Glide源码分析-网络图片加载主流程分析

第一节，我们主要分析
Glide.with(this)
.load(url)
.into(imageView)
这三步，也就是最简单的将一个网络图片展示在ImageView上的三步。

1 Glide.with

Glide.with 重载方法较多

public class Glide implements ComponentCallbacks2 {
    ...
    @NonNull
    public static RequestManager with(@NonNull Context context) {
        return getRetriever(context).get(context);
    }

    @NonNull
    public static RequestManager with(@NonNull Activity activity) {
        return getRetriever(activity).get(activity);
    }

    @NonNull
    public static RequestManager with(@NonNull FragmentActivity activity) {
        return getRetriever(activity).get(activity);
    }

    @NonNull
    public static RequestManager with(@NonNull Fragment fragment) {
        return getRetriever(fragment.getActivity()).get(fragment);
    }

    @SuppressWarnings("deprecation")
    @Deprecated
    @NonNull
    public static RequestManager with(@NonNull android.app.Fragment fragment) {
        return getRetriever(fragment.getActivity()).get(fragment);
    }

    @NonNull
    public static RequestManager with(@NonNull View view) {
        return getRetriever(view.getContext()).get(view);
    }
}

每个重载方法内部都首先调用getRetriever(@Nullable Context context)方法获取一个RequestManagerRetriever对象，然后调用其get方法来返回RequestManager。先来看Glide.getRetriever

  @NonNull
  private static RequestManagerRetriever getRetriever(@Nullable Context context) {
    // 此处省略对context判空代码
    // ...
    
    return Glide.get(context).getRequestManagerRetriever();
  }

方法里调用了Glide.get(context)创建了一个Glide，接着调用 Glide的getRequestManagerRetriever()返回RequestManagerRetriever对象。看下Glide.get(context)

  @NonNull
  public static Glide get(@NonNull Context context) {
    if (glide == null) {
      synchronized (Glide.class) {
        if (glide == null) {
          checkAndInitializeGlide(context);
        }
      }
    }

    return glide;
  }
  
    private static void checkAndInitializeGlide(@NonNull Context context) {
    // 保证只创建一个Glide实例
    if (isInitializing) {
      throw new IllegalStateException("You cannot call Glide.get() in registerComponents(),"
          + " use the provided Glide instance instead");
    }
    isInitializing = true;
    initializeGlide(context);
    isInitializing = false;
  }

这里是创建一个 Glide 单例对象，具体创建过程在initializeGlide(context)

  private static void initializeGlide(@NonNull Context context) {
    initializeGlide(context, new GlideBuilder());
  }

  @SuppressWarnings("deprecation")
  private static void initializeGlide(@NonNull Context context, @NonNull GlideBuilder builder) {
    Context applicationContext = context.getApplicationContext();
    
    // 如果有配置@GlideModule注解，那么会反射构造kapt生成的GeneratedAppGlideModuleImpl类
    GeneratedAppGlideModule annotationGeneratedModule = getAnnotationGeneratedGlideModules();
    
    // 如果Impl存在，且允许解析manifest文件，则遍历manifest中的meta-data，解析出所有的GlideModule类
    List<com.bumptech.glide.module.GlideModule> manifestModules = Collections.emptyList();
    if (annotationGeneratedModule == null || annotationGeneratedModule.isManifestParsingEnabled()) {
      manifestModules = new ManifestParser(applicationContext).parse();
    }

    // ... 此处省略根据Impl的黑名单，剔除manifest中的GlideModule类相关代码

    // 如果Impl存在，那么设置为该类的RequestManagerFactory； 否则，设置为null
    RequestManagerRetriever.RequestManagerFactory factory =
        annotationGeneratedModule != null
            ? annotationGeneratedModule.getRequestManagerFactory() : null;
    builder.setRequestManagerFactory(factory);
    
    // 依次调用manifest中GlideModule类的applyOptions方法，将配置写到builder里
    for (com.bumptech.glide.module.GlideModule module : manifestModules) {
      module.applyOptions(applicationContext, builder);
    }
    
    // 写入Impl的配置，会覆盖掉manifest中的配置
    if (annotationGeneratedModule != null) {
      annotationGeneratedModule.applyOptions(applicationContext, builder);
    }
    
    // 调用GlideBuilder.build方法创建Glide
    Glide glide = builder.build(applicationContext);
    
    // 依次调用manifest中GlideModule类的registerComponents方法，来替换Glide的默认配置
    for (com.bumptech.glide.module.GlideModule module : manifestModules) {
      module.registerComponents(applicationContext, glide, glide.registry);
    }
    // 调用Impl中替换Glide配置的方法
    if (annotationGeneratedModule != null) {
        annotationGeneratedModule.registerComponents(applicationContext, glide, glide.registry);
    }
    
    // 注册内存管理的回调，因为Glide实现了ComponentCallbacks2接口
    applicationContext.registerComponentCallbacks(glide);
    // 保存glide实例到静态变量中
    Glide.glide = glide;
  }

我们主要分析其中三步主流程，并没有 在 AndroidManifest 中配置meta-data，也没有配置@GlideModule 注解，所以初始化的代码可以简化如下：

  @SuppressWarnings("deprecation")
  private static void initializeGlide(@NonNull Context context, @NonNull GlideBuilder builder) {
    Context applicationContext = context.getApplicationContext();
    // 调用GlideBuilder.build方法创建Glide
    Glide glide = builder.build(applicationContext);
    // 注册内存管理的回调，Glide实现了ComponentCallbacks2
    applicationContext.registerComponentCallbacks(glide);
    // 保存glide实例到静态变量中
    Glide.glide = glide;
  }

继续跟进下GlideBuilder.build方法

@NonNull
  Glide build(@NonNull Context context) {
  
    // 此处省略构造Glide需要依赖的其他对象的实例化代码
    // ...
    
    RequestManagerRetriever requestManagerRetriever =
        new RequestManagerRetriever(requestManagerFactory);

    return new Glide(
        context,
        engine,
        memoryCache,
        bitmapPool,
        arrayPool,
        requestManagerRetriever,
        connectivityMonitorFactory,
        logLevel,
        defaultRequestOptions.lock(),
        defaultTransitionOptions,
        defaultRequestListeners,
        isLoggingRequestOriginsEnabled);
  }

这里重点看下requestManagerRetriever的初始化，requestManagerRetriever通过其构造器初始化并传入的requestManagerFactory，然后作为入参传入Glide构造器。跟进RequestManagerRetriever构造器

  public RequestManagerRetriever(@Nullable RequestManagerFactory factory) {
    this.factory = factory != null ? factory : DEFAULT_FACTORY;
    handler = new Handler(Looper.getMainLooper(), this /* Callback */);
  }

当传进来的factory为null时会用默认的 DEFAULT_FACTORY

  private static final RequestManagerFactory DEFAULT_FACTORY = new RequestManagerFactory() {
    @NonNull
    @Override
    public RequestManager build(@NonNull Glide glide, @NonNull Lifecycle lifecycle,
        @NonNull RequestManagerTreeNode requestManagerTreeNode, @NonNull Context context) {
      return new RequestManager(glide, lifecycle, requestManagerTreeNode, context);
    }
  };

DEFAULT_FACTORY 中build方法会构造 RequestManager 对象。回到 Glide.with 方法中，接着调用RequestManagerRetriever.get方法，并传入对生命周期可感知的入参，这里的入参有 Context、Activity、Fragment、View

@NonNull
private RequestManager getApplicationManager(@NonNull Context context) {
  // Either an application context or we're on a background thread.
  if (applicationManager == null) {
    synchronized (this) {
      if (applicationManager == null) {
        Glide glide = Glide.get(context.getApplicationContext());
        applicationManager =
            factory.build(
                glide,
                new ApplicationLifecycle(),
                new EmptyRequestManagerTreeNode(),
                context.getApplicationContext());
      }
    }
  }
  return applicationManager;
}

@NonNull
public RequestManager get(@NonNull Context context) {
  if (context == null) {
    throw new IllegalArgumentException("You cannot start a load on a null Context");
  } else if (Util.isOnMainThread() && !(context instanceof Application)) {
    if (context instanceof FragmentActivity) {
      return get((FragmentActivity) context);
    } else if (context instanceof Activity) {
      return get((Activity) context);
    } else if (context instanceof ContextWrapper) {
      return get(((ContextWrapper) context).getBaseContext());
    }
  }
  return getApplicationManager(context);
}

@NonNull
public RequestManager get(@NonNull FragmentActivity activity) {
  if (Util.isOnBackgroundThread()) {
    return get(activity.getApplicationContext());
  } else {
    assertNotDestroyed(activity);
    FragmentManager fm = activity.getSupportFragmentManager();
    return supportFragmentGet(
        activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
  }
}

