前言 Glide归根结底是一个图片加载框架，它一定会涉及到BitmapFactory相关API把Bitmap读取到内存；可能大家已经很熟悉如何高效的加载Bitmap(比如使用inSample等)，这一章还是要看一看Glide是如何玩转的；
本文主要分析这两个类：

• DownsampleStrategy
• Downsampler

从"Glide会对原图进行放大"案例开始 假设我们有一张宽高100x200的网络图片，需要加载到300x300像素的ImageView上(scaleType属性为CenterCrop)，用Glide加载，不做任何处理会得到多大的Bitmap?

Glide.with(MainActivity.this).load(URL).listener(new RequestListener() { @Override public boolean onLoadFailed(@Nullable GlideException e, Object model, Target target, boolean isFirstResource) { return false; }@Override public boolean onResourceReady(Drawable resource, Object model, Target target, DataSource dataSource, boolean isFirstResource) { if (resource instanceof BitmapDrawable){ Bitmap bitmap = ((BitmapDrawable) resource).getBitmap(); Log.d("onResourceReady",bitmap.toString()); } return false; } }).into(vh.imageView); 复制代码

不出意外会得到300*300的Bitmap，Bitmap比例显然被放大了；
我怀疑是进行了Transformation操作，所以我们要禁用Transformation;
【Glide4.8源码拆解（四）Bitmap解析之"下采样"浅析】假设我们对RequestOptions加上noTransformation的属性，比如这个样子:

RequestOptions requestOptions = RequestOptions.noTransformation(); Glide.with(MainActivity.this).load(URL).apply(requestOptions).listener(xxx).into(vh.imageView); 复制代码

不出意外会得到300*600的Bitmap，Bitmap比例依然在被放大，甚至更大了；
如果我们不想让Bitmap被放大，可以用加载原图尺寸的方式，比如设置override(Target.SIZE_ORIGINAL,Target.SIZE_ORIGINAL)或者用SimpleTarget这样的Target,但是加载原图终究不是解决方案，为什么？内存的原因，大部分情况下我们还是希望Glide来对图片进行缩小，加载原图的操作等于是直接把APP往OOM送近了一步；
Bitmap被放大不是Glide发明的，是Android官方带的头，比如同一张图片，放在res中不同drawable文件夹下，得到的尺寸不一样；既然如此，Bitmap被放大肯定是有积极意义的，也难怪Glide很少提及这事；
但是，总会有强迫症患者不想Bitmap被放大，Glide肯定也想到了这一点，具体怎么限制Bitmap不会放大，答案的使用RequestOptions.downsample()方法;
RequestOptions.downsample()方法接受参数类型为：DownsampleStrategy，那么我们从DownsampleStrategy开始分析；
DownsampleStrategy DownsampleStrategy顾名思义就是下采样策略，下采样是图像进行缩小的一种方式；
DownsampleStrategy.java

public abstract class DownsampleStrategy { //获取缩放比例 public abstract float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight); //获取SampleSize策略 public abstract SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight); }public enum SampleSizeRounding { //内存优先 MEMORY, //图片质量优先 QUALITY, } 复制代码

DownsampleStrategy是抽象类，提供两个抽象方法，getScaleFactor()顾名思义是获取缩放比例的，getSampleSizeRounding()可能是获取SampleSize的，DownsampleStrategy真是的实现类在其内部，几个嵌套内部类FitCenter,CenterOutside,AtLeast,AtMost,None,CenterInside,除此之外，DownsampleStrategy还定义对应类的静态变量；
DownsampleStrategy.java

public static final DownsampleStrategy FIT_CENTER = new FitCenter(); public static final DownsampleStrategy CENTER_OUTSIDE = new CenterOutside(); public static final DownsampleStrategy AT_LEAST = new AtLeast(); public static final DownsampleStrategy AT_MOST = new AtMost(); public static final DownsampleStrategy CENTER_INSIDE = new CenterInside(); public static final DownsampleStrategy NONE = new None(); public static final DownsampleStrategy DEFAULT = CENTER_OUTSIDE; 复制代码

其中DEFAULT应该是Glide默认的策略，它指向的是CENTER_OUTSIDE,我们简单看一下CenterOutside的逻辑；
CenterOutside.java

private static class CenterOutside extends DownsampleStrategy {@Synthetic CenterOutside() { }@Override public float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight) { //宽度比例(控件/图片) float widthPercentage = requestedWidth / (float) sourceWidth; //高度比例(控件/图片) float heightPercentage = requestedHeight / (float) sourceHeight; //谁大取谁 return Math.max(widthPercentage, heightPercentage); }@Override public SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight) { return SampleSizeRounding.QUALITY; } } 复制代码

