深入理解ART虚拟机—ImageSpace的加载过程分析

上一篇《深入理解ART虚拟机—虚拟机的启动》分析了art虚拟机的启动过程，不过跳过了一个技术点，就是ImageSpace。由之前的分析可知，ClassLinker是用来加载类对象的，有三类类对象，一是ClassRoot，包括java.lang包下的类，这类的类对象是语言自己的，不需要import，ART虚拟机是定义在mirror目录下的。二是boot_class_path，包括jdk和android.jar，这类的类对象是通过指定class path或者系统的Image文件。三是apk自己的dex/oat。ClassLinker的InitFromImage是通过ImageSpace来初始化ClassRoot和boot class path，InitWithoutImage的ClassRoot是直接AllocClass，boot class path是由直接由class path的DexFile生成DexCache加入dex_caches来生成的。第三类类对象是和应用自身相关的，都是走DexCache。

Runtime初始化的时候会创建gc::Heap对象，在Heap的构造函数会创建ImageSpace的实例。

    auto* image_space = space::ImageSpace::Create(image_file_name.c_str(), image_instruction_set, &error_msg);

ImageSpace::Create是ImageSpace的静态构造方法，先找到image文件的路径，之后会生成load镜像文件，具体代码如下：

ImageSpace* ImageSpace::Create(const char* image_location,
                               const InstructionSet image_isa,
                               std::string* error_msg) {
  std::string system_filename;
  bool has_system = false;
  std::string cache_filename;
  bool has_cache = false;
  bool dalvik_cache_exists = false;
  bool is_global_cache = true;
  const bool found_image = FindImageFilename(image_location, image_isa, &system_filename,
                                             &has_system, &cache_filename, &dalvik_cache_exists,
                                             &has_cache, &is_global_cache);

  if (Runtime::Current()->IsZygote()) {
    MarkZygoteStart(image_isa, Runtime::Current()->GetZygoteMaxFailedBoots());
  }

  ImageSpace* space;
  bool relocate = Runtime::Current()->ShouldRelocate();
  bool can_compile = Runtime::Current()->IsImageDex2OatEnabled();
  if (found_image) {
    const std::string* image_filename;
    bool is_system = false;
    bool relocated_version_used = false;
    if (relocate) {
      if (!dalvik_cache_exists) {
        *error_msg = StringPrintf("Requiring relocation for image '%s' at '%s' but we do not have "
                                  "any dalvik_cache to find/place it in.",
                                  image_location, system_filename.c_str());
        return nullptr;
      }
      if (has_system) {
        if (has_cache && ChecksumsMatch(system_filename.c_str(), cache_filename.c_str())) {
          // We already have a relocated version
          image_filename = &cache_filename;
          relocated_version_used = true;
        } else {
          // We cannot have a relocated version, Relocate the system one and use it.

          std::string reason;
          bool success;

          // Check whether we are allowed to relocate.
          if (!can_compile) {
            reason = "Image dex2oat disabled by -Xnoimage-dex2oat.";
            success = false;
          } else if (!ImageCreationAllowed(is_global_cache, &reason)) {
            // Whether we can write to the cache.
            success = false;
          } else {
            // Try to relocate.
            success = RelocateImage(image_location, cache_filename.c_str(), image_isa, &reason);
          }