@NonNull
public RequestManager get(@NonNull Fragment fragment) {
  Preconditions.checkNotNull(fragment.getActivity(),
        "You cannot start a load on a fragment before it is attached or after it is destroyed");
  if (Util.isOnBackgroundThread()) {
    return get(fragment.getActivity().getApplicationContext());
  } else {
    FragmentManager fm = fragment.getChildFragmentManager();
    return supportFragmentGet(fragment.getActivity(), fm, fragment, fragment.isVisible());
  }
}

@SuppressWarnings("deprecation")
@NonNull
public RequestManager get(@NonNull Activity activity) {
  if (Util.isOnBackgroundThread()) {
    return get(activity.getApplicationContext());
  } else {
    assertNotDestroyed(activity);
    android.app.FragmentManager fm = activity.getFragmentManager();
    return fragmentGet(
        activity, fm, /*parentHint=*/ null, isActivityVisible(activity));
  }
}

@SuppressWarnings("deprecation")
@NonNull
public RequestManager get(@NonNull View view) {
  if (Util.isOnBackgroundThread()) {
    return get(view.getContext().getApplicationContext());
  }

  // 省略 checkNotNull

  // 找出View所在的Fragment或Activity
  if (activity instanceof FragmentActivity) {
    Fragment fragment = findSupportFragment(view, (FragmentActivity) activity);
    return fragment != null ? get(fragment) : get(activity);
  }

  // Standard Fragments.
  android.app.Fragment fragment = findFragment(view, activity);
  if (fragment == null) {
    return get(activity);
  }
  return get(fragment);
}

这些get方法中先判断是否是后台线程，如果是后台线程，最终会直接调用 getApplicationManager(context) 给 applicationManager 赋值并将其返回，applicationManager是RequestManager类型的对象。初始化 applicationManager 的代码如下：

        Glide glide = Glide.get(context.getApplicationContext());
        applicationManager =
            factory.build(
                glide,
                new ApplicationLifecycle(),
                new EmptyRequestManagerTreeNode(),
                context.getApplicationContext());

其中 factory 就是 DEFAULT_FACTORY，DEFAULT_FACTORY 的 build方法会构造并返回的是一个 RequestManager 对象，所以 applicationManager 初始化代码可以简化如下：

applicationManager =
            RequestManager(
                glide,
                new ApplicationLifecycle(),
                new EmptyRequestManagerTreeNode(),
                context.getApplicationContext());

继续回到RequestManagerRetriever.get重载方法，如果当前线程不是后台线程，get(View)和get(Context)会根据情况调用get(Fragment)或get(FragmentActivity)，get(View)最终也会通过一系列的操作去找到上层的Fragment或者Activity 然后调用get(Fragment)或get(FragmentActivity)，由于通过View寻找上层的Fragment或者Activity 的过程开销相对较大，不建议传入View对象。get(Fragment)和get(FragmentActivity)方法都会调用supportFragmentGet方法，supportFragmentGet方法如下：

  @NonNull
  private RequestManager supportFragmentGet(
      @NonNull Context context,
      @NonNull FragmentManager fm,
      @Nullable Fragment parentHint,
      boolean isParentVisible) {
    SupportRequestManagerFragment current =
        getSupportRequestManagerFragment(fm, parentHint, isParentVisible);
    RequestManager requestManager = current.getRequestManager();
    if (requestManager == null) {
      // TODO(b/27524013): Factor out this Glide.get() call.
      Glide glide = Glide.get(context);
      requestManager =
          factory.build(
              glide, current.getGlideLifecycle(), current.getRequestManagerTreeNode(), context);
      current.setRequestManager(requestManager);
    }
    return requestManager;
  }

Glide会使用一个加载目标所在的宿主Activity或Fragment的子Fragment来安全保存一个RequestManager，而RequestManager被Glide用来开始、停止、管理Glide请求。也就是说利用给宿主添加一个子Fragment来间接的获取宿主的生命周期，从而保证Glide请求与生命周期同步。

supportFragmentGet就是创建/获取这个子Fragment，即 SupportRequestManagerFragment，并返回 RequestManager 实例。跟进RequestManagerRetriever.getSupportRequestManagerFragment

@NonNull
  private SupportRequestManagerFragment getSupportRequestManagerFragment(
      @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) {
    SupportRequestManagerFragment current =
        (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
    if (current == null) {
      current = pendingSupportRequestManagerFragments.get(fm);
      if (current == null) {
        current = new SupportRequestManagerFragment();
        // 对于fragment来说，此方法会以Activity为host创建另外一个SupportRequestManagerFragment
        // 作为rootRequestManagerFragment
        // 并会将current加入到rootRequestManagerFragment的childRequestManagerFragments中
        // 在RequestManager递归管理请求时会使用到
        current.setParentFragmentHint(parentHint);
        // 如果当前页面是可见的，那么调用其lifecycle的onStart方法
        if (isParentVisible) {
          current.getGlideLifecycle().onStart();
        }
        pendingSupportRequestManagerFragments.put(fm, current);
        // 将fragment添加到页面中
        fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
        handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
      }
    }
    return current;
  }

如果之前已经添加过SupportRequestManagerFragment则直接返回添加过的Fragment，否则从 map中获取获取，取到了也直接返回，取不到就通过构造器创建一个对象，然后将此Fragment添加到宿主Activity或Fragment中。

到这里 Glide.with方法分析完毕，主要就是创建 Glide 单例，并通过向宿主（Fragment/Activity）添加一个空的Fragment来关联宿主的生命周期用来管理Glide请求，然后返回一个具体管理Glide请求的 RequestManager 对象。

2 RequestManager.load

RequestManager.load也有许多重载的方法

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable Bitmap bitmap) {
  return asDrawable().load(bitmap);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable Drawable drawable) {
  return asDrawable().load(drawable);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable String string) {
  return asDrawable().load(string);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable Uri uri) {
  return asDrawable().load(uri);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable File file) {
  return asDrawable().load(file);
}

@SuppressWarnings("deprecation")
@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@RawRes @DrawableRes @Nullable Integer resourceId) {
  return asDrawable().load(resourceId);
}

@SuppressWarnings("deprecation")
@CheckResult
@Override
@Deprecated
public RequestBuilder<Drawable> load(@Nullable URL url) {
  return asDrawable().load(url);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable byte[] model) {
  return asDrawable().load(model);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<Drawable> load(@Nullable Object model) {
  return asDrawable().load(model);
}

所有的重载方法中都调用了asDrawable()方法得到一个RequestBuilder对象，然后再调用RequestBuilder.load方法。asDrawable()方法与asGif()、asBitmap()、asFile()等方法一样，都会调用RequestManager.as()方法生成一个RequestBuilder<ResourceType>对象，然后再调用apply方法添加不同的requestOptions，代码如下：

@NonNull
@CheckResult
public RequestBuilder<Bitmap> asBitmap() {
  return as(Bitmap.class).apply(DECODE_TYPE_BITMAP);
}

@NonNull
@CheckResult
public RequestBuilder<GifDrawable> asGif() {
  return as(GifDrawable.class).apply(DECODE_TYPE_GIF);
}  

@NonNull
@CheckResult
public RequestBuilder<Drawable> asDrawable() {
  return as(Drawable.class);
}

@NonNull
@CheckResult
public RequestBuilder<File> asFile() {
  return as(File.class).apply(skipMemoryCacheOf(true));
}

@NonNull
@CheckResult
public <ResourceType> RequestBuilder<ResourceType> as(
    @NonNull Class<ResourceType> resourceClass) {
  return new RequestBuilder<>(glide, this, resourceClass, context);
}

在RequestBuilder的构造器方法方法中将Drawable.class这样的入参保存到了transcodeClass变量中。继续跟进RequestBuilder.load，RequestBuilder.load 也有很多方法的重载：

@NonNull
@CheckResult
@SuppressWarnings("unchecked")
@Override
public RequestBuilder<TranscodeType> load(@Nullable Object model) {
  return loadGeneric(model);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable Bitmap bitmap) {
  return loadGeneric(bitmap)
      .apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable Drawable drawable) {
  return loadGeneric(drawable)
      .apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable Uri uri) {
  return loadGeneric(uri);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable File file) {
  return loadGeneric(file);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@RawRes @DrawableRes @Nullable Integer resourceId) {
  return loadGeneric(resourceId).apply(signatureOf(ApplicationVersionSignature.obtain(context)));
}

@Deprecated
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable URL url) {
  return loadGeneric(url);
}