主要关注是CenterOutside的getScaleFactor()逻辑,该逻辑非常简单，主要是拿控件宽高/图片宽高，得到尺寸取最大值，我们回到文章开头的那个例子，假设此时控件宽高是300x300，图片宽高100x200：
widthPercentage = 300/100 = 3.0f;
heightPercentage = 300/200 = 1.5f;
Math.max(widthPercentage, heightPercentage) = 3.0f；
从这个推理来看，文章开头那个图片确实会被放大3倍；
文章开头试图找控制缩放比例不超过1的，就是只缩小不放大的，有没有这样的策略，其实是有的，比如AtLeast,AtMost；
AtLeast.java

private static class AtLeast extends DownsampleStrategy {@Synthetic AtLeast() { }@Override public float getScaleFactor(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight) { int minIntegerFactor = Math.min(sourceHeight / requestedHeight, sourceWidth / requestedWidth); return minIntegerFactor == 0 ? 1f : 1f / Integer.highestOneBit(minIntegerFactor); }@Override public SampleSizeRounding getSampleSizeRounding(int sourceWidth, int sourceHeight, int requestedWidth, int requestedHeight) { return SampleSizeRounding.QUALITY; } } 复制代码

AtLeast对getScaleFactor的逻辑简要分析：
获取图片宽高/布局宽高的比例的最小值，转成int值；
如果这个值等于0，直接返回1，等于0其实就意味着图片宽高/布局宽高至少有一个是小于1的，也就屏蔽了需要放大的情况；
如果minIntegerFactor不等一0，肯定是需要缩小的，最后返回1f / Integer.highestOneBit(minIntegerFactor),其中Integer.highestOneBit(minIntegerFactor)是取minIntegerFactor的二进制形式最左边的最高一位且高位后面全部补零，最后返回int型的结果；
其余的DownsampleStrategy实现类就不讲解了，DownsampleStrategy是负责计算缩放比例和SampleSize策略，那么真正去进行Bitmap计算的是Downsampler;
重头戏Downsampler Downsampler.java
Downsampler这个类在我们之前分析Decode流程时已经遇到过它，该类的主要职责是从输入流中解析出Bitmap，核心功能的入口方法在decode();

public Resource decode(InputStream is, int requestedWidth, int requestedHeight, Options options, DecodeCallbacks callbacks) throws IOException { Preconditions.checkArgument(is.markSupported(), "You must provide an InputStream that supports" + " mark()"); byte[] bytesForOptions = byteArrayPool.get(ArrayPool.STANDARD_BUFFER_SIZE_BYTES, byte[].class); //获取默认的BitmapFactory BitmapFactory.Options bitmapFactoryOptions = getDefaultOptions(); bitmapFactoryOptions.inTempStorage = bytesForOptions; //decodeFormat主要是RGB_8888||RGB_565 DecodeFormat decodeFormat = options.get(DECODE_FORMAT); //获取DownsampleStrategy DownsampleStrategy downsampleStrategy = options.get(DownsampleStrategy.OPTION); //是否fixedBitmapSize boolean fixBitmapToRequestedDimensions = options.get(FIX_BITMAP_SIZE_TO_REQUESTED_DIMENSIONS); //是否支持硬件位图 boolean isHardwareConfigAllowed = options.get(ALLOW_HARDWARE_CONFIG) != null && options.get(ALLOW_HARDWARE_CONFIG); try { //解析出Bitmap Bitmap result = decodeFromWrappedStreams(is, bitmapFactoryOptions, downsampleStrategy, decodeFormat, isHardwareConfigAllowed, requestedWidth, requestedHeight, fixBitmapToRequestedDimensions, callbacks); //BitmapResource包装Bitmap return BitmapResource.obtain(result, bitmapPool); } finally { releaseOptions(bitmapFactoryOptions); byteArrayPool.put(bytesForOptions); } } 复制代码