          if (success) {
            relocated_version_used = true;
            image_filename = &cache_filename;
          } else {
            *error_msg = StringPrintf("Unable to relocate image '%s' from '%s' to '%s': %s",
                                      image_location, system_filename.c_str(),
                                      cache_filename.c_str(), reason.c_str());
            // We failed to create files, remove any possibly garbage output.
            // Since ImageCreationAllowed was true above, we are the zygote
            // and therefore the only process expected to generate these for
            // the device.
            PruneDalvikCache(image_isa);
            return nullptr;
          }
        }
      } else {
        CHECK(has_cache);
        // We can just use cache's since it should be fine. This might or might not be relocated.
        image_filename = &cache_filename;
      }
    } else {
      if (has_system && has_cache) {
        // Check they have the same cksum. If they do use the cache. Otherwise system.
        if (ChecksumsMatch(system_filename.c_str(), cache_filename.c_str())) {
          image_filename = &cache_filename;
          relocated_version_used = true;
        } else {
          image_filename = &system_filename;
          is_system = true;
        }
      } else if (has_system) {
        image_filename = &system_filename;
        is_system = true;
      } else {
        CHECK(has_cache);
        image_filename = &cache_filename;
      }
    }
    {
      // Note that we must not use the file descriptor associated with
      // ScopedFlock::GetFile to Init the image file. We want the file
      // descriptor (and the associated exclusive lock) to be released when
      // we leave Create.
      ScopedFlock image_lock;
      image_lock.Init(image_filename->c_str(), error_msg);
      VLOG(startup) << "Using image file " << image_filename->c_str() << " for image location "
                    << image_location;
      // If we are in /system we can assume the image is good. We can also
      // assume this if we are using a relocated image (i.e. image checksum
      // matches) since this is only different by the offset. We need this to
      // make sure that host tests continue to work.
      space = ImageSpace::Init(image_filename->c_str(), image_location,
                               !(is_system || relocated_version_used), error_msg);
    }
    if (space != nullptr) {
      return space;
    }

    if (relocated_version_used) {
      // Something is wrong with the relocated copy (even though checksums match). Cleanup.
      // This can happen if the .oat is corrupt, since the above only checks the .art checksums.
      // TODO: Check the oat file validity earlier.
      *error_msg = StringPrintf("Attempted to use relocated version of %s at %s generated from %s "
                                "but image failed to load: %s",
                                image_location, cache_filename.c_str(), system_filename.c_str(),
                                error_msg->c_str());
      PruneDalvikCache(image_isa);
      return nullptr;
    } else if (is_system) {
      // If the /system file exists, it should be up-to-date, don't try to generate it.
      *error_msg = StringPrintf("Failed to load /system image '%s': %s",
                                image_filename->c_str(), error_msg->c_str());
      return nullptr;
    } else {
      // Otherwise, log a warning and fall through to GenerateImage.
      LOG(WARNING) << *error_msg;
    }
  }

  if (!can_compile) {
    *error_msg = "Not attempting to compile image because -Xnoimage-dex2oat";
    return nullptr;
  } else if (!dalvik_cache_exists) {
    *error_msg = StringPrintf("No place to put generated image.");
    return nullptr;
  } else if (!ImageCreationAllowed(is_global_cache, error_msg)) {
    return nullptr;
  } else if (!GenerateImage(cache_filename, image_isa, error_msg)) {
    *error_msg = StringPrintf("Failed to generate image '%s': %s",
                              cache_filename.c_str(), error_msg->c_str());
    // We failed to create files, remove any possibly garbage output.
    // Since ImageCreationAllowed was true above, we are the zygote
    // and therefore the only process expected to generate these for
    // the device.
    PruneDalvikCache(image_isa);
    return nullptr;
  } else {
    // Check whether there is enough space left over after we have generated the image.
    if (!CheckSpace(cache_filename, error_msg)) {
      // No. Delete the generated image and try to run out of the dex files.
      PruneDalvikCache(image_isa);
      return nullptr;
    }

    // Note that we must not use the file descriptor associated with
    // ScopedFlock::GetFile to Init the image file. We want the file
    // descriptor (and the associated exclusive lock) to be released when
    // we leave Create.
    ScopedFlock image_lock;
    image_lock.Init(cache_filename.c_str(), error_msg);
    space = ImageSpace::Init(cache_filename.c_str(), image_location, true, error_msg);
    if (space == nullptr) {
      *error_msg = StringPrintf("Failed to load generated image '%s': %s",
                                cache_filename.c_str(), error_msg->c_str());
    }
    return space;
  }
}

FindImageFilename会查找系统和data区是否有image file，系统区镜像的目录是/system/framework/<image_isa>/boot.art，data区的image file的路径是/data/dalvik-cache/<image_isa>/boot.art。/system 下面的是在系统区，是ROM带的，而/data 下的image file是系统运行时生成的，GenerateImage就是生成这个cache image file的函数。image file存在的情况下，就能调用ImageSpace::Init来初始化ImageSpace了。

ImageSpace* ImageSpace::Init(const char* image_filename, const char* image_location,
                             bool validate_oat_file, std::string* error_msg) {
  CHEC