• Bitmap 是怎么开辟内存的？内存是怎么复用和销毁的？本地资源图片应该怎么去做适配？
• 打开我们自己的 APP 发现占用内存较大的一般都是本地资源图片，我们该如何去优化这些内存？
• 大家以后如果有涉及直播这一块的业务，直播间会有各种活动和各种复杂动画，线上 buggly 肯定会有大量的 OOM ，我们怎样才能在 OOM 前去 dump 线上内存来做优化分析？

Bitmap 我们是再熟悉不过了，首先抛几个问题让我们一起来思考一下，如果以上几个问题大家都能找到解决方案，相信我们在以后的开发过程中会省下许多事。

1. Bitmap 的内存大小

不知大家是否还记得在 Android图片压缩加密上传 - JPEG压缩算法解析 这篇文章中有一个题目：一张 864x582 的 PNG 图片，我把它放到 drawable-xhdpi 目录下，在红米 Note3 上加载，占用内存是多少(1920x1080像素 ，5.5英寸 )？ 我们清晰的知道 图片大小 = 宽 * 高 * 单个像素点所占字节数，那么这么一算大小应该是 864x582x4 = 2011392 ，但最终调用代码 Bitmap.getByteCount() 发现是 3465504。 难道我们刚刚所讲的公式不对？其实这里的宽高是 Bitmap 的宽高并不是资源图片的宽高：

Bitmap 大小 = bitmap.getWidth() * bitmap.getHeight() * 单个像素点所占字节数 = 1134 * 764 * 4 = 3465504。

那这里的宽高是怎样计算而来的？我们通过跟踪源码发现 width 和 height 都是在 Bitmap 的构造函数中赋值的：

    /**
     * Private constructor that must received an already allocated native bitmap
     * int (pointer).
     */
    // called from JNI
    Bitmap(long nativeBitmap, int width, int height, int density,
            boolean isMutable, boolean requestPremultiplied,
            byte[] ninePatchChunk, NinePatch.InsetStruct ninePatchInsets) {
        if (nativeBitmap == 0) {
            throw new RuntimeException("internal error: native bitmap is 0");
        }

        mWidth = width;
        mHeight = height;
        mIsMutable = isMutable;
        mRequestPremultiplied = requestPremultiplied;

        mNinePatchChunk = ninePatchChunk;
        mNinePatchInsets = ninePatchInsets;
        if (density >= 0) {
            mDensity = density;
        }

        mNativePtr = nativeBitmap;
        long nativeSize = NATIVE_ALLOCATION_SIZE + getAllocationByteCount();
        NativeAllocationRegistry registry = new NativeAllocationRegistry(
            Bitmap.class.getClassLoader(), nativeGetNativeFinalizer(), nativeSize);
        registry.registerNativeAllocation(this, nativeBitmap);

        if (ResourcesImpl.TRACE_FOR_DETAILED_PRELOAD) {
            sPreloadTracingNumInstantiatedBitmaps++;
            sPreloadTracingTotalBitmapsSize += nativeSize;
        }
    }

called from JNI 这个解释其实已经很明确了，也就是说这个对象是 Native 层构建返回的。因此我们跟踪到 BitmapFactory.decodeResource() 中去看看：

    public static Bitmap decodeResource(Resources res, int id, Options opts) {
        validate(opts);
        Bitmap bm = null;
        InputStream is = null; 
        
        try {
            final TypedValue value = new TypedValue();
            is = res.openRawResource(id, value);

            bm = decodeResourceStream(res, value, is, null, opts);
        } catch (Exception e) {
            /*  do nothing.
                If the exception happened on open, bm will be null.
                If it happened on close, bm is still valid.
            */
        } finally {
            try {
                if (is != null) is.close();
            } catch (IOException e) {
                // Ignore
            }
        }

        if (bm == null && opts != null && opts.inBitmap != null) {
            throw new IllegalArgumentException("Problem decoding into existing bitmap");
        }

        return bm;
    }

    public static Bitmap decodeResourceStream(@Nullable Resources res, @Nullable TypedValue value,
            @Nullable InputStream is, @Nullable Rect pad, @Nullable Options opts) {
        validate(opts);
        if (opts == null) {
            opts = new Options();
        }
       
        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {
                opts.inDensity = density;
            }
        }
        // 获取当前手机设备的 dpi 
        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }
        
        return decodeStream(is, pad, opts);
    }

    // 省略部分跟踪代码 ......

    private static native Bitmap nativeDecodeStream(InputStream is, byte[] storage,
            Rect padding, Options opts);

最终调用的是 native 方法 nativeDecodeStream ，这部分源码建议大家在 http://androidxref.com/ 上看，因为不同版本之间有差异，我们不能只看一个版本。当然也可以每个版本都去下载，但需要一百多G的内存。这里以 Android N 版本为例:

/frameworks/base/core/jni/android/graphics/BitmapFactory.cpp

static jobject nativeDecodeStream(JNIEnv *env, jobject clazz, jobject is, jbyteArray storage,
                                  jobject padding, jobject options) {
    jobject bitmap = NULL;
    std::unique_ptr<SkStream> stream(CreateJavaInputStreamAdaptor(env, is, storage));

    if (stream.get()) {
        std::unique_ptr<SkStreamRewindable> bufferedStream(
                SkFrontBufferedStream::Create(stream.release(), SkCodec::MinBufferedBytesNeeded()));
        SkASSERT(bufferedStream.get() != NULL);
        bitmap = doDecode(env, bufferedStream.release(), padding, options);
    }
    return bitmap;
}

static jobject doDecode(JNIEnv *env, SkStreamRewindable *stream, jobject padding, jobject options) {
    // This function takes ownership of the input stream.  Since the SkAndroidCodec
    // will take ownership of the stream, we don't necessarily need to take ownership
    // here.  This is a precaution - if we were to return before creating the codec,
    // we need to make sure that we delete the stream.
    std::unique_ptr<SkStreamRewindable> streamDeleter(stream);