首先调用getDefaultOptions()获取默认的BitmapFactory;
默认的BitmapFactory
默认的BitmapFactory使用一个队列缓存，初始化设置在resetOptions()方法；

private static void resetOptions(BitmapFactory.Options decodeBitmapOptions) { decodeBitmapOptions.inTempStorage = null; decodeBitmapOptions.inDither = false; decodeBitmapOptions.inScaled = false; decodeBitmapOptions.inSampleSize = 1; decodeBitmapOptions.inPreferredConfig = null; decodeBitmapOptions.inJustDecodeBounds = false; decodeBitmapOptions.inDensity = 0; decodeBitmapOptions.inTargetDensity = 0; decodeBitmapOptions.outWidth = 0; decodeBitmapOptions.outHeight = 0; decodeBitmapOptions.outMimeType = null; decodeBitmapOptions.inBitmap = null; decodeBitmapOptions.inMutable = true; } 复制代码

继续decode()方法分析，首先从Option中获取decodeFormat，fixBitmapToRequestedDimensions，isHardwareConfigAllowed等，这个Option是从RequestOptions传递过来，代表用户的配置;

• decodeFormat表示解析格式，RGB_565或ARGB_8888;
• fixBitmapToRequestedDimensions表示是否填充尺寸，不进行缩放
• isHardwareConfigAllowed是否允许硬件位图，详细了解可以看Glide官方文档：muyangmin.github.io/glide-docs-…
• 用BitmapResource包装Bitmap返回； 真正的加载Bitmap逻辑在decodeFromWrappedStreams()方法：

decodeFromWrappedStreams
decodeFromWrappedStreams()

private Bitmap decodeFromWrappedStreams(InputStream is, BitmapFactory.Options options, DownsampleStrategy downsampleStrategy, DecodeFormat decodeFormat, boolean isHardwareConfigAllowed, int requestedWidth, int requestedHeight, boolean fixBitmapToRequestedDimensions, DecodeCallbacks callbacks) throws IOException { long startTime = LogTime.getLogTime(); //解析出输入流图片尺寸 int[] sourceDimensions = getDimensions(is, options, callbacks, bitmapPool); int sourceWidth = sourceDimensions[0]; int sourceHeight = sourceDimensions[1]; //得到mimeType String sourceMimeType = options.outMimeType; if (sourceWidth == -1 || sourceHeight == -1) { isHardwareConfigAllowed = false; } //获取图片旋转方向 int orientation = ImageHeaderParserUtils.getOrientation(parsers, is, byteArrayPool); int degreesToRotate = TransformationUtils.getExifOrientationDegrees(orientation); boolean isExifOrientationRequired = TransformationUtils.isExifOrientationRequired(orientation); //判断目标控件尺寸 int targetWidth = requestedWidth == Target.SIZE_ORIGINAL ? sourceWidth : requestedWidth; int targetHeight = requestedHeight == Target.SIZE_ORIGINAL ? sourceHeight : requestedHeight; //获取image类型 ImageType imageType = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool); //计算缩放 calculateScaling(); //计算其他config calculateConfig(); boolean isKitKatOrGreater = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT; //处理inBitmap if ((options.inSampleSize == 1 || isKitKatOrGreater) && shouldUsePool(imageType)) { int expectedWidth; int expectedHeight; if (sourceWidth >= 0 && sourceHeight >= 0 && fixBitmapToRequestedDimensions && isKitKatOrGreater) { expectedWidth = targetWidth; expectedHeight = targetHeight; } else { float densityMultiplier = isScaling(options) ? (float) options.inTargetDensity / options.inDensity : 1f; int sampleSize = options.inSampleSize; int downsampledWidth = (int) Math.ceil(sourceWidth / (float) sampleSize); int downsampledHeight = (int) Math.ceil(sourceHeight / (float) sampleSize); expectedWidth = Math.round(downsampledWidth * densityMultiplier); expectedHeight = Math.round(downsampledHeight * densityMultiplier); } //设置inBitmap if (expectedWidth > 0 && expectedHeight > 0) { setInBitmap(options, bitmapPool, expectedWidth, expectedHeight); } } //BitmapFactory.Option配置完毕，调用decodeStream解析 Bitmap downsampled = decodeStream(is, options, callbacks, bitmapPool); callbacks.onDecodeComplete(bitmapPool, downsampled); Bitmap rotated = null; if (downsampled != null) { //重新设置density downsampled.setDensity(displayMetrics.densityDpi); //处理旋转信息 rotated = TransformationUtils.rotateImageExif(bitmapPool, downsampled, orientation); if (!downsampled.equals(rotated)) { //添加进BitmapPool bitmapPool.put(downsampled); } }return rotated; } 复制代码

