docs/html/resources/articles/avoiding-memory-leaks.jd

page.title=Avoiding Memory Leaks
@jd:body


<p>Android applications are, at least on the T-Mobile G1, limited
to 16 MB of heap. It's both a lot of memory for a phone and yet very
little for what some developers want to achieve. Even if you do not
plan on using all of this memory, you should use as little as possible
to let other applications run without getting them killed. The more
applications Android can keep in memory, the faster it will be for the
user to switch between his apps. As part of my job, I ran into memory
leaks issues in Android applications and they are most of the time due
to the same mistake: keeping a long-lived reference to a 
{@link android.content.Context Context}.</p>

<p>On Android, a <code>Context</code> is used for many operations
 but mostly to load and access resources. This is why all the widgets 
receive a <code>Context</code> parameter in their constructor. In a 
regular Android application, you usually have two kinds of 
<code>Context</code>, {@link android.app.Activity} and 
{@link android.app.Application}. It's usually the first one that 
the developer passes to classes and methods that need a <code>Context</code>:</p>

<pre class="prettyprint">&#64;Override
protected void onCreate(Bundle state) {
  super.onCreate(state);
  
  TextView label = new TextView(this);
  label.setText("Leaks are bad");
  
  setContentView(label);
}
</pre>

<p>This means that views have a reference to the entire activity and
therefore to anything your activity is holding onto; usually the entire
View hierarchy and all its resources. Therefore, if you leak the <code>Context</code>
("leak" meaning you keep a reference to it thus preventing the GC from
collecting it), you leak a lot of memory. Leaking an entire activity
can be really easy if you're not careful.</p>

<p>When the screen orientation changes the system will, by default,
destroy the current activity and create a new one while preserving its
state. In doing so, Android will reload the application's UI from the
resources. Now imagine you wrote an application with a large bitmap
that you don't want to load on every rotation. The easiest way to keep
it around and not having to reload it on every rotation is to keep in a
static field:</p>

<pre class="prettyprint">private static Drawable sBackground;
  
&#64;Override
protected void onCreate(Bundle state) {
  super.onCreate(state);
  
  TextView label = new TextView(this);
  label.setText("Leaks are bad");
  
  if (sBackground == null) {
    sBackground = getDrawable(R.drawable.large_bitmap);
  }
  label.setBackgroundDrawable(sBackground);
  
  setContentView(label);
}
</pre> 

<p>This code is very fast and also very wrong; it leaks the first activity 
created upon the first screen orientation change. When a 
{@link android.graphics.drawable.Drawable Drawable} is attached to a view, the view is set as a 
{@link android.graphics.drawable.Drawable#setCallback(android.graphics.drawable.Drawable.Callback) callback} 
on the drawable. In the code snippet above, this means the drawable has a 
reference to the <code>TextView</code> which itself has a reference to the 
activity (the <code>Context</code>) which in turns has references to 
pretty much anything (depending on your code.)</p> 
 
<p>This example is one of the simplest cases of leaking the 
<code>Context</code> and you can see how we worked around it in the 
<a href="http://android.git.kernel.org/?p=platform/packages/apps/Launcher.git;a=blob;f=src/com/android/launcher/LauncherModel.java;h=0ef2a806b767142b28b2ff3b37f21f4ca16c355d;hb=cupcake">Home screen's source code</a>
(look for the <code>unbindDrawables()</code> method) by setting the stored 
drawables' callbacks to null when the activity is destroyed. Interestingly 
enough, there are cases where you can create a chain of leaked contexts, 
and they are bad. They make you run out of memory rather quickly.</p> 
 
<p>There are two easy ways to avoid context-related memory leaks. The most 
obvious one is to avoid escaping the context outside of its own scope. The 
example above showed the case of a static reference but inner classes and 
their implicit reference to the outer class can be equally dangerous. The 
second solution is to use the <code>Application</code> context. This 
context will live as long as your application is alive and does not depend 
on the activities life cycle. If you plan on keeping long-lived objects 
that need a context, remember the application object. You can obtain it 
easily by calling 
{@link android.content.Context#getApplicationContext() Context.getApplicationContext()} 
or {@link android.app.Activity#getApplication() Activity.getApplication()}.</p> 
 
<p>In summary, to avoid context-related memory leaks, remember the following:</p> 
<ul> 
<li>Do not keep long-lived references to a context-activity (a reference 
to an activity should have the same life cycle as the activity itself)</li> 
<li>Try using the context-application instead of a context-activity</li> 
<li>Avoid non-static inner classes in an activity if you don't control 
their life cycle, use a static inner class and make a weak reference to 
the activity inside. The solution to this issue is to use a static inner 
class with a {@link java.lang.ref.WeakReference WeakReference} to the 
outer class, as done in <a href="http://android.git.kernel.org/?p=platform/frameworks/base.git;a=blob;f=core/java/android/view/ViewRoot.java;h=9d7a124cb01ab94bf53e34f6e5e8a07f81e2423c;hb=master">ViewRoot</a>
and its W inner class for instance</li> 
<li>A garbage collector is not an insurance against memory leaks</li> 
</ul>