    // Set default values for the options parameters.
    int sampleSize = 1;
    // 是否只是获取图片的大小
    bool onlyDecodeSize = false;
    SkColorType prefColorType = kN32_SkColorType;
    bool isMutable = false;
    float scale = 1.0f;
    bool requireUnpremultiplied = false;
    jobject javaBitmap = NULL;

    // Update with options supplied by the client.
    // 解析 options 参数
    if (options != NULL) {
        sampleSize = env->GetIntField(options, gOptions_sampleSizeFieldID);
        // Correct a non-positive sampleSize.  sampleSize defaults to zero within the
        // options object, which is strange.
        if (sampleSize <= 0) {
            sampleSize = 1;
        }

        if (env->GetBooleanField(options, gOptions_justBoundsFieldID)) {
            onlyDecodeSize = true;
        }

        // initialize these, in case we fail later on
        env->SetIntField(options, gOptions_widthFieldID, -1);
        env->SetIntField(options, gOptions_heightFieldID, -1);
        env->SetObjectField(options, gOptions_mimeFieldID, 0);
        // 解析 ColorType ，复用参数等等
        jobject jconfig = env->GetObjectField(options, gOptions_configFieldID);
        prefColorType = GraphicsJNI::getNativeBitmapColorType(env, jconfig);
        isMutable = env->GetBooleanField(options, gOptions_mutableFieldID);
        requireUnpremultiplied = !env->GetBooleanField(options, gOptions_premultipliedFieldID);
        javaBitmap = env->GetObjectField(options, gOptions_bitmapFieldID);
        // 计算缩放的比例
        if (env->GetBooleanField(options, gOptions_scaledFieldID)) {
            // 获取图片当前 xhdpi 的 density
            const int density = env->GetIntField(options, gOptions_densityFieldID);
            // 获取当前设备的 dpi
            const int targetDensity = env->GetIntField(options, gOptions_targetDensityFieldID);
            const int screenDensity = env->GetIntField(options, gOptions_screenDensityFieldID);
            if (density != 0 && targetDensity != 0 && density != screenDensity) {
                // scale = 当前设备的 dpi / xhdpi 的 density
                // scale = 420/320 = 1.3125
                scale = (float) targetDensity / density;
            }
        }
    }

    // Create the codec.
    NinePatchPeeker peeker;
    std::unique_ptr<SkAndroidCodec> codec(SkAndroidCodec::NewFromStream(streamDeleter.release(),
                                                                        280 & peeker));
    if (!codec.get()) {
        return nullObjectReturn("SkAndroidCodec::NewFromStream returned null");
    }

    // Do not allow ninepatch decodes to 565.  In the past, decodes to 565
    // would dither, and we do not want to pre-dither ninepatches, since we
    // know that they will be stretched.  We no longer dither 565 decodes,
    // but we continue to prevent ninepatches from decoding to 565, in order
    // to maintain the old behavior.
    if (peeker.mPatch && kRGB_565_SkColorType == prefColorType) {
        prefColorType = kN32_SkColorType;
    }
    // 获取当前图片的大小
    // Determine the output size.
    SkISize size = codec->getSampledDimensions(sampleSize);

    int scaledWidth = size.width();
    int scaledHeight = size.height();
    bool willScale = false;
    // 处理 simpleSize 压缩，我们这里没穿，上面默认是 1 
    // Apply a fine scaling step if necessary.
    if (needsFineScale(codec->getInfo().dimensions(), size, sampleSize)) {
        willScale = true;
        scaledWidth = codec->getInfo().width() / sampleSize;
        scaledHeight = codec->getInfo().height() / sampleSize;
    }

    // Set the options and return if the client only wants the size.
    if (options != NULL) {
        jstring mimeType = encodedFormatToString(env, codec->getEncodedFormat());
        if (env->ExceptionCheck()) {
            return nullObjectReturn("OOM in encodedFormatToString()");
        }
        // 设置 options 对象中的 outWidth 和 outHeight
        env->SetIntField(options, gOptions_widthFieldID, scaledWidth);
        env->SetIntField(options, gOptions_heightFieldID, scaledHeight);
        env->SetObjectField(options, gOptions_mimeFieldID, mimeType);
        // 如果只是获取大小直接 return null 这里是 nullptr 而不是 NULL
        if (onlyDecodeSize) {
            return nullptr;
        }
    }

    // Scale is necessary due to density differences.
    if (scale != 1.0f) {
        willScale = true;
        // 计算 scaledWidth 和 scaledHeight
        // scaledWidth = 864 * 1.3125 + 0.5f = 1134 + 0.5f = 1134
        scaledWidth = static_cast<int>(scaledWidth * scale + 0.5f);
        // scaledHeight = 582 * 1.3125 + 0.5f = 763.875 + 0.5f = 764
        scaledHeight = static_cast<int>(scaledHeight * scale + 0.5f);
    }
    // 判断是否有复用的 Bitmap
    android::Bitmap *reuseBitmap = nullptr;
    unsigned int existingBufferSize = 0;
    if (javaBitmap != NULL) {
        reuseBitmap = GraphicsJNI::getBitmap(env, javaBitmap);
        if (reuseBitmap->peekAtPixelRef()->isImmutable()) {
            // 无法重用一个不变的位图图像解码器的目标。
            ALOGW("Unable to reuse an immutable bitmap as an image decoder target.");
            javaBitmap = NULL;
            reuseBitmap = nullptr;
        } else {
            existingBufferSize = GraphicsJNI::getBitmapAllocationByteCount(env, javaBitmap);
        }
    }
    
    JavaPixelAllocator javaAllocator(env);
    RecyclingPixelAllocator recyclingAllocator(reuseBitmap, existingBufferSize);
    ScaleCheckingAllocator scaleCheckingAllocator(scale, existingBufferSize);
    SkBitmap::HeapAllocator heapAllocator;
    SkBitmap::Allocator *decodeAllocator;
    if (javaBitmap != nullptr && willScale) {
        // This will allocate pixels using a HeapAllocator, since there will be an extra
        // scaling step that copies these pixels into Java memory.  This allocator
        // also checks that the recycled javaBitmap is large enough.
        decodeAllocator = &scaleCheckingAllocator;
    } else if (javaBitmap != nullptr) {
        decodeAllocator = &recyclingAllocator;
    } else if (willScale) {
        // This will allocate pixels using a HeapAllocator, since there will be an extra
        // scaling step that copies these pixels into Java memory.
        decodeAllocator = &heapAllocator;
    } else {
        decodeAllocator = &javaAllocator;
    }