• decodeFromWrappedStreams()方法首先调用getDimensions()获取输入流图片尺寸和mimeType；
• 获取图片的方向和旋转角度；
• 确认目标控件的宽高，获取图片的ImageType;
• 调用calculateScaling()计算缩放信息；
• 调用calculateConfig()计算其他配置信息；
• 根据配置，调用setInBitmap()设置inBitmap;
• 上诉流程完成BitmapFactory.Option配置，调用decodeStream()解析输入流；
• 得到Bitmap，对Bitmap做最后的操作；

getDimensions()

private static int[] getDimensions(InputStream is, BitmapFactory.Options options, DecodeCallbacks decodeCallbacks, BitmapPool bitmapPool) throws IOException { //只解析Bounds options.inJustDecodeBounds = true; decodeStream(is, options, decodeCallbacks, bitmapPool); //重置为false; options.inJustDecodeBounds = false; return new int[] { options.outWidth, options.outHeight }; } 复制代码

getDimensions()使用options.inJustDecodeBounds = true读取输入流图片信息；
计算缩放
calculateScaling()

private static void calculateScaling( ImageType imageType, InputStream is, DecodeCallbacks decodeCallbacks, BitmapPool bitmapPool, DownsampleStrategy downsampleStrategy, int degreesToRotate, int sourceWidth, int sourceHeight, int targetWidth, int targetHeight, BitmapFactory.Options options) throws IOException { //尺寸不能为0 if (sourceWidth <= 0 || sourceHeight <= 0) { return; }final float exactScaleFactor; //根据方向和角度获取exactScaleFactor if (degreesToRotate == 90 || degreesToRotate == 270) { exactScaleFactor = downsampleStrategy.getScaleFactor(sourceHeight, sourceWidth, targetWidth, targetHeight); } else { exactScaleFactor = downsampleStrategy.getScaleFactor(sourceWidth, sourceHeight, targetWidth, targetHeight); } //exactScaleFactor不能小于等于0 if (exactScaleFactor <= 0f) { throw new IllegalArgumentException(""); } //获取SampleSizeRounding SampleSizeRounding rounding = downsampleStrategy.getSampleSizeRounding(sourceWidth, sourceHeight, targetWidth, targetHeight); if (rounding == null) { throw new IllegalArgumentException("Cannot round with null rounding"); } //获取Bitmap输出宽高 int outWidth = round(exactScaleFactor * sourceWidth); int outHeight = round(exactScaleFactor * sourceHeight); //转成int类型的factor int widthScaleFactor = sourceWidth / outWidth; int heightScaleFactor = sourceHeight / outHeight; //根据SampleSizeRounding得到scaleFactor int scaleFactor = rounding == SampleSizeRounding.MEMORY ? Math.max(widthScaleFactor, heightScaleFactor) : Math.min(widthScaleFactor, heightScaleFactor); int powerOfTwoSampleSize; //不支持下采样 if (Build.VERSION.SDK_INT <= 23 && NO_DOWNSAMPLE_PRE_N_MIME_TYPES.contains(options.outMimeType)) { powerOfTwoSampleSize = 1; } else { //在scaleFactor再进行处理，保证是2的指数幂 powerOfTwoSampleSize = Math.max(1, Integer.highestOneBit(scaleFactor)); if (rounding == SampleSizeRounding.MEMORY && powerOfTwoSampleSize < (1.f / exactScaleFactor)) { powerOfTwoSampleSize = powerOfTwoSampleSize << 1; } } //设置到inSampleSize options.inSampleSize = powerOfTwoSampleSize; int powerOfTwoWidth; int powerOfTwoHeight; //针对不同图片格式，重新计算采样后的宽高 if (imageType == ImageType.JPEG) { //libjpeg引擎最大采样size=8，skia会进行二次采样 int nativeScaling = Math.min(powerOfTwoSampleSize, 8); powerOfTwoWidth = (int) Math.ceil(sourceWidth / (float) nativeScaling); powerOfTwoHeight = (int) Math.ceil(sourceHeight / (float) nativeScaling); //如大大于8，skia会对剩下的进行二次采样计算逻辑 int secondaryScaling = powerOfTwoSampleSize / 8; if (secondaryScaling > 0) { powerOfTwoWidth = powerOfTwoWidth / secondaryScaling; powerOfTwoHeight = powerOfTwoHeight / secondaryScaling; } } else if (imageType == ImageType.PNG || imageType == ImageType.PNG_A) { powerOfTwoWidth = (int) Math.floor(sourceWidth / (float) powerOfTwoSampleSize); powerOfTwoHeight = (int) Math.floor(sourceHeight / (float) powerOfTwoSampleSize); } else if (imageType == ImageType.WEBP || imageType == ImageType.WEBP_A) { //不同版本采样不同的计算方式round或者floor if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) { powerOfTwoWidth = Math.round(sourceWidth / (float) powerOfTwoSampleSize); powerOfTwoHeight = Math.round(sourceHeight / (float) powerOfTwoSampleSize); } else { powerOfTwoWidth = (int) Math.floor(sourceWidth / (float) powerOfTwoSampleSize); powerOfTwoHeight = (int) Math.floor(sourceHeight / (float) powerOfTwoSampleSize); } } else if ( sourceWidth % powerOfTwoSampleSize != 0 || sourceHeight % powerOfTwoSampleSize != 0) { int[] dimensions = getDimensions(is, options, decodeCallbacks, bitmapPool); powerOfTwoWidth = dimensions[0]; powerOfTwoHeight = dimensions[1]; } else { powerOfTwoWidth = sourceWidth / powerOfTwoSampleSize; powerOfTwoHeight = sourceHeight / powerOfTwoSampleSize; } //计算采样后进行缩放的比例 double adjustedScaleFactor = downsampleStrategy.getScaleFactor( powerOfTwoWidth, powerOfTwoHeight, targetWidth, targetHeight); //计算inTargetDensity和inDensity进行缩放 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) { //这一块计算方法没有弄明白 options.inTargetDensity = adjustTargetDensityForError(adjustedScaleFactor); options.inDensity = getDensityMultiplier(adjustedScaleFactor); } //设置inScaled if (isScaling(options)){ options.inScaled = true; } else { options.inDensity = options.inTargetDensity = 0; } } 复制代码

