>Creating the app
   <a href="{docRoot}guide/components/processes-and-threads.html#Processes">
   process.</a></li>
</ol>
<br/>
<p>
As soon as the system creates the app process, the app process is responsible
for the next stages. These stages are:
</p>

<ol style="1">
   <li>Creating the app object.</li>
   <li>Launching the main thread.</li>
   <li>Creating the main activity.</li>
   <li>Inflating views.</li>
   <li>Laying out the screen.</li>
   <li>Performing the initial draw.</li>
</ol>

<p>
Once the app process has completed the first draw, the system process swaps
out the currently displayed background window, replacing it with the main
activity. At this point, the user can start using the app.
</p>

<p>
Figure 1 shows how the system and app processes hand off work between each
other.
</p>
<br/>

  <img src="{@docRoot}topic/performance/images/cold-launch.png">
  <p class="img-caption">
    <strong>Figure 1.</strong> A visual representation of the important parts of
    a cold application launch.
  </p>

<p>
Performance issues can arise during creation of the app and
creation of the activity.
</p>

<h4 id="app-creation">Application creation</h4>

<p>
When your application launches, the blank starting window remains on the screen
until the system finishes drawing the app for the first time. At that point,
the system process swaps out the starting window for your app, allowing the
user to start interacting with the app.
</p>

<p>
If you’ve overloaded {@link android.app.Application#onCreate() Application.oncreate()}
in your own app, the app starts by calling this
method on your app object. Afterwards, the app spawns the main thread, also
known as the UI thread, and tasks it with creating your main activity.
</p>

<p>
From this point, system- and app-level processes proceed in accordance with
the <a href="{docRoot}guide/topics/processes/process-lifecycle.html">
app lifecycle stages</a>.
</p>

<h4 id="act-creation">Activity creation</h4>

<p>
After the app process creates your activity, the activity performs the
following operations:
</p>

<ol style="1">
   <li>Initializes values.</li>
   <li>Calls constructors.</li>
   <li>Calls the callback method, such as
   {@link android.app.Activity#onCreate(android.os.Bundle) Activity.onCreate()},
   appropriate to the current lifecycle state of the activity.</li>
</ol>

<p>
Typically, the
{@link android.app.Activity#onCreate(android.os.Bundle) onCreate()}
method has the greatest impact on load time, because it performs the work with
the highest overhead: loading and inflating views, and initializing the objects
needed for the activity to run.
</p>

<h3 id="warm">Warm start</h3>

<p>
A warm start of your application is much simpler and lower-overhead than a
cold start. In a warm start, all the system does is bring your activity to
the foreground. If all of your application’s activities are still resident in
memory, then the app can avoid having to repeat object initialization, layout
inflation, and rendering.
</p>

<p>
However, if some memory has been purged in response to memory trimming
events, such as
{@link android.content.ComponentCallbacks2#onTrimMemory(int) onTrimMemory()},
then those objects will need to be recreated in
response to the warm start event.
</p>

<p>
A warm start displays the same on-screen behavior as a cold start scenario:
The system process displays a blank screen until the app has finished rendering
the activity.
</p>

<h3 id="lukewarm">Lukewarm start</h3>

<p>
A lukewarm start encompasses some subset of the operations that
take place during a cold start; at the same time, it represents less overhead
than a warm start. There are many potential states that could be considered
lukewarm starts. For instance:
</p>

<ul>
   <li>The user backs out of your app, but then re-launches it. The process may
       have continued to run, but the app must recreate the activity from scratch
       via a call to
       {@link android.app.Activity#onCreate(android.os.Bundle) onCreate()}.</li>

   <li>The system evicts your app from memory, and then the user re-launches it.
       The process and the Activity need to be restarted, but the task can
       benefit somewhat from the saved instance state bundle passed into
       {@link android.app.Activity#onCreate(android.os.Bundle) onCreate()}.</li>
</ul>

<h2 id="profiling">Profiling Launch Performance</h2>

<p>
In order to properly diagnose start time performance, you can track metrics
that show how long it takes your application to start.
</p>

<h3 id="time-initial">Time to initial display</h3>

<p>
From Android 4.4 (API level 19), logcat includes an output line containing
a value called {@code Displayed}. This value represents
the amount of time elapsed between launching the process and finishing drawing
the corresponding activity on the screen. The elapsed time encompasses the
following sequence of events:
</p>

<ol style="1">
   <li>Launch the process.</li>
   <li>Initialize the objects.</li>
   <li>Create and initialize the activity.</li>
   <li>Inflate the layout.</li>
   <li>Draw your application for the first time.</li>
</ol>

<p>
The reported log line looks similar to the following example:
</p>

<pre class="no-pretty-print">
ActivityManager: Displayed com.android.myexample/.StartupTiming: +3s534ms
</pre>

<p>
If you’re tracking logcat output from the command line, or in a terminal,
finding the elapsed time is straightforward. To find elapsed time in
Android Studio, you must disable filters in your logcat view. Disabling the
filters is necessary because the system server, not the app itself, serves
this log.
</p>