    // Set the decode colorType.  This is necessary because we can't always support
    // the requested colorType.
    SkColorType decodeColorType = codec->computeOutputColorType(prefColorType);

    // Construct a color table for the decode if necessary
    SkAutoTUnref <SkColorTable> colorTable(nullptr);
    SkPMColor *colorPtr = nullptr;
    int *colorCount = nullptr;
    int maxColors = 256;
    SkPMColor colors[256];
    if (kIndex_8_SkColorType == decodeColorType) {
        colorTable.reset(new SkColorTable(colors, maxColors));

        // SkColorTable expects us to initialize all of the colors before creating an
        // SkColorTable.  However, we are using SkBitmap with an Allocator to allocate
        // memory for the decode, so we need to create the SkColorTable before decoding.
        // It is safe for SkAndroidCodec to modify the colors because this SkBitmap is
        // not being used elsewhere.
        colorPtr = const_cast<SkPMColor *>(colorTable->readColors());
        colorCount = &maxColors;
    }

    // Set the alpha type for the decode.
    SkAlphaType alphaType = codec->computeOutputAlphaType(requireUnpremultiplied);
    // 创建 SkImageInfo 信息，宽，高，ColorType，alphaType
    const SkImageInfo decodeInfo = SkImageInfo::Make(size.width(), size.height(), decodeColorType,
                                                     alphaType);
    SkImageInfo bitmapInfo = decodeInfo;
    if (decodeColorType == kGray_8_SkColorType) {
        // The legacy implementation of BitmapFactory used kAlpha8 for
        // grayscale images (before kGray8 existed).  While the codec
        // recognizes kGray8, we need to decode into a kAlpha8 bitmap
        // in order to avoid a behavior change.
        bitmapInfo = SkImageInfo::MakeA8(size.width(), size.height());
    }
    // 解析 SkBitmap 设置 bitmapInfo，tryAllocPixels 开辟内存，具体分析在后面 
    SkBitmap decodingBitmap;
    if (!decodingBitmap.setInfo(bitmapInfo) ||
        !decodingBitmap.tryAllocPixels(decodeAllocator, colorTable)) {
        // SkAndroidCodec should recommend a valid SkImageInfo, so setInfo()
        // should only only fail if the calculated value for rowBytes is too
        // large.
        // tryAllocPixels() can fail due to OOM on the Java heap, OOM on the
        // native heap, or the recycled javaBitmap being too small to reuse.
        return nullptr;
    }

    // Use SkAndroidCodec to perform the decode.
    SkAndroidCodec::AndroidOptions codecOptions;
    codecOptions.fZeroInitialized = (decodeAllocator == &javaAllocator) ?
    SkCodec::kYes_ZeroInitialized : SkCodec::kNo_ZeroInitialized;
    codecOptions.fColorPtr = colorPtr;
    codecOptions.fColorCount = colorCount;
    codecOptions.fSampleSize = sampleSize;
    // 解析获取像素值
    SkCodec::Result result = codec->getAndroidPixels(decodeInfo, decodingBitmap.getPixels(),
                                                     decodingBitmap.rowBytes(), &codecOptions);
    switch (result) {
        case SkCodec::kSuccess:
        case SkCodec::kIncompleteInput:
            break;
        default:
            return nullObjectReturn("codec->getAndroidPixels() failed.");
    }

    jbyteArray ninePatchChunk = NULL;
    if (peeker.mPatch != NULL) {
        if (willScale) {
            scaleNinePatchChunk(peeker.mPatch, scale, scaledWidth, scaledHeight);
        }

        size_t ninePatchArraySize = peeker.mPatch->serializedSize();
        ninePatchChunk = env->NewByteArray(ninePatchArraySize);
        if (ninePatchChunk == NULL) {
            return nullObjectReturn("ninePatchChunk == null");
        }

        jbyte *array = (jbyte *) env->GetPrimitiveArrayCritical(ninePatchChunk, NULL);
        if (array == NULL) {
            return nullObjectReturn("primitive array == null");
        }

        memcpy(array, peeker.mPatch, peeker.mPatchSize);
        env->ReleasePrimitiveArrayCritical(ninePatchChunk, array, 0);
    }

    jobject ninePatchInsets = NULL;
    if (peeker.mHasInsets) {
        ninePatchInsets = env->NewObject(gInsetStruct_class, gInsetStruct_constructorMethodID,
        peeker.mOpticalInsets[0], peeker.mOpticalInsets[1], peeker.mOpticalInsets[2], peeker.mOpticalInsets[3],
                peeker.mOutlineInsets[0], peeker.mOutlineInsets[1], peeker.mOutlineInsets[2], peeker.mOutlineInsets[3],
                peeker.mOutlineRadius, peeker.mOutlineAlpha, scale);
        if (ninePatchInsets == NULL) {
            return nullObjectReturn("nine patch insets == null");
        }
        if (javaBitmap != NULL) {
            env->SetObjectField(javaBitmap, gBitmap_ninePatchInsetsFieldID, ninePatchInsets);
        }
    }
    // 构建 SkBitmap 这个才是最终的
    SkBitmap outputBitmap;
    if (willScale) {
        // 如果需要缩放，那需要重新创建一张图片，上面加载的是图片的本身大小
        // This is weird so let me explain: we could use the scale parameter
        // directly, but for historical reasons this is how the corresponding
        // Dalvik code has always behaved. We simply recreate the behavior here.
        // The result is slightly different from simply using scale because of
        // the 0.5f rounding bias applied when computing the target image size
        const float sx = scaledWidth / float(decodingBitmap.width());
        const float sy = scaledHeight / float(decodingBitmap.height());

