training/displaying-bitmaps/manage-memory.jd

page.title=Managing Bitmap Memory
parent.title=Displaying Bitmaps Efficiently
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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#recycle">Manage Memory on Android 2.3.3 and Lower</a></li>
  <li><a href="#inBitmap">Manage Memory on Android 3.0 and Higher</a></li>
</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="http://android-developers.blogspot.com/2011/03/memory-analysis-for-android.html">Memory Analysis for Android Applications</a> blog post</li>
  <li><a href="http://www.google.com/events/io/2011/sessions/memory-management-for-android-apps.html">Memory management for Android Apps</a> Google I/O presentation</li>
  <li><a href="{@docRoot}design/patterns/swipe-views.html">Android Design: Swipe Views</a></li>
  <li><a href="{@docRoot}design/building-blocks/grid-lists.html">Android Design: Grid Lists</a></li>
</ul>

<h2>Try it out</h2>

<div class="download-box">
  <a href="{@docRoot}shareables/training/BitmapFun.zip" class="button">Download the sample</a>
  <p class="filename">BitmapFun.zip</p>
</div>

</div>
</div>

<p>In addition to the steps described in <a href="cache-bitmap.html">Caching Bitmaps</a>,
there are  specific things you can do to facilitate garbage collection
and bitmap reuse. The recommended strategy depends on which version(s)
of Android you are targeting. The {@code BitmapFun} sample app included with
this class shows you how to design your app to work efficiently across
different versions of Android.</p>

<p>To set the stage for this lesson, here is how Android's management of
bitmap memory has evolved:</p>
<ul>
  <li>
On Android Android 2.2 (API level 8) and lower, when garbage
collection occurs, your app's threads get stopped. This causes a lag that
can degrade performance.
<strong>Android 2.3 adds concurrent garbage collection, which means that
the memory is reclaimed soon after a bitmap is no longer referenced.</strong>
</li>

  <li>On Android 2.3.3 (API level 10) and lower, the backing pixel data for a
bitmap is stored in native memory. It is separate from the bitmap itself,
which is stored in the Dalvik heap. The pixel data in native memory is
not released in a predictable manner, potentially causing an application
to briefly exceed its memory limits and crash.
<strong>As of Android 3.0 (API Level 11), the pixel data is stored on the
Dalvik heap along with the associated bitmap.</strong></li>

</ul>

<p>The following sections describe how to optimize bitmap memory
management for different Android versions.</p>

<h2 id="recycle">Manage Memory on Android 2.3.3 and Lower</h2>

<p>On Android 2.3.3 (API level 10) and lower, using
{@link android.graphics.Bitmap#recycle recycle()}
is recommended. If you're displaying large amounts of bitmap data in your app,
you're likely to run into
{@link java.lang.OutOfMemoryError} errors. The
{@link android.graphics.Bitmap#recycle recycle()} method allows an app
to reclaim memory as soon as possible.</p>

<p class="note"><strong>Caution:</strong> You should use
{@link android.graphics.Bitmap#recycle recycle()} only when you are sure that the
bitmap is no longer being used. If you call {@link android.graphics.Bitmap#recycle recycle()}
and later attempt to draw the bitmap, you will get the error:
{@code &quot;Canvas: trying to use a recycled bitmap&quot;}.</p>

<p>The following code snippet gives an example of calling
{@link android.graphics.Bitmap#recycle recycle()}. It uses reference counting
(in the variables {@code mDisplayRefCount} and {@code mCacheRefCount}) to track
whether a bitmap is currently being displayed or in the cache. The
code recycles the bitmap when these conditions are met:</p>

<ul>
<li>The reference count for both {@code mDisplayRefCount} and
{@code mCacheRefCount} is 0.</li>
<li>The bitmap is not {@code null}, and it hasn't been recycled yet.</li>
</ul>

<pre>private int mCacheRefCount = 0;
private int mDisplayRefCount = 0;
...
// Notify the drawable that the displayed state has changed.
// Keep a count to determine when the drawable is no longer displayed.
public void setIsDisplayed(boolean isDisplayed) {
    synchronized (this) {
        if (isDisplayed) {
            mDisplayRefCount++;
            mHasBeenDisplayed = true;
        } else {
            mDisplayRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

// Notify the drawable that the cache state has changed.
// Keep a count to determine when the drawable is no longer being cached.
public void setIsCached(boolean isCached) {
    synchronized (this) {
        if (isCached) {
            mCacheRefCount++;
        } else {
            mCacheRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

private synchronized void checkState() {
    // If the drawable cache and display ref counts = 0, and this drawable
    // has been displayed, then recycle.
    if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
            && hasValidBitmap()) {
        getBitmap().recycle();
    }
}

private synchronized boolean hasValidBitmap() {
    Bitmap bitmap = getBitmap();
    return bitmap != null && !bitmap.isRecycled();
}</pre>