calculateScaling代码看似复杂，但是仔细分析流程还是很清晰的，该方法主要计算BitmapFactory.Factory的inSampleSize和inTargetDensity、inDensity，inSampleSize主要是进行向下采样，采样后自然会对图片进行缩小，但是可能不满足目标缩放比例，所以再配合inTargetDensity、inDensity进行二次缩放；主要针对两个缩放变量powerOfTwoSampleSize和adjustedScaleFactor；具体流程分析:
流程一

1. 计算powerOfTwoSampleSize过程:powerOfTwoSampleSize是最终赋值给options.inSampleSize的；

• 根据图片角度不同，分别调用downSampleStategy.getScaleFactor()方法，得到exactScaleFactor;
• 通过exactScaleFactor计算出outWidth和outHeight,再重新计算widthScaleFactor和heightScaleFactor(ps:widthScaleFactor和heightScaleFactor是int类型，而exactScaleFactor是浮点型，结果是不一样的);
• 通过策略得到int类型的比例scaleFactor
• 判断是否支持下采样，计算出最终的缩放比例powerOfTwoSampleSize
• 赋值给options.inSampleSize

流程二

2. 计算adjustedScaleFactor过程：adjustedScaleFactor是最终计算options.inTargetDensity和options.inDensity的；

• 不同格式和版本的下采样逻辑不同，Glide分别实现采样逻辑的计算，然后生成采样后的宽高尺寸powerOfTwoWidth和powerOfTwoHeight;
• 再此调用downsampleStrategy.getScaleFactor()得到缩放比adjustedScaleFactor;
• 调用adjustTargetDensityForError()和getDensityMultiplier()得到inTargetDensity和inDensity并赋值；
• 上面inDensity依赖options.inScaled = true，最终还得判断是否能够进行scale;

计算硬件位图/Bitmap.Config
calculateConfig()

private void calculateConfig( InputStream is, DecodeFormat format, boolean isHardwareConfigAllowed, boolean isExifOrientationRequired, BitmapFactory.Options optionsWithScaling, int targetWidth, int targetHeight) { //如果支持硬件位图 if (hardwareConfigState.setHardwareConfigIfAllowed( targetWidth, targetHeight, optionsWithScaling, format, isHardwareConfigAllowed, isExifOrientationRequired)) { return; } //这种情况，直接配置ARGB_8888 if (format == DecodeFormat.PREFER_ARGB_8888 || Build.VERSION.SDK_INT == Build.VERSION_CODES.JELLY_BEAN) { optionsWithScaling.inPreferredConfig = Bitmap.Config.ARGB_8888; return; } //是否有透明的通道 boolean hasAlpha = false; try { hasAlpha = ImageHeaderParserUtils.getType(parsers, is, byteArrayPool).hasAlpha(); } catch (IOException e) {} //有透明的通道，需要配置成ARGB_8888 optionsWithScaling.inPreferredConfig = hasAlpha ? Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565; if (optionsWithScaling.inPreferredConfig == Config.RGB_565) { optionsWithScaling.inDither = true; //565支持抖动界面 } } 复制代码