        // Set the allocator for the outputBitmap.
        SkBitmap::Allocator *outputAllocator;
        if (javaBitmap != nullptr) {
            outputAllocator = &recyclingAllocator;
        } else {
            outputAllocator = &javaAllocator;
        }

        SkColorType scaledColorType = colorTypeForScaledOutput(decodingBitmap.colorType());
        // FIXME: If the alphaType is kUnpremul and the image has alpha, the
        // colors may not be correct, since Skia does not yet support drawing
        // to/from unpremultiplied bitmaps.
        // 设置 SkImageInfo ，注意这里是 scaledWidth ，scaledHeight 
        outputBitmap.setInfo(SkImageInfo::Make(scaledWidth, scaledHeight,
        scaledColorType, decodingBitmap.alphaType()));
        // 开辟当前 Bitmap 图片的内存
        if (!outputBitmap.tryAllocPixels(outputAllocator, NULL)) {
            // This should only fail on OOM.  The recyclingAllocator should have
            // enough memory since we check this before decoding using the
            // scaleCheckingAllocator.
            return nullObjectReturn("allocation failed for scaled bitmap");
        }

        SkPaint paint;
        // kSrc_Mode instructs us to overwrite the unininitialized pixels in
        // outputBitmap.  Otherwise we would blend by default, which is not
        // what we want.
        paint.setXfermodeMode(SkXfermode::kSrc_Mode);
        paint.setFilterQuality(kLow_SkFilterQuality);
        // decodingBitmap -> 画到 outputBitmap
        SkCanvas canvas(outputBitmap);
        canvas.scale(sx, sy);
        canvas.drawBitmap(decodingBitmap, 0.0f, 0.0f, &paint);
    } else {
        outputBitmap.swap(decodingBitmap);
    }

    if (padding) {
        if (peeker.mPatch != NULL) {
            GraphicsJNI::set_jrect(env, padding,
            peeker.mPatch->paddingLeft, peeker.mPatch->paddingTop,
            peeker.mPatch->paddingRight, peeker.mPatch->paddingBottom);
        } else {
            GraphicsJNI::set_jrect(env, padding, -1, -1, -1, -1);
        }
    }

    // If we get here, the outputBitmap should have an installed pixelref.
    if (outputBitmap.pixelRef() == NULL) {
        return nullObjectReturn("Got null SkPixelRef");
    }
    
    if (!isMutable && javaBitmap == NULL) {
        // promise we will never change our pixels (great for sharing and pictures)
        outputBitmap.setImmutable();
    }
    // 如果有复用返回原来的 javaBitmap
    bool isPremultiplied = !requireUnpremultiplied;
    if (javaBitmap != nullptr) {
        GraphicsJNI::reinitBitmap(env, javaBitmap, outputBitmap.info(), isPremultiplied);
        outputBitmap.notifyPixelsChanged();
        // If a java bitmap was passed in for reuse, pass it back
        return javaBitmap;
    }

    int bitmapCreateFlags = 0x0;
    if (isMutable) bitmapCreateFlags |= GraphicsJNI::kBitmapCreateFlag_Mutable;
    if (isPremultiplied) bitmapCreateFlags |= GraphicsJNI::kBitmapCreateFlag_Premultiplied;
    // 没有复用的 Bitmap 创建一个新的 Bitmap
    // now create the java bitmap
    return GraphicsJNI::createBitmap(env, javaAllocator.getStorageObjAndReset(),
    bitmapCreateFlags, ninePatchChunk, ninePatchInsets, -1);
}

jobject GraphicsJNI::createBitmap(JNIEnv *env, android::Bitmap *bitmap,
                                  int bitmapCreateFlags, jbyteArray ninePatchChunk,
                                  jobject ninePatchInsets,
                                  int density) {
    bool isMutable = bitmapCreateFlags & kBitmapCreateFlag_Mutable;
    bool isPremultiplied = bitmapCreateFlags & kBitmapCreateFlag_Premultiplied;
    // The caller needs to have already set the alpha type properly, so the
    // native SkBitmap stays in sync with the Java Bitmap.
    assert_premultiplied(bitmap->info(), isPremultiplied);

    jobject obj = env->NewObject(gBitmap_class, gBitmap_constructorMethodID,
    reinterpret_cast<jlong>(bitmap), bitmap->javaByteArray(),
            bitmap->width(), bitmap->height(), density, isMutable, isPremultiplied,
            ninePatchChunk, ninePatchInsets);
    hasException(env); // For the side effect of logging.
    return obj;
}

上面的代码看起来比较长，其实是非常简单的，相信大家都能看得懂，这里我对上面的流程再做一些总结：

1. 解析 java 层传递过来的 Options 的参数，如 simpleSize ，isMutable，javaBitmap 等等，同时计算出 scale 。
2. 获取当前图片的大小，根据 sampleSize 判断是否需要压缩，同时计算出 scaledWidth ，scaledHeight。
3. 设置 options 宽高为 scaledWidth ，scaledHeight ，如果只是解析宽高那么就直接返回，也就是 options.inJustDecodeBounds = true 时，但是这里需要注意返回的是，资源图片的宽高并不是 Bitmap 最终的宽高。（我们大部分人对这个有误解）
4. 创建 native 层的 SkImageInfo 和 SkBitmap ，然后调用 tryAllocPixels 去开辟图片的内存空间，然后调用 getAndroidPixels 去解析像素值 ，这里的 decodingBitmap 也并不是最终需要返回的 Bitmap ，而是原资源图片的 Bitmap 。
5. 构建需要返回的 outputBitmap ，如果需要缩放那么重新去开辟一块内存空间，如果不需要缩放直接调用 swap 方法即可。最后判断有没有复用的 JavaBitmap ,如果有复用调用 reinitBitmap 然后直接返回，如果没有则调用 createBitmap 去创建一个新的 Bitmap 。

通过上面的分析，我们可能会有疑问？我们调用了两次 tryAllocPixels ，那如果加载一张 (1440x2560) 10M 的图片，岂不是需要 20M 的内存？