@NonNull
@CheckResult
@Override
public RequestBuilder<TranscodeType> load(@Nullable byte[] model) {
  RequestBuilder<TranscodeType> result = loadGeneric(model);
  if (!result.isDiskCacheStrategySet()) {
      result = result.apply(diskCacheStrategyOf(DiskCacheStrategy.NONE));
  }
  if (!result.isSkipMemoryCacheSet()) {
    result = result.apply(skipMemoryCacheOf(true /*skipMemoryCache*/));
  }
  return result;
}

@NonNull
private RequestBuilder<TranscodeType> loadGeneric(@Nullable Object model) {
  this.model = model;
  isModelSet = true;
  return this;
}

每个重载方法中都会调用loadGeneric保存传递的参数到 model 中并设置isModelSet=true，部分方法会apply额外的requestOptions。

总结一下：RequestManager.load方法中主要就是构造RequestBuilder并将load的资源保存到RequestBuilder中的 model 变量中。

3 RequestBuilder.into

RequestBuilder.into方法的重载也比较多，我们常用的是into(imageView)，代码如下：

@NonNull
  public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
    Util.assertMainThread();
    Preconditions.checkNotNull(view);

    BaseRequestOptions<?> requestOptions = this;
    
    // 若没有指定transform，isTransformationSet()为false
    // isTransformationAllowed()一般为true，除非主动调用了dontTransform()方法
    if (!requestOptions.isTransformationSet()
        && requestOptions.isTransformationAllowed()
        && view.getScaleType() != null) {
      
      // 根据ImageView的ScaleType设置不同的 downsample 和transform 选项对图片进行剪裁
      switch (view.getScaleType()) {
        case CENTER_CROP:
          requestOptions = requestOptions.clone().optionalCenterCrop();
          break;
        case CENTER_INSIDE:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case FIT_CENTER:
        case FIT_START:
        case FIT_END:
          requestOptions = requestOptions.clone().optionalFitCenter();
          break;
        case FIT_XY:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case CENTER:
        case MATRIX:
        default:
          // Do nothing.
      }
    }

    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
        /*targetListener=*/ null,
        requestOptions,
        Executors.mainThreadExecutor());
  }

into(ImageView)方法里面会先判断需不需要对图片进行裁切，然后调用into重载方法。into重载方法第一个参数glideContext.buildImageViewTarget(view, transcodeClass)代码如下：

// GlideContext
@NonNull
public <X> ViewTarget<ImageView, X> buildImageViewTarget(
    @NonNull ImageView imageView, @NonNull Class<X> transcodeClass) {
  // imageViewTargetFactory是ImageViewTargetFactory的一个实例
  // transcodeClass在RequestManager.load方法中确定了，就是Drawable.class
  return imageViewTargetFactory.buildTarget(imageView, transcodeClass);
}

// ImageViewTargetFactory
@NonNull
@SuppressWarnings("unchecked")
public <Z> ViewTarget<ImageView, Z> buildTarget(@NonNull ImageView view,
    @NonNull Class<Z> clazz) {
  if (Bitmap.class.equals(clazz)) {
    return (ViewTarget<ImageView, Z>) new BitmapImageViewTarget(view);
  } else if (Drawable.class.isAssignableFrom(clazz)) {
    // 返回的是(ViewTarget<ImageView, Drawable>) new DrawableImageViewTarget(view);
    return (ViewTarget<ImageView, Z>) new DrawableImageViewTarget(view);
  } else {
    throw new IllegalArgumentException(
        "Unhandled class: " + clazz + ", try .as*(Class).transcode(ResourceTranscoder)");
  }
}

因为我们这里要转码的类型为：Drawable.class，所以buildTarget返回的是DrawableImageViewTarget对象，所以第一个参数等价于

(ViewTarget<ImageView, Drawable>) new DrawableImageViewTarget(view)

第四个参数Executors.mainThreadExecutor()是一个Executor，内部使用MainLooper的Handler，当执行Executor.execute(Runnable)方法时将Runnable使用此 Handler post 出去。

  /** Posts executions to the main thread. */
  public static Executor mainThreadExecutor() {
    return MAIN_THREAD_EXECUTOR;
  }

    private static final Executor MAIN_THREAD_EXECUTOR =
      new Executor() {
        private final Handler handler = new Handler(Looper.getMainLooper());

        @Override
        public void execute(@NonNull Runnable command) {
          handler.post(command);
        }
    };

分析完了参数，继续分析into 重载方法内部实现

private <Y extends Target<TranscodeType>> Y into(
    @NonNull Y target,
    @Nullable RequestListener<TranscodeType> targetListener,
    BaseRequestOptions<?> options,
    Executor callbackExecutor) {

  if (!isModelSet) {
    throw new IllegalArgumentException("You must call #load() before calling #into()");
  }

  // 创建了一个SingleRequest
  Request request = buildRequest(target, targetListener, options, callbackExecutor);

  // 这里会判断需不需要重新开始任务
  // 如果当前request和target上之前的request previous相等
  // 且设置了忽略内存缓存或previous还没有完成
  // 那么会进入if分支
  Request previous = target.getRequest();
  if (request.isEquivalentTo(previous)
      && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
    request.recycle();
    
    // 如果正在运行，就不管它；如果已经失败了，就重新开始
    if (!Preconditions.checkNotNull(previous).isRunning()) {
        previous.begin();
    }
    return target;
  }

  // 如果不能复用previous
  // 先清除target上之前的Request
  requestManager.clear(target);
  // 将Request作为tag设置到view中
  target.setRequest(request);
  // 真正开始网络图片的加载
  requestManager.track(target, request);

  return target;
}

先看一下buildRequest(target, targetListener, options, callbackExecutor)如何创建SingleRequest，方法调用链为：buildRequest->buildRequestRecursive->buildThumbnailRequestRecursive->obtainRequest：

private Request obtainRequest(
    Target<TranscodeType> target,
    RequestListener<TranscodeType> targetListener,
    BaseRequestOptions<?> requestOptions,
    RequestCoordinator requestCoordinator,
    TransitionOptions<?, ? super TranscodeType> transitionOptions,
    Priority priority,
    int overrideWidth,
    int overrideHeight,
    Executor callbackExecutor) {
  return SingleRequest.obtain(
      context,
      glideContext,
      model,
      transcodeClass,
      requestOptions,
      overrideWidth,
      overrideHeight,
      priority,
      target,
      targetListener,
      requestListeners,
      requestCoordinator,
      glideContext.getEngine(),
      transitionOptions.getTransitionFactory(),
      callbackExecutor);
}

最终通过SingleRequest.obtain方法传递参数构建SingleRequest对象。继续回到into重载方法中调用requestManager.track(target, request)进行网络图片的加载。

  synchronized void track(@NonNull Target<?> target, @NonNull Request request) {
    targetTracker.track(target);
    requestTracker.runRequest(request);
  }

targetTracker成员变量在声明的时候直接初始化为TargetTracker类的无参数实例，该类的作用是保存所有的Target并向它们转发生命周期事件.requestTracker在RequestManager的构造器中传入了new RequestTracker()，该类的作用管理所有状态的请求。继续跟进requestTracker.runRequest(request)

  /**
   * Starts tracking the given request.
   */
  public void runRequest(@NonNull Request request) {
    requests.add(request);
    if (!isPaused) {
      request.begin();
    } else {
      request.clear();
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Paused, delaying request");
      }
      pendingRequests.add(request);
    }
  }

isPaused默认为false，只有调用了RequestTracker.pauseRequests或RequestTracker.pauseAllRequests后才会为true。因此，下面会执行request.begin()方法，这里的request就是在into重载方法里面构造出来的 SingleRequest，跟进SingleRequest.begin

@Override
  public synchronized void begin() {
  
    //...
    
    // 如果model为空，会调用监听器的onLoadFailed处理
    // 若无法处理，则展示失败时的占位图
    if (model == null) {
      ...
      onLoadFailed(new GlideException("Received null model"), logLevel);
      return;
    }

    if (status == Status.RUNNING) {
      throw new IllegalArgumentException("Cannot restart a running request");
    }

    // 如果已经加载过，再次在相同的目标或视图中加载相同的请求，在重新来加载前清除View或Target
    if (status == Status.COMPLETE) {
      onResourceReady(resource, DataSource.MEMORY_CACHE);
      return;
    }

    // 如果指定了overrideWidth和overrideHeight，那么直接调用onSizeReady方法
    // 否则会获取ImageView的宽、高，然后调用onSizeReady方法
    // 在该方法中会创建图片加载的Job并开始执行
    status = Status.WAITING_FOR_SIZE;
    if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
      onSizeReady(overrideWidth, overrideHeight);
    } else {
      target.getSize(this);
    }

    // 显示加载中的占位符
    if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
        && canNotifyStatusChanged()) {
      target.onLoadStarted(getPlaceholderDrawable());
    }
    if (IS_VERBOSE_LOGGABLE) {
      logV("finished run method in " + LogTime.getElapsedMillis(startTime));
    }
  }

如果 model == null会调用onLoadFailed方法，在其内部会调用监听器的onLoadFailed处理，若无法处理，则展示失败时的占位图。如果指定了overrideWidth和overrideHeight，那么直接调用onSizeReady方法，否则会获取ImageView的宽、高，然后调用onSizeReady方法，在该方法中会创建图片加载的Job并开始执行。我们先来看下 SingleRequest.onSizeReady：

@Override
  public synchronized void onSizeReady(int width, int height) {
    
    ...
    