calculateConfig()主要是对BitmapFactory.Option的硬件位图和inPreferredConfig进行计算；如果用户配置8888直接配置，如果用户配置565，还需要判断是否有透明度通道，如果有透明度通道，依然采用8888解码器；
硬件位图
HardwareConfigState.java

boolean setHardwareConfigIfAllowed( int targetWidth, int targetHeight, BitmapFactory.Options optionsWithScaling, DecodeFormat decodeFormat, boolean isHardwareConfigAllowed, boolean isExifOrientationRequired) { if (!isHardwareConfigAllowed || Build.VERSION.SDK_INT < Build.VERSION_CODES.O || isExifOrientationRequired) { Android O以上支持 return false; } //对尺寸有要求； boolean result = targetWidth >= MIN_HARDWARE_DIMENSION && targetHeight >= MIN_HARDWARE_DIMENSION && isFdSizeBelowHardwareLimit(); if (result) { optionsWithScaling.inPreferredConfig = Bitmap.Config.HARDWARE; optionsWithScaling.inMutable = false; } return result; } 复制代码

硬件位图Bitmap.Config.HARDWARE 是一种 Android O 添加的新的位图格式。硬件位图仅在显存 (graphic memory) 里存储像素数据，并对图片仅在屏幕上绘制的场景做了优化。
###设置InBitmap
setInBitmap()

private static void setInBitmap( BitmapFactory.Options options, BitmapPool bitmapPool, int width, int height) { @Nullable Bitmap.Config expectedConfig = null; // Avoid short circuiting, it appears to break on some devices. if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) { //硬件位图不设置 if (options.inPreferredConfig == Config.HARDWARE) { return; } //inJustDecodeBoudes时候有可能可以获取到 expectedConfig = options.outConfig; }if (expectedConfig == null) { expectedConfig = options.inPreferredConfig; }options.inBitmap = bitmapPool.getDirty(width, height, expectedConfig); } 复制代码

设置成硬件位图的不能进行inBitmap操作，最终会设置options.inBitmap，其中缓存的位图从bitmapPool中获取;
解析输出流
decodeStream()
万事具体只差东风，设置完BitmapFactory.Options之后，正在解析输入流的代码就在decodeStream():

private static Bitmap decodeStream(InputStream is, BitmapFactory.Options options, DecodeCallbacks callbacks, BitmapPool bitmapPool) throws IOException { if (options.inJustDecodeBounds) { is.mark(MARK_POSITION); } else { callbacks.onObtainBounds(); } int sourceWidth = options.outWidth; int sourceHeight = options.outHeight; String outMimeType = options.outMimeType; final Bitmap result; TransformationUtils.getBitmapDrawableLock().lock(); try { //真正的解析调用 result = BitmapFactory.decodeStream(is, null, options); } catch (IllegalArgumentException e) { if (options.inBitmap != null) { try { //发生异常要做inBitmap回收 is.reset(); bitmapPool.put(options.inBitmap); options.inBitmap = null; //不适用inBitmap return decodeStream(is, options, callbacks, bitmapPool); } catch (IOException resetException) { throw bitmapAssertionException; } } throw bitmapAssertionException; } finally { TransformationUtils.getBitmapDrawableLock().unlock(); } if (options.inJustDecodeBounds) { is.reset(); } return result; } 复制代码

decodeStream()核心的方法是调用BitmapFactory.decodeStream(is, null, options)做解析工作，剩下的代码都是对异常解析，如果发现异常时inBitmap不为空，设置inBitmap为null并重试一次；
总结 本文主要是对Glide下采样做了简单介绍，从代码流程上分析，可以分为对BitmapFactory.Options的配置、调用BitmapFactory解析输入流以及对Bitmap最后的处理三个步骤，其中Options配置阶段在计算缩放上最为重要，其中在缩放因子计算阶段，涉及native以及libjpeg底层的逻辑尤为重要，值得学习；

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