温馨提示：有 Java 内存和 Native 内存之分。

2. Bitmap 的内存申请

Bitmap 的内存申请不同版本间有些许差异，在 3.0-7.0 的 bitmap 像素内存都是存放在 Java heap 中的，而 8.0 以后则是放在 Native heap 中的，我们可能会想这有啥区别？请看一个简单的事例：

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        logMemory();

        Bitmap bitmap = Bitmap.createBitmap(1024, 1024 * 500, Bitmap.Config.ARGB_8888);

        logMemory();
    }
    
    private void logMemory() {
        ActivityManager activityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
        ActivityManager.MemoryInfo memoryInfo = new ActivityManager.MemoryInfo();
        activityManager.getMemoryInfo(memoryInfo);
        Log.e("TAG", "AvailMem :" + memoryInfo.availMem / 1024 / 1024);
        Log.e("TAG", "lowMemory：" + memoryInfo.lowMemory);
        Log.e("TAG", "NativeHeapAllocatedSize :" + Debug.getNativeHeapAllocatedSize() / 1024 / 1024);
    }

上面我们创建了一张 2G 大小的 bitmap 我们在 8.0 以下的版本运行是会 OOM 的，而我们在 8.0 以上的版本运行是完全没问题，但 Native 内存多了 2G 的内存。

E/TAG: AvailMem :1654
E/TAG: lowMemory：false
E/TAG: NativeHeapAllocatedSize :4

E/TAG: AvailMem :1656
E/TAG: lowMemory：false
E/TAG: NativeHeapAllocatedSize :2052

通过之前的源码分析可知 bitmap 的内存创建都是通过 tryAllocPixels 方法来申请的，我们通过源码来对比一下他们之间的区别，我们首先来看下 7.0 的代码：

/frameworks/base/core/jni/android/graphics/Bitmap.cpp

bool SkBitmap::tryAllocPixels(Allocator *allocator, SkColorTable *ctable) {
    HeapAllocator stdalloc;

    if (nullptr == allocator) {
        allocator = &stdalloc;
    }
    return allocator->allocPixelRef(this, ctable);
}

bool JavaPixelAllocator::allocPixelRef(SkBitmap *bitmap, SkColorTable *ctable) {
    JNIEnv *env = vm2env(mJavaVM);

    mStorage = GraphicsJNI::allocateJavaPixelRef(env, bitmap, ctable);
    return mStorage != nullptr;
}

android::Bitmap *GraphicsJNI::allocateJavaPixelRef(JNIEnv *env, SkBitmap *bitmap,
                                                    SkColorTable *ctable) {
    const SkImageInfo &info = bitmap->info();
    if (info.colorType() == kUnknown_SkColorType) {
        doThrowIAE(env, "unknown bitmap configuration");
        return NULL;
    }

    size_t size;
    if (!computeAllocationSize(*bitmap, &size)) {
        return NULL;
    }

    // we must respect the rowBytes value already set on the bitmap instead of
    // attempting to compute our own.
    const size_t rowBytes = bitmap->rowBytes();

    jbyteArray arrayObj = (jbyteArray) env->CallObjectMethod(gVMRuntime,
                                                             gVMRuntime_newNonMovableArray,
                                                             gByte_class, size);
    if (env->ExceptionCheck() != 0) {
        return NULL;
    }
    SkASSERT(arrayObj);
    jbyte *addr = (jbyte *) env->CallLongMethod(gVMRuntime, gVMRuntime_addressOf, arrayObj);
    if (env->ExceptionCheck() != 0) {
        return NULL;
    }
    SkASSERT(addr);
    android::Bitmap *wrapper = new android::Bitmap(env, arrayObj, (void *) addr, info, rowBytes,
                                                   ctable);
    wrapper->getSkBitmap(bitmap);
    // since we're already allocated, we lockPixels right away
    // HeapAllocator behaves this way too
    bitmap->lockPixels();

    return wrapper;
}

从上面就可以看到, new android::Bitmap 见：
frameworks/base/core/jni/android/graphics/Bitmap.cpp

Bitmap::Bitmap(JNIEnv *env, jbyteArray storageObj, void *address,
               const SkImageInfo &info, size_t rowBytes, SkColorTable *ctable)
        : mPixelStorageType(PixelStorageType::Java) {
    env->GetJavaVM(&mPixelStorage.java.jvm);
    mPixelStorage.java.jweakRef = env->NewWeakGlobalRef(storageObj);
    mPixelStorage.java.jstrongRef = nullptr;
    mPixelRef.reset(new WrappedPixelRef(this, address, info, rowBytes, ctable));
    // Note: this will trigger a call to onStrongRefDestroyed(), but
    // we want the pixel ref to have a ref count of 0 at this point
    mPixelRef->unref();
}

address 获取的是 arrayObj 的地址，而 arrayObj 是 jbyteArray 数据类型，也就是说这里是通过 JNI 世界进入了 Java 世界开辟了内存，好比 Zygote 进入 Java 世界是通过 JNI 调用 com.android.internal.os.ZygoteInit 类的 main 函数是一个道理~ 我们还可以继续跟到 gVMRuntime_newNonMovableArray 中去看看实现，最后是 runtime->GetHeap() 上分配内存也就是 Java heap 内存。这里我就不再贴具体的代码了。

我们还得看下 8.0 的源码，比较一下它与 7.0 之间的区别：
external/skia/src/core/SkBitmap.cpp

bool SkBitmap::tryAllocPixels(Allocator *allocator, SkColorTable *ctable) {
    HeapAllocator stdalloc;

    if (nullptr == allocator) {
        allocator = &stdalloc;
    }
    return allocator->allocPixelRef(this, ctable);
}

bool HeapAllocator::allocPixelRef(SkBitmap *bitmap, SkColorTable *ctable) {
    mStorage = android::Bitmap::allocateHeapBitmap(bitmap, ctable);
    return !!mStorage;
}

allocateHeapBitmap方法会最终new Bitmap,分配内存 ,见:
/frameworks/base/libs/hwui/hwui/Bitmap.cpp

sk_sp <Bitmap> Bitmap::allocateHeapBitmap(SkBitmap *bitmap, SkColorTable *ctable) {
    return allocateBitmap(bitmap, ctable, &android::allocateHeapBitmap);
}

static sk_sp <Bitmap> allocateBitmap(SkBitmap *bitmap, SkColorTable *ctable, AllocPixeRef alloc) {
    const SkImageInfo &info = bitmap->info();
    if (info.colorType() == kUnknown_SkColorType) {
        LOG_ALWAYS_FATAL("unknown bitmap configuration");
        return nullptr;
    }

    size_t size;