<h2 id="inBitmap">Manage Memory on Android 3.0 and Higher</h2>

<p>Android 3.0 (API Level 11) introduces the
{@link android.graphics.BitmapFactory.Options#inBitmap BitmapFactory.Options.inBitmap}
field. If this option is set, decode methods that take the
{@link android.graphics.BitmapFactory.Options Options} object
will attempt to reuse an existing bitmap when loading content. This means
that the bitmap's memory is reused, resulting in improved performance, and
removing both memory allocation and de-allocation. There are some caveats in using
{@link android.graphics.BitmapFactory.Options#inBitmap}:</p>
<ul>
  <li>The reused bitmap must be of the same size as the source content (to make
sure that the same amount of memory is used), and in JPEG or PNG format
(whether as a resource or as a stream).</li>


<li>The {@link android.graphics.Bitmap.Config configuration} of the reused bitmap
overrides the setting of
{@link android.graphics.BitmapFactory.Options#inPreferredConfig}, if set. </li>

  <li>You should always use the returned bitmap of the decode method,
because you can't assume that reusing the bitmap worked (for example, if there is
a size mismatch).</li>

<h3>Save a bitmap for later use</h3>

<p>The following snippet demonstrates how an existing bitmap is stored for possible
later use in the sample app. When an app is running on Android 3.0 or higher and
a bitmap is evicted from the {@link android.util.LruCache},
a soft reference to the bitmap is placed
in a {@link java.util.HashSet}, for possible reuse later with
{@link android.graphics.BitmapFactory.Options#inBitmap}:

<pre>HashSet&lt;SoftReference&lt;Bitmap&gt;&gt; mReusableBitmaps;
private LruCache&lt;String, BitmapDrawable&gt; mMemoryCache;

// If you're running on Honeycomb or newer, create
// a HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    mReusableBitmaps = new HashSet&lt;SoftReference&lt;Bitmap&gt;&gt;();
}

mMemoryCache = new LruCache&lt;String, BitmapDrawable&gt;(mCacheParams.memCacheSize) {

    // Notify the removed entry that is no longer being cached.
    &#64;Override
    protected void entryRemoved(boolean evicted, String key,
            BitmapDrawable oldValue, BitmapDrawable newValue) {
        if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been removed from the memory cache.
            ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
        } else {
            // The removed entry is a standard BitmapDrawable.
            if (Utils.hasHoneycomb()) {
                // We're running on Honeycomb or later, so add the bitmap
                // to a SoftReference set for possible use with inBitmap later.
                mReusableBitmaps.add
                        (new SoftReference&lt;Bitmap&gt;(oldValue.getBitmap()));
            }
        }
    }
....
}</pre>


<h3>Use an existing bitmap</h3>
<p>In the running app, decoder methods check to see if there is an existing
bitmap they can use. For example:</p>

<pre>public static Bitmap decodeSampledBitmapFromFile(String filename,
        int reqWidth, int reqHeight, ImageCache cache) {

    final BitmapFactory.Options options = new BitmapFactory.Options();
    ...
    BitmapFactory.decodeFile(filename, options);
    ...

    // If we're running on Honeycomb or newer, try to use inBitmap.
    if (Utils.hasHoneycomb()) {
        addInBitmapOptions(options, cache);
    }
    ...
    return BitmapFactory.decodeFile(filename, options);
}</pre

<p>The next snippet shows the {@code addInBitmapOptions()} method that is called in the
above snippet. It looks for an existing bitmap to set as the value for
{@link android.graphics.BitmapFactory.Options#inBitmap}. Note that this
method only sets a value for {@link android.graphics.BitmapFactory.Options#inBitmap}
if it finds a suitable match (your code should never assume that a match will be found):</p>

<pre>private static void addInBitmapOptions(BitmapFactory.Options options,
        ImageCache cache) {
    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true;

    if (cache != null) {
        // Try to find a bitmap to use for inBitmap.
        Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

        if (inBitmap != null) {
            // If a suitable bitmap has been found, set it as the value of
            // inBitmap.
            options.inBitmap = inBitmap;
        }
    }
}

// This method iterates through the reusable bitmaps, looking for one
// to use for inBitmap:
protected Bitmap getBitmapFromReusableSet(BitmapFactory.Options options) {
        Bitmap bitmap = null;

    if (mReusableBitmaps != null && !mReusableBitmaps.isEmpty()) {
        final Iterator&lt;SoftReference&lt;Bitmap&gt;&gt; iterator
                = mReusableBitmaps.iterator();
        Bitmap item;

        while (iterator.hasNext()) {
            item = iterator.next().get();

            if (null != item && item.isMutable()) {
                // Check to see it the item can be used for inBitmap.
                if (canUseForInBitmap(item, options)) {
                    bitmap = item;

                    // Remove from reusable set so it can't be used again.
                    iterator.remove();
                    break;
                }
            } else {
                // Remove from the set if the reference has been cleared.
                iterator.remove();
            }
        }
    }
    return bitmap;
}</pre>

<p>Finally, this method determines whether a candidate bitmap
satisfies the size criteria to be used for
{@link android.graphics.BitmapFactory.Options#inBitmap}:</p>

<pre>private static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {
    int width = targetOptions.outWidth / targetOptions.inSampleSize;
    int height = targetOptions.outHeight / targetOptions.inSampleSize;

    // Returns true if "candidate" can be used for inBitmap re-use with
    // "targetOptions".
    return candidate.getWidth() == width && candidate.getHeight() == height;
}</pre>

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