    // 在SingleRequest.begin方法中已经将status设置为WAITING_FOR_SIZE状态了
    if (status != Status.WAITING_FOR_SIZE) {
      return;
    }
    
    // 设置状态为RUNNING
    status = Status.RUNNING;

    // 将原始尺寸与0～1之间的系数相乘，取最接近的整数值，得到新的尺寸
    float sizeMultiplier = requestOptions.getSizeMultiplier();
    this.width = maybeApplySizeMultiplier(width, sizeMultiplier);
    this.height = maybeApplySizeMultiplier(height, sizeMultiplier);

    ...
    
    loadStatus =
        engine.load(
            glideContext,
            model,
            requestOptions.getSignature(),
            this.width,
            this.height,
            requestOptions.getResourceClass(),
            transcodeClass,
            priority,
            requestOptions.getDiskCacheStrategy(),
            requestOptions.getTransformations(),
            requestOptions.isTransformationRequired(),
            requestOptions.isScaleOnlyOrNoTransform(),
            requestOptions.getOptions(),
            requestOptions.isMemoryCacheable(),
            requestOptions.getUseUnlimitedSourceGeneratorsPool(),
            requestOptions.getUseAnimationPool(),
            requestOptions.getOnlyRetrieveFromCache(),
            this,
            callbackExecutor);

    ...
  }

最后会调用engine.load方法，Engine是负责加载，管理active、cached状态资源的类。在GlideBuilder.build中创建Glide时，若没有主动设置engine，会使用下面的参数进行创建

if (sourceExecutor == null) {
  sourceExecutor = GlideExecutor.newSourceExecutor();
}

if (diskCacheExecutor == null) {
  diskCacheExecutor = GlideExecutor.newDiskCacheExecutor();
}

if (memoryCache == null) {
  memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());
}

if (diskCacheFactory == null) {
  diskCacheFactory = new InternalCacheDiskCacheFactory(context);
}

if (engine == null) {
  engine =
      new Engine(
          memoryCache,
          diskCacheFactory,
          diskCacheExecutor,
          sourceExecutor,
          GlideExecutor.newUnlimitedSourceExecutor(),
          GlideExecutor.newAnimationExecutor(),
          isActiveResourceRetentionAllowed);
}

继续跟进Engine.load方法

public synchronized <R> LoadStatus load(
      GlideContext glideContext,
      Object model,
      Key signature,
      int width,
      int height,
      Class<?> resourceClass,
      Class<R> transcodeClass,
      Priority priority,
      DiskCacheStrategy diskCacheStrategy,
      Map<Class<?>, Transformation<?>> transformations,
      boolean isTransformationRequired,
      boolean isScaleOnlyOrNoTransform,
      Options options,
      boolean isMemoryCacheable,
      boolean useUnlimitedSourceExecutorPool,
      boolean useAnimationPool,
      boolean onlyRetrieveFromCache,
      ResourceCallback cb,
      Executor callbackExecutor) {
    long startTime = VERBOSE_IS_LOGGABLE ? LogTime.getLogTime() : 0;

    // 根据部分参数生成 key 
    EngineKey key = keyFactory.buildKey(model, signature, width, height, transformations,
        resourceClass, transcodeClass, options);

    // 从active资源中进行加载
    EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
    if (active != null) {
      cb.onResourceReady(active, DataSource.MEMORY_CACHE);
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from active resources", startTime, key);
      }
      return null;
    }

    // 从内存cache资源中进行加载
    EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
    if (cached != null) {
      cb.onResourceReady(cached, DataSource.MEMORY_CACHE);
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Loaded resource from cache", startTime, key);
      }
      return null;
    }

    // 从正在进行的jobs中进行加载
    EngineJob<?> current = jobs.get(key, onlyRetrieveFromCache);
    if (current != null) {
      current.addCallback(cb, callbackExecutor);
      if (VERBOSE_IS_LOGGABLE) {
        logWithTimeAndKey("Added to existing load", startTime, key);
      }
      return new LoadStatus(cb, current);
    }

    // 构建出一个EngineJob
    EngineJob<R> engineJob =
        engineJobFactory.build(
            key,
            isMemoryCacheable,
            useUnlimitedSourceExecutorPool,
            useAnimationPool,
            onlyRetrieveFromCache);

    // 构建出一个DecodeJob，该类实现了Runnable接口
    DecodeJob<R> decodeJob =
        decodeJobFactory.build(
            glideContext,
            model,
            key,
            signature,
            width,
            height,
            resourceClass,
            transcodeClass,
            priority,
            diskCacheStrategy,
            transformations,
            isTransformationRequired,
            isScaleOnlyOrNoTransform,
            onlyRetrieveFromCache,
            options,
            engineJob);

    // 根据engineJob.onlyRetrieveFromCache的值是否为true
    // 将engineJob保存到onlyCacheJobs或者jobs HashMap中
    jobs.put(key, engineJob);

    // 添加资源加载状态回调，参数会包装成ResourceCallbackAndExecutor类型
    // 并保存到ResourceCallbacksAndExecutors.callbacksAndExecutors中
    engineJob.addCallback(cb, callbackExecutor);
    
    // 开始执行decodeJob任务
    engineJob.start(decodeJob);

    ...
    
    return new LoadStatus(cb, engineJob);
  }

Engine.load方法中会以一些参数作为key，依次从active状态、cached状态和进行中的 jobs 里寻找。若没有找到，则会创建对应的job并开始执行。engineJobFactory 和 decodeJobFactory 需要注意的是里面使用了对象池 Pools.Pool 来复用 Job对象内存，Pools.Pool里面通过Object[]来保存Job。继续跟进 engineJob.start(decodeJob)：

  // EngineJob
  public synchronized void start(DecodeJob<R> decodeJob) {
    this.decodeJob = decodeJob;
    GlideExecutor executor = decodeJob.willDecodeFromCache()
        ? diskCacheExecutor
        : getActiveSourceExecutor();
    executor.execute(decodeJob);
  }

由于我们没有配置缓存策略，默认会使用缓存，所以decodeJob.willDecodeFromCache()为true，那么就使用diskCacheExecutor 来执行decodeJob。diskCacheExecutor 默认值为GlideExecutor.newDiskCacheExecutor()，这是类似于一个SingleThreadExecutor的线程池，这里使用了设计模式中的代理模式：

/**
 * A prioritized {@link ThreadPoolExecutor} for running jobs in Glide.
 */
public final class GlideExecutor implements ExecutorService {
  private static final String DEFAULT_DISK_CACHE_EXECUTOR_NAME = "disk-cache";
  private static final int DEFAULT_DISK_CACHE_EXECUTOR_THREADS = 1;

  private final ExecutorService delegate;

  public static GlideExecutor newDiskCacheExecutor() {
    return newDiskCacheExecutor(
        DEFAULT_DISK_CACHE_EXECUTOR_THREADS,
        DEFAULT_DISK_CACHE_EXECUTOR_NAME,
        UncaughtThrowableStrategy.DEFAULT);
  }

  public static GlideExecutor newDiskCacheExecutor(
      int threadCount, String name, UncaughtThrowableStrategy uncaughtThrowableStrategy) {
    return new GlideExecutor(
        new ThreadPoolExecutor(
            threadCount /* corePoolSize */,
            threadCount /* maximumPoolSize */,
            0 /* keepAliveTime */,
            TimeUnit.MILLISECONDS,
            new PriorityBlockingQueue<Runnable>(),
            new DefaultThreadFactory(name, uncaughtThrowableStrategy, true)));
  }

  @VisibleForTesting
  GlideExecutor(ExecutorService delegate) {
    this.delegate = delegate;
  }

  @Override
  public void execute(@NonNull Runnable command) {
    delegate.execute(command);
  }

  @NonNull
  @Override
  public Future<?> submit(@NonNull Runnable task) {
    return delegate.submit(task);
  }
  ...
}