    // we must respect the rowBytes value already set on the bitmap instead of
    // attempting to compute our own.
    const size_t rowBytes = bitmap->rowBytes();
    if (!computeAllocationSize(rowBytes, bitmap->height(), &size)) {
        return nullptr;
    }

    auto wrapper = alloc(size, info, rowBytes, ctable);
    if (wrapper) {
        wrapper->getSkBitmap(bitmap);
        // since we're already allocated, we lockPixels right away
        // HeapAllocator behaves this way too
        bitmap->lockPixels();
    }
    return wrapper;
}

3. Bitmap 的内存回收

通过上面的源码分析可知，Bitmap 其实占两部分对象，一个是 Java Bitmap 对象，还有一个是 Native Bitmap 对象，Java Bitmap 对象当然是垃圾回收机制来管理了，那 Native Bitmap 对象会在什么时候回收呢？

我们的 Bitmap 提供了一个 recycle 方法可以被开发者调用，但是这个方法我们在真正的开发过程中并没有调用它，那它到底有啥用呢？我们该在什么时候调用呢？

首先我先把结论写出来然后再去看源码，在 Android 2.3.3 之前开发者必须手动调用 recycle 方法去释放 Native 内存，因为那个时候管理Bitmap内存比较复杂,需要手动维护引用计数器,在官网上有如下一段解释:

On Android 2.3.3 (API level 10) and lower, using recycle() is recommended. If you're displaying large amounts of bitmap data in your app, you're likely to run into OutOfMemoryError errors. The recycle()method allows an app to reclaim memory as soon as possible.
Caution: You should use recycle() only when you are sure that the bitmap is no longer being used. If you call recycle() and later attempt to draw the bitmap, you will get the error: "Canvas: trying to use a recycled bitmap".
The following code snippet gives an example of calling recycle(). It uses reference counting (in the variables mDisplayRefCount and mCacheRefCount) to track whether a bitmap is currently being displayed or in the cache. The code recycles the bitmap when these conditions are met:
The reference count for both mDisplayRefCount and mCacheRefCount is 0.
The bitmap is not null, and it hasn't been recycled yet.

在 Android 2.3.3 以后不需要开发者主动调用 recycle 方法来回收内存了，但 Android K,L,M,N,O 版本上,都还能看到 recycle 方法，为什么没有干掉呢? 调用它会不会真正的释放内存呢？既然不需要手动释放 Native Bitmap ，那 Native 层的对象是怎么自动释放的？我们先来看下 7.0 和 8.0 中 recycle 的方法实现。

  /**
  * Free the native object associated with this bitmap, and clear the
  * reference to the pixel data. This will not free the pixel data synchronously;
  * it simply allows it to be garbage collected if there are no other references.
  * The bitmap is marked as "dead", meaning it will throw an exception if
  * getPixels() or setPixels() is called, and will draw nothing. This operation
  * cannot be reversed, so it should only be called if you are sure there are no
  * further uses for the bitmap. This is an advanced call, and normally need
  * not be called, since the normal GC process will free up this memory when
  * there are no more references to this bitmap.
  */
  public void recycle() {
    if (!mRecycled && mNativePtr != 0) {
      if (nativeRecycle(mNativePtr)) {
        // return value indicates whether native pixel object was actually recycled.
        // false indicates that it is still in use at the native level and these
        // objects should not be collected now. They will be collected later when the
        // Bitmap itself is collected.
        mNinePatchChunk = null;
      }
      mRecycled = true;
    }
  }

  private static native boolean nativeRecycle(long nativeBitmap);

8.0 见：
/xref/frameworks/base/core/jni/android/graphics/Bitmap.cpp

static jboolean Bitmap_recycle(JNIEnv *env, jobject, jlong bitmapHandle) {
    LocalScopedBitmap bitmap(bitmapHandle);
    bitmap->freePixels();
    return JNI_TRUE;
}

void freePixels() {
    mInfo = mBitmap->info();
    mHasHardwareMipMap = mBitmap->hasHardwareMipMap();
    mAllocationSize = mBitmap->getAllocationByteCount();
    mRowBytes = mBitmap->rowBytes();
    mGenerationId = mBitmap->getGenerationID();
    mIsHardware = mBitmap->isHardware();
    // 清空了数据
    mBitmap.reset();
}

7.0 见：
/xref/frameworks/base/core/jni/android/graphics/Bitmap.cpp

static jboolean Bitmap_recycle(JNIEnv *env, jobject, jlong bitmapHandle) {
    LocalScopedBitmap bitmap(bitmapHandle);
    bitmap->freePixels();
    return JNI_TRUE;
}

void Bitmap::doFreePixels() {
    switch (mPixelStorageType) {
        case PixelStorageType::Invalid:
            // already free'd, nothing to do
            break;
        case PixelStorageType::External:
            mPixelStorage.external.freeFunc(mPixelStorage.external.address,
                                            183
            mPixelStorage.external.context);
            break;
        case PixelStorageType::Ashmem:
            munmap(mPixelStorage.ashmem.address, mPixelStorage.ashmem.size);
            close(mPixelStorage.ashmem.fd);
            break;
        case PixelStorageType::Java:
            // 只是释放了 Java 层之前创建的引用
            JNIEnv *env = jniEnv();
            LOG_ALWAYS_FATAL_IF(mPixelStorage.java.jstrongRef,
                                192
            "Deleting a bitmap wrapper while there are outstanding strong "
                    "references! mPinnedRefCount = %d", mPinnedRefCount);
            env->DeleteWeakGlobalRef(mPixelStorage.java.jweakRef);
            break;
    }

    if (android::uirenderer::Caches::hasInstance()) {
        android::uirenderer::Caches::getInstance().textureCache.releaseTexture(
                mPixelRef->getStableID());
    }
}