<p>
Once you’ve made the appropriate settings, you can easily search for the
correct term to see the time. Figure 2 shows how to disable filters, and,
in the second line of output from the bottom, an example of logcat output of
the {@code Displayed} time.
</p>
<br/>

  <img src="{@docRoot}topic/performance/images/displayed-logcat.png">
  <p class="img-caption">
    <strong>Figure 2.</strong> Disabling filters, and
    finding the {@code Displayed} value in logcat.
  </p>

<p>
The {@code Displayed} metric in the logcat output does not necessarily capture
the amount of time until all resources are loaded and displayed: it leaves out
resources that are not referenced in the layout file or that the app creates
as part of object initialization. It excludes these resources because loading
them is an inline process, and does not block the app’s initial display.
</p>

<h3 id="time-full">Time to full display</h3>

<p>
You can use the {@link android.app.Activity#reportFullyDrawn()} method to
measure the elapsed time
between application launch and complete display of all resources and view
hierarchies. This can be valuable in cases where an app performs lazy loading.
In lazy loading, an app does not block the initial drawing of the window, but
instead asynchronously loads resources and updates the view hierarchy.
</p>

<p>
If, due to lazy loading, an app’s initial display does not include all
resources, you might consider the completed loading and display of all
resources and views as a separate metric: For example, your UI might be
fully loaded, with some text drawn, but not yet display images that the
app must fetch from the network.
</p>

<p>
To address this concern, you can manually call
{@link android.app.Activity#reportFullyDrawn()}
to let the system know that your activity is
finished with its lazy loading. When you use this method, the value
that logcat displays is the time elapsed
since the creation of the application object, and the moment
{@link android.app.Activity#reportFullyDrawn()} is called.
</p>

<p>
If you learn that your display times are slower than you’d like, you can
go on to try to identify the bottlenecks in the startup process.
</p>

<h4 id="bottlenecks">Identifying bottlenecks</h4>

<p>
Two good ways to look for bottlenecks are Android Studio’s Method Tracer tool
and inline tracing. To learn about Method Tracer, see that tool’s
<a href="{docRoot}studio/profile/am-methodtrace.html">documentation</a>.
</p>

<p>
If you do not have access to the Method Tracer tool, or cannot start the tool
at the correct time to gain log information, you can gain similar insight
through inline tracing inside of your apps’ and activities’ {@code onCreate()}
methods. To learn about inline tracing, see the reference documentation for
the {@link android.os.Trace} functions, and for the
<a href="{docRoot}studio/profile/systrace-commandline.html">Systrace</a> tool.
</p>

<h2 id="common">Common Issues</h2>

<p>
This section discusses several issues that often affect apps’ startup
performance. These issues chiefly concern initializing app and activity
objects, as well as the loading of screens.
</p>

<h3 id="heavy-app">Heavy app initialization</h3>

<p>
Launch performance can suffer when your code overrides the {@code Application}
object, and executes heavy work or complex logic when initializing that object.
Your app may waste time during startup if your Application subclasses perform
initializations that don’t need to be done yet. Some initializations may be
completely unnecessary: for example, initializing state information for the
main activity, when the app has actually started up in response to an intent.
With an intent, the app uses only a subset of the previously initialized state
data.
</p>

<p>
Other challenges during app initialization include garbage-collection events
that are impactful or numerous, or disk I/O happening concurrently with
initialization, further blocking the initialization process. Garbage collection
is especially a consideration with the Dalvik runtime; the Art runtime performs
garbage collection concurrently, minimizing that operation's impact.
</p>

<h4 id="diagnosing-1">Diagnosing the problem</h4>

<p>
You can use method tracing or inline tracing to try to diagnose the problem.
</p>

<h5>Method tracing</h5>

<p>
Running the Method Tracer tool reveals that the
{@link android.app.Instrumentation#callApplicationOnCreate(android.app.Application) callApplicationOnCreate()}
method eventually calls your {@code com.example.customApplication.onCreate}
method. If the tool shows that these
methods are taking a long time to finish executing, you should explore further
to see what work is occurring there.
</p>

<h5>Inline tracing</h5>

<p>
Use inline tracing to investigate likely culprits including:
</p>

<ul>
   <li>Your app’s initial {@link android.app.Application#onCreate()}
   function.</li>
   <li>Any global singleton objects your app initializes.</li>
   <li>Any disk I/O, deserialization, or tight loops that might be occurring
   during the bottleneck.
</ul>


<h4 id="solutions-1">Solutions to the problem</h4>

<p>
Whether the problem lies with unnecessary initializations or disk I/O,
the solution calls for lazy-initializing objects: initializing only those
objects that are immediately needed. For example, rather than creating global
static objects, instead, move to a singleton pattern, where the app initalizes
objects only the first time it accesses them.
</p>