至此，decodeJob已经提交到了线程池中。回到SingleRequest.begin 继续跟进后面的代码

if ((status == Status.RUNNING || status == Status.WAITING_FOR_SIZE)
    && canNotifyStatusChanged()) {
  target.onLoadStarted(getPlaceholderDrawable());
}

由于此时status == Status.RUNNING为true，现在开始展示placeholder。

前面我们已经将decodeJob已经提交到了线程池中，那么继续分析DecodeJob.run方法

@Override
  public void run() {
    ...
    try {
      if (isCancelled) {
        notifyFailed();
        return;
      }
      
      runWrapped();
    } catch (CallbackException e) {
      throw e;
    } catch (Throwable t) {
      ...
      throw t;
    } finally {
       ...
    }
  }

方法里面主要就是执行了 runWrapped 方法

  private void runWrapped() {
    switch (runReason) {
      case INITIALIZE:
        stage = getNextStage(Stage.INITIALIZE);
        currentGenerator = getNextGenerator();
        runGenerators();
        break;
      case SWITCH_TO_SOURCE_SERVICE:
        ...
        break;
      case DECODE_DATA:
        ...
        break;
      ...
    }
  }

runReason在DecodeJob.init方法中被初始化为INITIALIZE，所以我们需要关注的的代码为

// 获取下一个状态RESOURCE_CACHE并赋值给stage
stage = getNextStage(Stage.INITIALIZE);
// getNextGenerator()返回的是 ResourceCacheGenerator
currentGenerator = getNextGenerator();
runGenerators();

  private Stage getNextStage(Stage current) {
    switch (current) {
      case INITIALIZE:
        return diskCacheStrategy.decodeCachedResource()
            ? Stage.RESOURCE_CACHE : getNextStage(Stage.RESOURCE_CACHE);
      case RESOURCE_CACHE:
        ...
      case DATA_CACHE:
        ...
      case SOURCE:
      case FINISHED:
        ...
      ...
    }
  }
  
  private DataFetcherGenerator getNextGenerator() {
    switch (stage) {
      case RESOURCE_CACHE:
        return new ResourceCacheGenerator(decodeHelper, this);
      case DATA_CACHE:
        return new DataCacheGenerator(decodeHelper, this);
      case SOURCE:
        return new SourceGenerator(decodeHelper, this);
      case FINISHED:
        return null;
      default:
        throw new IllegalStateException("Unrecognized stage: " + stage);
    }
  }

stage 的状态被修改为 RESOURCE_CACHE，currentGenerator 为 ResourceCacheGenerator，继续跟进 runGenerators 方法：

  private void runGenerators() {
    currentThread = Thread.currentThread();
    startFetchTime = LogTime.getLogTime();
    boolean isStarted = false;
    while (!isCancelled && currentGenerator != null
        && !(isStarted = currentGenerator.startNext())) {
      stage = getNextStage(stage);
      currentGenerator = getNextGenerator();

      if (stage == Stage.SOURCE) {
        reschedule();
        return;
      }
    }
    // We've run out of stages and generators, give up.
    if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
      notifyFailed();
    }

  }

该方法中会依次调用各个状态生成的 DataFetcherGenerator 的 startNext() 尝试fetch数据，取数据成功了该方法就结束。若状态到了Stage.FINISHED 或 job 被取消，且所有状态的DataFetcherGenerator.startNext()都无法满足条件，则调用SingleRequest.onLoadFailed进行错误处理。根据getNextGenerator()方法代码可发现：DataFetcherGenerator 有三个子类：

• ResourceCacheGenerator
  获取downsample、transform后的资源文件的缓存文件

• DataCacheGenerator
  获取原始的没有修改过的资源文件的缓存文件

• SourceGenerator
  获取原始源数据

先跟进下 ResourceCacheGenerator.startNext：

@Override
  public boolean startNext() {
    // list里面只有一个GlideUrl对象
    List<Key> sourceIds = helper.getCacheKeys();
    if (sourceIds.isEmpty()) {
      return false;
    }
    // 获得了三个可以到达的registeredResourceClasses
    // GifDrawable、Bitmap、BitmapDrawable
    List<Class<?>> resourceClasses = helper.getRegisteredResourceClasses();
    if (resourceClasses.isEmpty()) {
      if (File.class.equals(helper.getTranscodeClass())) {
        return false;
      }
      throw new IllegalStateException(
         "Failed to find any load path from " + helper.getModelClass() + " to "
             + helper.getTranscodeClass());
    }
    
    // 遍历sourceIds中的每一个key、resourceClasses中每一个class，以及其他的一些值组成key
    // 尝试在磁盘缓存中以key找到缓存文件
    while (modelLoaders == null || !hasNextModelLoader()) {
      resourceClassIndex++;
      if (resourceClassIndex >= resourceClasses.size()) {
        sourceIdIndex++;
        if (sourceIdIndex >= sourceIds.size()) {
          return false;
        }
        resourceClassIndex = 0;
      }

      Key sourceId = sourceIds.get(sourceIdIndex);
      Class<?> resourceClass = resourceClasses.get(resourceClassIndex);
      Transformation<?> transformation = helper.getTransformation(resourceClass);
      // PMD.AvoidInstantiatingObjectsInLoops Each iteration is comparatively expensive anyway,
      // we only run until the first one succeeds, the loop runs for only a limited
      // number of iterations on the order of 10-20 in the worst case.
      currentKey =
          new ResourceCacheKey(// NOPMD AvoidInstantiatingObjectsInLoops
              helper.getArrayPool(),
              sourceId,
              helper.getSignature(),
              helper.getWidth(),
              helper.getHeight(),
              transformation,
              resourceClass,
              helper.getOptions());
      cacheFile = helper.getDiskCache().get(currentKey);
      
      // 如果找到了缓存文件，循环条件则会为false，退出循环
      if (cacheFile != null) {
        sourceKey = sourceId;
        // 1. 找出注入时以File.class为modelClass的注入代码
        // 2. 调用所有注入的factory.build方法得到ModelLoader
        // 3 .过滤掉不可能处理model的ModelLoader
        // 此时的modelLoaders值为：
        // [ByteBufferFileLoader, FileLoader, FileLoader, UnitModelLoader]
        modelLoaders = helper.getModelLoaders(cacheFile);
        modelLoaderIndex = 0;
      }
    }

    // 如果找到了缓存文件，hasNextModelLoader()方法则会为true，可以执行循环
    // 没有找到缓存文件，则不会进入循环，会直接返回false
    loadData = null;
    boolean started = false;
    while (!started && hasNextModelLoader()) {
      ModelLoader<File, ?> modelLoader = modelLoaders.get(modelLoaderIndex++);
      // 在循环中会依次判断某个ModelLoader能不能加载此文件
      loadData = modelLoader.buildLoadData(cacheFile,
          helper.getWidth(), helper.getHeight(), helper.getOptions());
      if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
        started = true;
        
        // 如果某个ModelLoader可以，那么就调用其fetcher进行加载数据
        // 加载成功或失败会通知自身
        loadData.fetcher.loadData(helper.getPriority(), this);
      }
    }

    return started;
  }

如果已经找到一个一条可以加载的路径，那么就调用此fetcher.loadData方法进行加载。同时，该方法ResourceCacheGenerator.startNext返回true，这就意味着DecodeJob无需在尝试另外的 DataFetcherGenerator 进行加载，整个into过程已经大致完成，剩下的就是等待资源加载完毕后触发回调。续分析 loadData.fetcher.loadData(helper.getPriority(), this)，这里的 fetcher 是 ByteBufferFetcher：

    // ByteBufferFetcher
    @Override
    public void loadData(@NonNull Priority priority,
        @NonNull DataCallback<? super ByteBuffer> callback) {
      ByteBuffer result;
      try {
        // 这里的file就是缓存下来的source file
        result = ByteBufferUtil.fromFile(file);
      } catch (IOException e) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
          Log.d(TAG, "Failed to obtain ByteBuffer for file", e);
        }
        callback.onLoadFailed(e);
        return;
      }

      callback.onDataReady(result);
    }

ByteBufferUtil.fromFile使用了RandomAccessFile和FileChannel进行文件操作。如果操作失败，调用callback.onLoadFailed(e)通知ResourceCacheGenerator类，该类会将操作转发给DecodeJob；callback.onDataReady操作类似。这样程序就回到了DecodeJob回调方法中了。

但是按照我们的流程，第一次加载时没有缓存的，所以 ResourceCacheGenerator.startNext 中找不到缓存文件，继续交给DataCacheGenerator.startNext 处理：

public boolean startNext() {
    while (modelLoaders == null || !hasNextModelLoader()) {
      sourceIdIndex++;
      if (sourceIdIndex >= cacheKeys.size()) {
        return false;
      }