从上面的源码可以看出，如果是 8.0 我们手动调用 recycle 方法，数据是会立即释放的，因为像素数据本身就是在 Native 层开辟的。但如果是在 8.0 以下，就算我们手动调用 recycle 方法，数据也是不会立即释放的，而是 DeleteWeakGlobalRef 交由 Java GC 来回收。建议大家翻译一下 recycle 方法注释。注意：以上的所说的释放数据仅代表释放像素数据，并未释放 Native 层的 Bitmap 对象。

最后只剩下一个问题了，我们在开发的过程中一般情况下并不会手动去调用 recycle 方法，那 Native 层的 Bitmap 是怎么回收的呢？如果让我们来写这个代码，我们不妨思考一下该怎么下手？这里我就不卖关子了。在 new Bitmap 时,其实就已经指定了谁来控制 Bitmap 的内存回收。Android M 版本及以前的版本, Bitmap 的内存回收主要是通过 BitmapFinalizer 来完成的见:
/frameworks/base/graphics/java/android/graphics/Bitmap.java

    Bitmap(long nativeBitmap, byte[] buffer, int width, int height, int density,
            boolean isMutable, boolean requestPremultiplied,
            byte[] ninePatchChunk, NinePatchInsetStruct ninePatchInsets) {
        if (nativeBitmap == 0) {
            throw new RuntimeException("internal error: native bitmap is 0");
        }

        mWidth = width;
        mHeight = height;
        mIsMutable = isMutable;
        mRequestPremultiplied = requestPremultiplied;
        mBuffer = buffer;

        mNinePatchChunk = ninePatchChunk;
        mNinePatchInsets = ninePatchInsets;
        if (density >= 0) {
            mDensity = density;
        }

        mNativePtr = nativeBitmap;
        // 这个对象对象来回收
        mFinalizer = new BitmapFinalizer(nativeBitmap);
        int nativeAllocationByteCount = (buffer == null ? getByteCount() : 0);
        mFinalizer.setNativeAllocationByteCount(nativeAllocationByteCount);
    }

    private static class BitmapFinalizer {
        private long mNativeBitmap;

        // Native memory allocated for the duration of the Bitmap,
        // if pixel data allocated into native memory, instead of java byte[]
        private int mNativeAllocationByteCount;

        BitmapFinalizer(long nativeBitmap) {
            mNativeBitmap = nativeBitmap;
        }

        public void setNativeAllocationByteCount(int nativeByteCount) {
            if (mNativeAllocationByteCount != 0) {
                VMRuntime.getRuntime().registerNativeFree(mNativeAllocationByteCount);
            }
            mNativeAllocationByteCount = nativeByteCount;
            if (mNativeAllocationByteCount != 0) {
                VMRuntime.getRuntime().registerNativeAllocation(mNativeAllocationByteCount);
            }
        }

        @Override
        public void finalize() {
            try {
                super.finalize();
            } catch (Throwable t) {
                // Ignore
            } finally {
                // finalize 这里是 GC 回收该对象时会调用
                setNativeAllocationByteCount(0);
                nativeDestructor(mNativeBitmap);
                mNativeBitmap = 0;
            }
        }
    }

    private static native void nativeDestructor(long nativeBitmap);

在 Android N 和 Android O 上做了些改动,但改动不大。虽然没有了 BitmapFinalizer 类,但在 new Bitmap 时会注册 native 的 Finalizer 方法见: /frameworks/base/graphics/java/android/graphics/Bitmap.java

    /**
     * Private constructor that must received an already allocated native bitmap
     * int (pointer).
     */
    // called from JNI
    Bitmap(long nativeBitmap, int width, int height, int density,
            boolean isMutable, boolean requestPremultiplied,
            byte[] ninePatchChunk, NinePatch.InsetStruct ninePatchInsets) {
        if (nativeBitmap == 0) {
            throw new RuntimeException("internal error: native bitmap is 0");
        }

        mWidth = width;
        mHeight = height;
        mIsMutable = isMutable;
        mRequestPremultiplied = requestPremultiplied;

        mNinePatchChunk = ninePatchChunk;
        mNinePatchInsets = ninePatchInsets;
        if (density >= 0) {
            mDensity = density;
        }

        mNativePtr = nativeBitmap;
        long nativeSize = NATIVE_ALLOCATION_SIZE + getAllocationByteCount();
        NativeAllocationRegistry registry = new NativeAllocationRegistry(
                Bitmap.class.getClassLoader(), nativeGetNativeFinalizer(), nativeSize);
        registry.registerNativeAllocation(this, nativeBitmap);
    }

NativeAllocationRegistry 见：
/libcore/luni/src/main/java/libcore/util/NativeAllocationRegistry.java

    public class NativeAllocationRegistry {

        private final ClassLoader classLoader;
        private final long freeFunction;
        private final long size;

        /**
         * Constructs a NativeAllocationRegistry for a particular kind of native
         * allocation.
         * The address of a native function that can be used to free this kind
         * native allocation should be provided using the
         * <code>freeFunction</code> argument. The native function should have the
         * type:
         * <pre>
         *    void f(void* nativePtr);
         * </pre>
         * <p>
         * The <code>classLoader</code> argument should be the class loader used
         * to load the native library that freeFunction belongs to. This is needed
         * to ensure the native library doesn't get unloaded before freeFunction
         * is called.
         * <p>
         * The <code>size</code> should be an estimate of the total number of
         * native bytes this kind of native allocation takes up. Different
         * NativeAllocationRegistrys must be used to register native allocations
         * with different estimated sizes, even if they use the same
         * <code>freeFunction</code>.
         *
         * @param classLoader  ClassLoader that was used to load the native
         *                     library freeFunction belongs to.
         * @param freeFunction address of a native function used to free this
         *                     kind of native allocation
         * @param size         estimated size in bytes of this kind of native
         *                     allocation
         * @throws IllegalArgumentException If <code>size</code> is negative
         */
        public NativeAllocationRegistry(ClassLoader classLoader, long freeFunction, long size) {
            if (size < 0) {
                throw new IllegalArgumentException("Invalid native allocation size: " + size);
            }

            this.classLoader = classLoader;
            this.freeFunction = freeFunction;
            this.size = size;
        }