<h3 id="heavy-act">Heavy activity initialization</h4>

<p>
Activity creation often entails a lot of high-overhead work. Often, there are
opportunities to optimize this work to achieve performance improvements. Such
common issues include:
</p>

<ul>
   <li>Inflating large or complex layouts.</li>
   <li>Blocking screen drawing on disk, or network I/O.</li>
   <li>Loading and decoding bitmaps.</li>
   <li>Rasterizing {@link android.graphics.drawable.VectorDrawable VectorDrawable} objects.</li>
   <li>Initialization of other subsystems of the activity.</li>
</ul>

<h4 id="diagnosing-2">Diagnosing the problem</h4>

<p>
In this case, as well, both method tracing and inline tracing can prove useful.
</p>

<h5>Method tracing</h5>

<p>
When running the Method Tracer tool, the particular areas to
focus on your your app’s {@link android.app.Application} subclass constructors and
{@code com.example.customApplication.onCreate()} methods.
</p>

<p>
If the tool shows that these methods are taking a long time to finish
executing, you should explore further to see what work is occurring there.
</p>

<h5>Inline tracing</h5>

<p>
Use inline tracing to investigate likely culprits including:
</p>

<ul>
   <li>Your app’s initial {@link android.app.Application#onCreate()}
   function.</li>
   <li>Any global singleton objects it initializes.</li>
   <li>Any disk I/O, deserialization, or tight loops that might be occurring
   during the bottleneck.</li>
</ul>

<h4 id="solutions-2">Solutions to the problem</h4>

<p>
There are many potential bottlenecks, but two common problems and remedies
are as follows:
</p>

<ul>
   <li>The larger your view hierarchy, the more time the app takes to inflate
   it. Two steps you can take to address this issue are:

   <ul>
      <li>Flattening your view hierarchy by reducing redundant or nested
      layouts.</li>

      <li>Not inflating parts of the UI that do not need to be visible during
      launch. Instead, use use a {@link android.view.ViewStub} object as a
      placeholder for sub-hierarchies that the app can inflate at a more
      appropriate time.</li>
   </ul>
   </li>

   <li>Having all of your resource initialization on the main
       thread can also slow down startup. You can address this issue as follows:

   <ul>
      <li>Move all resource initialization so that the app can perform it
      lazily on a different thread.</li>
      <li>Allow the app to load and display your views, and then later
      update visual properties that are dependent on bitmaps and other
      resources.</li>
   </ul>
   </li>

<h3 id="themed">Themed launch screens</h3>


<p>
You may wish to theme your app’s loading experience, so that the app’s
launch screen is thematically consistent with the rest of the app, instead of
with the system theming. Doing so can hide a slow activity launch.
</p>

<p>
A common way to implement a themed launch screen is to use the the
{@link android.R.attr#windowDisablePreview} theme attribute to turn off
the initial blank screen
that the system process draws when launching the app. However, this approach
can result in a longer startup time than apps that don’t suppress the preview
window. Also, it forces the user to wait with no feedback while the activity
launches, making them wonder if the app is functioning properly.
</p>

<h4 id="diagnosing-3">Diagnosing the problem</h4>

<p>
You can often diagnose this problem by observing a slow response when a user
launches your app. In such a case, the screen may seem to be frozen, or to
have stopped responding to input.
</p>

<h4 id="solutions-3">Solutions to the problem</h4>

<p>
We recommend that, rather than disabling the preview window, you
follow the common
<a href="http://www.google.com/design/spec/patterns/launch-screens.html#">
Material Design</a> patterns. You can use the activity's
{@code windowBackground} theme attribute to provide a simple custom drawable
for the starting activity.
</p>

<p>
For example, you might create a new drawable file and reference it from the
layout XML and app manifest file as follows:
</p>

<p>Layout XML file:</p>

<pre>
&lt;layer-list xmlns:android="http://schemas.android.com/apk/res/android" android:opacity="opaque"&gt;
  &lt;!-- The background color, preferably the same as your normal theme --&gt;
  &lt;item android:drawable="@android:color/white"/&gt;
  &lt;!-- Your product logo - 144dp color version of your app icon --&gt;
  &lt;item&gt;
    &lt;bitmap
      android:src="@drawable/product_logo_144dp"
      android:gravity="center"/&gt;
  &lt;/item&gt;
&lt;/layer-list&gt;
</pre>

<p>Manifest file:</p>

<pre>
&lt;activity ...
android:theme="@style/AppTheme.Launcher" /&gt;
</pre>

<p>
The easiest way to transition back to your normal theme is to call
{@link android.view.ContextThemeWrapper#setTheme(int) setTheme(R.style.AppTheme)}
before calling {@code super.onCreate()} and {@code setContentView()}:
</p>

<pre class="no-pretty-print">
public class MyMainActivity extends AppCompatActivity {
  &#64;Override
  protected void onCreate(Bundle savedInstanceState) {
    // Make sure this is before calling super.onCreate
    setTheme(R.style.Theme_MyApp);
    super.onCreate(savedInstanceState);
    // ...
  }
}
</pre>