      Key sourceId = cacheKeys.get(sourceIdIndex);
      // PMD.AvoidInstantiatingObjectsInLoops The loop iterates a limited number of times
      // and the actions it performs are much more expensive than a single allocation.
      @SuppressWarnings("PMD.AvoidInstantiatingObjectsInLoops")
      Key originalKey = new DataCacheKey(sourceId, helper.getSignature());
      cacheFile = helper.getDiskCache().get(originalKey);
      if (cacheFile != null) {
        this.sourceKey = sourceId;
        modelLoaders = helper.getModelLoaders(cacheFile);
        modelLoaderIndex = 0;
      }
    }

    loadData = null;
    boolean started = false;
    while (!started && hasNextModelLoader()) {
      ModelLoader<File, ?> modelLoader = modelLoaders.get(modelLoaderIndex++);
      loadData =
          modelLoader.buildLoadData(cacheFile, helper.getWidth(), helper.getHeight(),
              helper.getOptions());
      if (loadData != null && helper.hasLoadPath(loadData.fetcher.getDataClass())) {
        started = true;
        loadData.fetcher.loadData(helper.getPriority(), this);
      }
    }
    return started;
  }

由于第一次加载，本地缓存没有，接着交给最后一个 SourceGenerator.startNext 来处理

@Override
  public boolean startNext() {
  
    // 首次运行dataToCache为null
    if (dataToCache != null) {
      Object data = dataToCache;
      dataToCache = null;
      cacheData(data);
    }

    // 首次运行sourceCacheGenerator为null
    if (sourceCacheGenerator != null && sourceCacheGenerator.startNext()) {
      return true;
    }
    sourceCacheGenerator = null;

    loadData = null;
    boolean started = false;
    while (!started && hasNextModelLoader()) {
      loadData = helper.getLoadData().get(loadDataListIndex++);
      if (loadData != null
          && (helper.getDiskCacheStrategy().isDataCacheable(loadData.fetcher.getDataSource())
          || helper.hasLoadPath(loadData.fetcher.getDataClass()))) {
        started = true;
        loadData.fetcher.loadData(helper.getPriority(), this);
      }
    }
    return started;
  }

helper.getLoadData()的值在ResourceCacheGenerator中就已经被获取并缓存下来了，这是一个MultiModelLoader对象生成的LoadData对象，LoadData对象里面有两个fetcher。

遍历LoadData list，找出符合条件的LoadData，然后调用loadData.fetcher.loadData加载数据。若loadData不为null，会判断Glide的缓存策略是否可以缓存此数据源，或者是否有加载路径。MultiModelLoader里面的fetcher是MultiFetcher，我们来看下MultiFetcher.loadData：

    // MultiFetcher
    @Override
    public void loadData(
        @NonNull Priority priority, @NonNull DataCallback<? super Data> callback) {
      this.priority = priority;
      this.callback = callback;
      exceptions = throwableListPool.acquire();
      // 类型是HttpUrlFetcher
      fetchers.get(currentIndex).loadData(priority, this);

      if (isCancelled) {
        cancel();
      }
    }

继续跟进 HttpUrlFetcher.loadData

  // HttpUrlFetcher
  @Override
  public void loadData(@NonNull Priority priority,
      @NonNull DataCallback<? super InputStream> callback) {
    long startTime = LogTime.getLogTime();
    try {
      InputStream result = loadDataWithRedirects(glideUrl.toURL(), 0, null, glideUrl.getHeaders());
      callback.onDataReady(result);
    } catch (IOException e) {
      if (Log.isLoggable(TAG, Log.DEBUG)) {
        Log.d(TAG, "Failed to load data for url", e);
      }
      callback.onLoadFailed(e);
    } finally {
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        Log.v(TAG, "Finished http url fetcher fetch in " + LogTime.getElapsedMillis(startTime));
      }
    }
  }

这里将请求操作放到了loadDataWithRedirects方法中，然后将请求结果通过回调返回上一层也就是MultiFetcher中。跟进HttpUrlFetcher.loadDataWithRedirects：

// HttpUrlFetcher
private InputStream loadDataWithRedirects(URL url, int redirects, URL lastUrl,
      Map<String, String> headers) throws IOException {
      
    // 检查重定向次数
    if (redirects >= MAXIMUM_REDIRECTS) {
      throw new HttpException("Too many (> " + MAXIMUM_REDIRECTS + ") redirects!");
    } else {
      try {
        // 检查是不是重定向到自身了
        if (lastUrl != null && url.toURI().equals(lastUrl.toURI())) {
          throw new HttpException("In re-direct loop");
        }
      } catch (URISyntaxException e) {
        // Do nothing, this is best effort.
      }
    }

    // connectionFactory默认是DefaultHttpUrlConnectionFactory
    // 其build方法就是调用了url.openConnection()
    urlConnection = connectionFactory.build(url);
    for (Map.Entry<String, String> headerEntry : headers.entrySet()) {
      urlConnection.addRequestProperty(headerEntry.getKey(), headerEntry.getValue());
    }
    urlConnection.setConnectTimeout(timeout);
    urlConnection.setReadTimeout(timeout);
    urlConnection.setUseCaches(false);
    urlConnection.setDoInput(true);

    // 禁止HttpUrlConnection自动重定向，重定向功能由本方法自己实现
    urlConnection.setInstanceFollowRedirects(false);

    // Connect explicitly to avoid errors in decoders if connection fails.
    urlConnection.connect();
    // Set the stream so that it's closed in cleanup to avoid resource leaks. See #2352.
    stream = urlConnection.getInputStream();
    if (isCancelled) {
      return null;
    }
    final int statusCode = urlConnection.getResponseCode();
    if (isHttpOk(statusCode)) {
      // statusCode=2xx，请求成功
      return getStreamForSuccessfulRequest(urlConnection);
    } else if (isHttpRedirect(statusCode)) {
      // statusCode=3xx，需要重定向
      String redirectUrlString = urlConnection.getHeaderField("Location");
      if (TextUtils.isEmpty(redirectUrlString)) {
        throw new HttpException("Received empty or null redirect url");
      }
      URL redirectUrl = new URL(url, redirectUrlString);
      // Closing the stream specifically is required to avoid leaking ResponseBodys in addition
      // to disconnecting the url connection below. See #2352.
      cleanup();
      return loadDataWithRedirects(redirectUrl, redirects + 1, url, headers);
    } else if (statusCode == INVALID_STATUS_CODE) {
      // -1 表示不是HTTP响应
      throw new HttpException(statusCode);
    } else {
      throw new HttpException(urlConnection.getResponseMessage(), statusCode);
    }
  }
  
private static boolean isHttpOk(int statusCode) {
  return statusCode / 100 == 2;
}

private static boolean isHttpRedirect(int statusCode) {
  return statusCode / 100 == 3;
}

现在我们已经获得网络图片的InputStream了，该资源会通过回调经过MultiFetcher到达SourceGenerator中。看下DataCallback回调在SourceGenerator中的实现：

  // SourceGenerator
  @Override
  public void onDataReady(Object data) {
    DiskCacheStrategy diskCacheStrategy = helper.getDiskCacheStrategy();
    if (data != null && diskCacheStrategy.isDataCacheable(loadData.fetcher.getDataSource())) {
      dataToCache = data;
      // We might be being called back on someone else's thread. Before doing anything, we should
      // reschedule to get back onto Glide's thread.
      cb.reschedule();
    } else {
      cb.onDataFetcherReady(loadData.sourceKey, data, loadData.fetcher,
          loadData.fetcher.getDataSource(), originalKey);
    }
  }

  @Override
  public void onLoadFailed(@NonNull Exception e) {
    cb.onDataFetcherFailed(originalKey, e, loadData.fetcher, loadData.fetcher.getDataSource());
  }

onDataReady方法会首先判data能不能缓存，若能缓存则缓存起来，然后调用DataCacheGenerator进行加载缓存；若不能缓存，则直接调用DecodeJob.onDataFetcherReady方法通知外界data已经准备好了。继续跟进
DecodeJob.onDataFetcherReady：

  // DecodeJob
  @Override
  public void onDataFetcherReady(Key sourceKey, Object data, DataFetcher<?> fetcher,
      DataSource dataSource, Key attemptedKey) {
    this.currentSourceKey = sourceKey;
    this.currentData = data;
    this.currentFetcher = fetcher;
    this.currentDataSource = dataSource;
    this.currentAttemptingKey = attemptedKey;
    if (Thread.currentThread() != currentThread) {
      runReason = RunReason.DECODE_DATA;
      callback.reschedule(this);
    } else {
      GlideTrace.beginSection("DecodeJob.decodeFromRetrievedData");
      try {
        decodeFromRetrievedData();
      } finally {
        GlideTrace.endSection();
      }
    }
  }