        /**
         * Registers a new native allocation and associated Java object with the
         * runtime.
         * This NativeAllocationRegistry's <code>freeFunction</code> will
         * automatically be called with <code>nativePtr</code> as its sole
         * argument when <code>referent</code> becomes unreachable. If you
         * maintain copies of <code>nativePtr</code> outside
         * <code>referent</code>, you must not access these after
         * <code>referent</code> becomes unreachable, because they may be dangling
         * pointers.
         * <p>
         * The returned Runnable can be used to free the native allocation before
         * <code>referent</code> becomes unreachable. The runnable will have no
         * effect if the native allocation has already been freed by the runtime
         * or by using the runnable.
         *
         * @param referent  java object to associate the native allocation with
         * @param nativePtr address of the native allocation
         * @return runnable to explicitly free native allocation
         * @throws IllegalArgumentException if either referent or nativePtr is null.
         * @throws OutOfMemoryError         if there is not enough space on the Java heap
         *                                  in which to register the allocation. In this
         *                                  case, <code>freeFunction</code> will be
         *                                  called with <code>nativePtr</code> as its
         *                                  argument before the OutOfMemoryError is
         *                                  thrown.
         */
        public Runnable registerNativeAllocation(Object referent, long nativePtr) {
            if (referent == null) {
                throw new IllegalArgumentException("referent is null");
            }
            if (nativePtr == 0) {
                throw new IllegalArgumentException("nativePtr is null");
            }

            try {
                registerNativeAllocation(this.size);
            } catch (OutOfMemoryError oome) {
                applyFreeFunction(freeFunction, nativePtr);
                throw oome;
            }

            Cleaner cleaner = Cleaner.create(referent, new CleanerThunk(nativePtr));
            return new CleanerRunner(cleaner);
        }


        private class CleanerThunk implements Runnable {
            private long nativePtr;

            public CleanerThunk() {
                this.nativePtr = 0;
            }

            public CleanerThunk(long nativePtr) {
                this.nativePtr = nativePtr;
            }

            public void run() {
                if (nativePtr != 0) {
                    applyFreeFunction(freeFunction, nativePtr);
                }
                registerNativeFree(size);
            }

            public void setNativePtr(long nativePtr) {
                this.nativePtr = nativePtr;
            }
        }

        private static class CleanerRunner implements Runnable {
            private final Cleaner cleaner;

            public CleanerRunner(Cleaner cleaner) {
                this.cleaner = cleaner;
            }

            public void run() {
                cleaner.clean();
            }
        }

        /**
         * Calls <code>freeFunction</code>(<code>nativePtr</code>).
         * Provided as a convenience in the case where you wish to manually free a
         * native allocation using a <code>freeFunction</code> without using a
         * NativeAllocationRegistry.
         */
        public static native void applyFreeFunction(long freeFunction, long nativePtr);
    }

总结：其实无论是 Android M 前还是之后，释放 Native 层的 Bitmap 对象的思想都是去监听 Java 层的 Bitmap 是否被释放，一旦当 Java 层的 Bitmap 对象被释放则立即去释放 Native 层的 Bitmap 。只不过 Android M 前是基于 Java 的 GC 机制，而 Android M 后是注册 native 的 Finalizer 方法。

4. Bitmap 的内存复用

Bitmap 绝对是我们 Android 开发中最容易引起 OOM 的对象之一，因为其占用的像素数据内存比较大，而加载图片又是很常见的操作。如果不断反复的去开辟和销毁 Bitmap 数据内存，势必可能会引起应用的内存抖动，因此 Google 的开发者也为我们想了一些办法，那就是允许 Bitmap 内存复用，具体如下：

• 被复用的 Bitmap 必须为 Mutable（通过 BitmapFactory.Options 设置）
• 4.4 之前，将要解码的图像（无论是资源还是流）必须是 jpeg 或 png 格式且和被复用的 Bitmap 大小一样，其中BitmapFactory.Options#inSampleSize 字段必须设置为 1，要求比较严苛
• 4.4 以后，将要解码的图像的内存需要小于等于要复用的 Bitmap 的内存

  // 不复用的写法，消耗内存 32 M
  logMemory();
  Bitmap bitmap1 = BitmapFactory.decodeResource(getResources(), R.drawable.test2);
  Bitmap bitmap2 = BitmapFactory.decodeResource(getResources(), R.drawable.test2);
  logMemory();
  // 复用的写法，消耗内存 16 M
  logMemory();
  BitmapFactory.Options options = new BitmapFactory.Options();
  options.inMutable = true;
  Bitmap bitmap1 = BitmapFactory.decodeResource(getResources(), R.drawable.test2, options);
  options.inBitmap = bitmap1;
  Bitmap bitmap2 = BitmapFactory.decodeResource(getResources(), R.drawable.test2, options);
  logMemory();

具体的实现源码上面已经讲过了，这里就不再做过多的啰嗦，最后建议大家去看看 Glide 的源码，看看这些比较知名的开源框架，到底是怎么复用 Bitmap 的。某些同学看 C++ 代码可能比较头疼，其实我们只需要了解一些基础语法，就能看得懂上面的代码了。打通我们的任督二脉的确需要一些时间，但换个角度来讲，这些时间可以拿来换更多的时间和金钱。

最后，我们不妨再来思考下，本地资源图片 xhdpi，xxhdpi 和 xxxhdpi 我们应该放哪个目录的文件夹下，加载效率才会更高？还是应该放多套图？

视频地址：https://pan.baidu.com/s/1CIfeKabrv9mOQ-tfClDe3w
视频密码：1xax