training/displaying-bitmaps/manage-memory.jd

page.title=Managing Bitmap Memory
parent.title=Displaying Bitmaps Efficiently
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@jd:body

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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}downloads/samples/DisplayingBitmaps.zip" class="button">Download the sample</a>
  <p class="filename">DisplayingBitmaps.zip</p>
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</div>
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<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;</code>.</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. However, there are certain restrictions with how
{@link android.graphics.BitmapFactory.Options#inBitmap} can be used. In particular, before Android
4.4 (API level 19), only equal sized bitmaps are supported. For details, please see the
{@link android.graphics.BitmapFactory.Options#inBitmap} documentation.

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

// If you're running on Honeycomb or newer, create a
// synchronized HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    mReusableBitmaps =
            Collections.synchronizedSet(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()) {
        synchronized (mReusableBitmaps) {
            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>static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {

    if (Build.VERSION.SDK_INT &gt;= Build.VERSION_CODES.KITKAT) {
        // From Android 4.4 (KitKat) onward we can re-use if the byte size of
        // the new bitmap is smaller than the reusable bitmap candidate
        // allocation byte count.
        int width = targetOptions.outWidth / targetOptions.inSampleSize;
        int height = targetOptions.outHeight / targetOptions.inSampleSize;
        int byteCount = width * height * getBytesPerPixel(candidate.getConfig());
        return byteCount &lt;= candidate.getAllocationByteCount();
    }

    // On earlier versions, the dimensions must match exactly and the inSampleSize must be 1
    return candidate.getWidth() == targetOptions.outWidth
            && candidate.getHeight() == targetOptions.outHeight
            && targetOptions.inSampleSize == 1;
}

/**
 * A helper function to return the byte usage per pixel of a bitmap based on its configuration.
 */
static int getBytesPerPixel(Config config) {
    if (config == Config.ARGB_8888) {
        return 4;
    } else if (config == Config.RGB_565) {
        return 2;
    } else if (config == Config.ARGB_4444) {
        return 2;
    } else if (config == Config.ALPHA_8) {
        return 1;
    }
    return 1;
}</pre>

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