确认执行线程后调用decodeFromRetrievedData()方法进行解码：

  // DecodeJob
  private void decodeFromRetrievedData() {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey("Retrieved data", startFetchTime,
          "data: " + currentData
              + ", cache key: " + currentSourceKey
              + ", fetcher: " + currentFetcher);
    }
    Resource<R> resource = null;
    try {
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      throwables.add(e);
    }
    if (resource != null) {
      notifyEncodeAndRelease(resource, currentDataSource);
    } else {
      runGenerators();
    }
  }

先调用decodeFromData方法进行解码，然后调用notifyEncodeAndRelease方法进行缓存，同时也会通知EngineJob资源已经准备好了，先看decodeFromData：

  // DecodeJob
  private <Data> Resource<R> decodeFromData(DataFetcher<?> fetcher, Data data,
      DataSource dataSource) throws GlideException {
    try {
      if (data == null) {
        return null;
      }
      long startTime = LogTime.getLogTime();
      Resource<R> result = decodeFromFetcher(data, dataSource);
      if (Log.isLoggable(TAG, Log.VERBOSE)) {
        logWithTimeAndKey("Decoded result " + result, startTime);
      }
      return result;
    } finally {
      fetcher.cleanup();
    }
  }
  
  private <Data> Resource<R> decodeFromFetcher(Data data, DataSource dataSource)
      throws GlideException {
    LoadPath<Data, ?, R> path = decodeHelper.getLoadPath((Class<Data>) data.getClass());
    return runLoadPath(data, dataSource, path);
  }

decodeFromData方法内部又会调用decodeFromFetcher，在decodeFromFetcher方法中首先会获取LoadPath。然后调用runLoadPath方法解析成资源：

// DecodeJob
private <Data, ResourceType> Resource<R> runLoadPath(Data data, DataSource dataSource,
    LoadPath<Data, ResourceType, R> path) throws GlideException {
  Options options = getOptionsWithHardwareConfig(dataSource);
  DataRewinder<Data> rewinder = glideContext.getRegistry().getRewinder(data);
  try {
    // ResourceType in DecodeCallback below is required for compilation to work with gradle.
    return path.load(
        rewinder, options, width, height, new DecodeCallback<ResourceType>(dataSource));
  } finally {
    rewinder.cleanup();
  }
}

注意runLoadPath方法使用到了DataRewinder，这是一个将数据流里面的指针重新指向开头的类，在调用ResourceDecoder对data进行编码时会尝试很多个编码器，所以每一次尝试后都需要重置索引。在path.load(rewinder, options, width, height, new DecodeCallback<ResourceType>(dataSource))这行代码中，最后传入了一个DecodeCallback回调，该类的回调方法会回调给DecodeJob对应的方法：

private final class DecodeCallback<Z> implements DecodePath.DecodeCallback<Z> {

  private final DataSource dataSource;

  @Synthetic
  DecodeCallback(DataSource dataSource) {
    this.dataSource = dataSource;
  }

  @NonNull
  @Override
  public Resource<Z> onResourceDecoded(@NonNull Resource<Z> decoded) {
    return DecodeJob.this.onResourceDecoded(dataSource, decoded);
  }
}

继续跟进LoadPath.load

  // LoadPath
  public Resource<Transcode> load(DataRewinder<Data> rewinder, @NonNull Options options, int width,
      int height, DecodePath.DecodeCallback<ResourceType> decodeCallback) throws GlideException {
    List<Throwable> throwables = Preconditions.checkNotNull(listPool.acquire());
    try {
      return loadWithExceptionList(rewinder, options, width, height, decodeCallback, throwables);
    } finally {
      listPool.release(throwables);
    }
  }

主要调用了loadWithExceptionList方法

// LoadPath
private Resource<Transcode> loadWithExceptionList(DataRewinder<Data> rewinder,
      @NonNull Options options,
      int width, int height, DecodePath.DecodeCallback<ResourceType> decodeCallback,
      List<Throwable> exceptions) throws GlideException {
    Resource<Transcode> result = null;
    //noinspection ForLoopReplaceableByForEach to improve perf
    for (int i = 0, size = decodePaths.size(); i < size; i++) {
      DecodePath<Data, ResourceType, Transcode> path = decodePaths.get(i);
      try {
        result = path.decode(rewinder, width, height, options, decodeCallback);
      } catch (GlideException e) {
        exceptions.add(e);
      }
      if (result != null) {
        break;
      }
    }

    if (result == null) {
      throw new GlideException(failureMessage, new ArrayList<>(exceptions));
    }

    return result;
  }

对于每条DecodePath，都调用其decode方法，直到有一个DecodePath可以decode出资源。继续跟进DecodePath.decode：

  // DecodePath
  public Resource<Transcode> decode(DataRewinder<DataType> rewinder, int width, int height,
      @NonNull Options options, DecodeCallback<ResourceType> callback) throws GlideException {
    // 使用ResourceDecoder List进行decode
    Resource<ResourceType> decoded = decodeResource(rewinder, width, height, options);
    // 将decoded的资源进行transform
    Resource<ResourceType> transformed = callback.onResourceDecoded(decoded);
    // 将transformed的资源进行transcode
    return transcoder.transcode(transformed, options);
  }

先看一下DecodePath.decodeResource

  @NonNull
  private Resource<ResourceType> decodeResource(DataRewinder<DataType> rewinder, int width,
      int height, @NonNull Options options) throws GlideException {
    List<Throwable> exceptions = Preconditions.checkNotNull(listPool.acquire());
    try {
      return decodeResourceWithList(rewinder, width, height, options, exceptions);
    } finally {
      listPool.release(exceptions);
    }
  }

继续跟进DecodePath.decodeResourceWithList

@NonNull
  private Resource<ResourceType> decodeResourceWithList(DataRewinder<DataType> rewinder, int width,
      int height, @NonNull Options options, List<Throwable> exceptions) throws GlideException {
    Resource<ResourceType> result = null;
    
    // decoders只有一条，就是ByteBufferBitmapDecoder
    for (int i = 0, size = decoders.size(); i < size; i++) {
      ResourceDecoder<DataType, ResourceType> decoder = decoders.get(i);
      try {
      
        // rewinder是ByteBufferRewind类型
        // data为ByteBuffer类型
        DataType data = rewinder.rewindAndGet();
        if (decoder.handles(data, options)) {
          // 调用ByteBuffer.position(0)复位
          data = rewinder.rewindAndGet();
          // 开始解码
          result = decoder.decode(data, width, height, options);
        }
       
      } catch (IOException | RuntimeException | OutOfMemoryError e) {
        ...
      }

      if (result != null) {
        break;
      }
    }
    ...
    return result;
  }

继续跟进ByteBufferBitmapDecoder.decode方法

  @Override
  public Resource<Bitmap> decode(@NonNull ByteBuffer source, int width, int height,
      @NonNull Options options)
      throws IOException {
    InputStream is = ByteBufferUtil.toStream(source);
    return downsampler.decode(is, width, height, options);
  }

先将ByteBuffer转换成InputStream，然后在调用Downsampler.decode方法进行解码。回到DecodePath.decode，解码后调用callback.onResourceDecoded(decoded);，DecodeJob中的DecodeCallback实现了DecodePath.DecodeCallback的onResourceDecoded方法，该方法里面调用了DecodeJob.onResourceDecoded(dataSource, decoded)：

// DecodeJob
<Z> Resource<Z> onResourceDecoded(DataSource dataSource,
      @NonNull Resource<Z> decoded) {

    Class<Z> resourceSubClass = (Class<Z>) decoded.get().getClass();
    Transformation<Z> appliedTransformation = null;
    Resource<Z> transformed = decoded;
    
    // dataSource为DATA_DISK_CACHE，所以满足条件
    if (dataSource != DataSource.RESOURCE_DISK_CACHE) {
      appliedTransformation = decodeHelper.getTransformation(resourceSubClass);
      transformed = appliedTransformation.transform(glideContext, decoded, width, height);
    }
    
    ...

    final EncodeStrategy encodeStrategy;
    final ResourceEncoder<Z> encoder;
    // Bitmap有注册对应的BitmapEncoder，所以是available的
    if (decodeHelper.isResourceEncoderAvailable(transformed)) {
      // encoder就是BitmapEncoder
      encoder = decodeHelper.getResultEncoder(transformed);
      // encodeStrategy为EncodeStrategy.TRANSFORMED
      encodeStrategy = encoder.getEncodeStrategy(options);
    } else {
      encoder = null;
      encodeStrategy = EncodeStrategy.NONE;
    }

    Resource<Z> result = transformed;
    
    ...

    return result;
  }

继续回到DecodePath.decode方法中的transcoder.transcode(transformed, options);
这里的transcoder就是BitmapDrawableTranscoder，该方法返回了一个LazyBitmapDrawableResource。至此，资源已经解码完毕。然后我们继续回到 DecodeJob.decodeFromRetrievedData方法中：

// DecodeJob
private void decodeFromRetrievedData() {
    if (Log.isLoggable(TAG, Log.VERBOSE)) {
      logWithTimeAndKey("Retrieved data", startFetchTime,
          "data: " + currentData
              + ", cache key: " + currentSourceKey
              + ", fetcher: " + currentFetcher);
    }
    Resource<R> resource = null;
    try {
      resource = decodeFromData(currentFetcher, currentData, currentDataSource);
    } catch (GlideException e) {
      e.setLoggingDetails(currentAttemptingKey, currentDataSource);
      throwables.add(e);
    }
    if (resource != null) {
      notifyEncodeAndRelease(resource, currentDataSource);
    } else {
      runGenerators();
    }
  }

拿到 resource 之后会调用notifyEncodeAndRelease(resource, currentDataSource);，跟进 DecodeJob.notifyEncodeAndRelease：

// DecodeJob
private void notifyEncodeAndRelease(Resource<R> resource, DataSource dataSource) {

    // resource是BitmapResource类型，实现了Initializable接口
    if (resource instanceof Initializable) {
      // initialize方法调用了bitmap.prepareToDraw()
      ((Initializable) resource).initialize();
    }

    Resource<R> result = resource;
    ...

    // 通知回调，资源已经就绪
    notifyComplete(result, dataSource);

    ...
    
    // 进行清理工作
    onEncodeComplete();
  }

notifyComplete方法中该方法内部会调用callback.onResourceReady(resource, dataSource)将结果传递给回调，这里的回调是EngineJob：

// EngineJob
  @Override
  public void onResourceReady(Resource<R> resource, DataSource dataSource) {
    synchronized (this) {
      this.resource = resource;
      this.dataSource = dataSource;
    }
    notifyCallbacksOfResult();
  }

这里存储资源后继续调用EngineJob.notifyCallbacksOfResult

// EngineJob
@Synthetic
  void notifyCallbacksOfResult() {
    ResourceCallbacksAndExecutors copy;
    Key localKey;
    EngineResource<?> localResource;
    synchronized (this) {
      stateVerifier.throwIfRecycled();
      if (isCancelled) {
        resource.recycle();
        release();
        return;
      } else if (cbs.isEmpty()) {
        throw new IllegalStateException("Received a resource without any callbacks to notify");
      } else if (hasResource) {
        throw new IllegalStateException("Already have resource");
      }
      
      // new EngineResource<>(resource, isMemoryCacheable, /*isRecyclable=*/ true)
      engineResource = engineResourceFactory.build(resource, isCacheable);
     
      hasResource = true;
      copy = cbs.copy();
      incrementPendingCallbacks(copy.size() + 1);

      localKey = key;
      localResource = engineResource;
    }

    // listener就是Engine，该方法会将资源保存到activeResources中
    listener.onEngineJobComplete(this, localKey, localResource);

    // 这里的ResourceCallbackAndExecutor就是之前创建EngineJob和DecodeJob时并在执行DecodeJob之前添加的回调
    // entry.executor就是Glide.with.load.into中出现的Executors.mainThreadExecutor()
    // entry.cb就是SingleRequest
    for (final ResourceCallbackAndExecutor entry : copy) {
      entry.executor.execute(new CallResourceReady(entry.cb));
    }
    decrementPendingCallbacks();
  }

先来看下listener.onEngineJobComplete(this, localKey, localResource)，listener就是Engine，跟进Engine.onEngineJobComplete：

  // Engine
  @Override
  public synchronized void onEngineJobComplete(
      EngineJob<?> engineJob, Key key, EngineResource<?> resource) {
    
    // 设置资源的回调为自己，这样在资源释放时会通知自己的回调方法
    if (resource != null) {
      resource.setResourceListener(key, this);

      // 将资源放入activeResources中，资源变为active状态
      if (resource.isCacheable()) {
        activeResources.activate(key, resource);
      }
    }

    // 将engineJob从Jobs中移除
    jobs.removeIfCurrent(key, engineJob);
  }

再回到EngineJob.notifyCallbacksOfResult，看下entry.executor.execute(new CallResourceReady(entry.cb))，entry.executor就是Glide.with.load.into中出现的Executors.mainThreadExecutor()，内部使用MainLooper的Handler，在execute Runnable时使用此Handler post出去，这里entry.cb就是SingleRequest，所以我们要看下CallResourceReady：

  // EngineJob
  private class CallResourceReady implements Runnable {

    private final ResourceCallback cb;

    CallResourceReady(ResourceCallback cb) {
      this.cb = cb;
    }

    @Override
    public void run() {
      synchronized (EngineJob.this) {
        if (cbs.contains(cb)) {
          engineResource.acquire();
          // 调用callback
          callCallbackOnResourceReady(cb);
          // 移除callback
          removeCallback(cb);
        }
        decrementPendingCallbacks();
      }
    }
  }

跟进EngineJob.callCallbackOnResourceReady

  // EngineJob
  @Synthetic
  synchronized void callCallbackOnResourceReady(ResourceCallback cb) {
    try {
      // 调用SingleRequest.onResourceReady
      cb.onResourceReady(engineResource, dataSource);
    } catch (Throwable t) {
      throw new CallbackException(t);
    }
  }

继续跟进SingleRequest.onResourceReady

// SingleRequest
@Override
  public synchronized void onResourceReady(Resource<?> resource, DataSource dataSource) {
    stateVerifier.throwIfRecycled();
    loadStatus = null;
    if (resource == null) {
      ...
      onLoadFailed(exception);
      return;
    }

    Object received = resource.get();
    if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
      releaseResource(resource);
      ...
      onLoadFailed(exception);
      return;
    }

    ...

    onResourceReady((Resource<R>) resource, (R) received, dataSource);
  }

这里进行一些资源的判空处理，然后调用onResourceReady((Resource<R>) resource, (R) received, dataSource);

// SingleRequest
private synchronized void onResourceReady(Resource<R> resource, R result, DataSource dataSource) {
    // We must call isFirstReadyResource before setting status.
    
    // 由于requestCoordinator为null，所以返回true
    boolean isFirstResource = isFirstReadyResource();
    // 将status状态设置为COMPLETE
    status = Status.COMPLETE;
    this.resource = resource;

    ...

    isCallingCallbacks = true;
    try {
      // 尝试调用各个listener的onResourceReady回调进行处理
      boolean anyListenerHandledUpdatingTarget = false;
      if (requestListeners != null) {
        for (RequestListener<R> listener : requestListeners) {
          anyListenerHandledUpdatingTarget |=
              listener.onResourceReady(result, model, target, dataSource, isFirstResource);
        }
      }
      anyListenerHandledUpdatingTarget |=
          targetListener != null
              && targetListener.onResourceReady(result, model, target, dataSource, isFirstResource);

      // 如果没有一个回调能够处理，那么自己处理
      if (!anyListenerHandledUpdatingTarget) {
        Transition<? super R> animation =
            animationFactory.build(dataSource, isFirstResource);
            
        // target为DrawableImageViewTarget
        target.onResourceReady(result, animation);
      }
    } finally {
      isCallingCallbacks = false;
    }

    notifyLoadSuccess();
  }

DrawableImageViewTarget的基类ImageViewTarget实现了onResourceReady(result, animation)方法：

  // ImageViewTarget
  @Override
  public void onResourceReady(@NonNull Z resource, @Nullable Transition<? super Z> transition) {
    // NO_ANIMATION.transition返回false
    if (transition == null || !transition.transition(resource, this)) {
      setResourceInternal(resource);
    } else {
      maybeUpdateAnimatable(resource);
    }
  }
  
  private void setResourceInternal(@Nullable Z resource) {
    // 设置资源图片
    setResource(resource);
    // 有动画则执行动画
    maybeUpdateAnimatable(resource);
  }
  
  protected abstract void setResource(@Nullable Z resource);

setResource 方法由 DrawableImageViewTarget 实现

  // DrawableImageViewTarget
  @Override
  protected void setResource(@Nullable Drawable resource) {
    // view 是 ImageView 类型
    view.setImageDrawable(resource);
  }

至此为止，Glide.with(this).load(url).into(imageView)已经将网络图片正确展示在ImageView上。

总结

Glide.with(this).load(url).into(imageView)整个源码调用时序图如下：

参考链接
https://muyangmin.github.io/glide-docs-cn/
https://blog.yorek.xyz/android/3rd-library/glide2/