編輯：關於Android編程
上一篇《深入理解ART虛擬機—虛擬機的啟動》分析了art虛擬機的啟動過程，不過跳過了一個技術點，就是ImageSpace。由之前的分析可知，ClassLinker是用來加載類對象的，有三類類對象，一是ClassRoot，包括java.lang包下的類，這類的類對象是語言自己的，不需要import，ART虛擬機是定義在mirror目錄下的。二是boot_class_path，包括jdk和android.jar，這類的類對象是通過指定class path或者系統的Image文件。三是apk自己的dex/oat。ClassLinker的InitFromImage是通過ImageSpace來初始化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來生成的。第三類類對象是和應用自身相關的，都是走DexCache。
Runtime初始化的時候會創建gc::Heap對象，在Heap的構造函數會創建ImageSpace的實例。
auto* image_space = space::ImageSpace::Create(image_file_name.c_str(), image_instruction_set, &error_msg);
ImageSpace::Create是ImageSpace的靜態構造方法，先找到image文件的路徑，之後會生成load鏡像文件，具體代碼如下：
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會查找系統和data區是否有image file，系統區鏡像的目錄是/system/framework/
ImageSpace* ImageSpace::Init(const char* image_filename, const char* image_location,
bool validate_oat_file, std::string* error_msg) {
CHECK(image_filename != nullptr);
CHECK(image_location != nullptr);
uint64_t start_time = 0;
if (VLOG_IS_ON(heap) || VLOG_IS_ON(startup)) {
start_time = NanoTime();
LOG(INFO) << "ImageSpace::Init entering image_filename=" << image_filename;
}
std::unique_ptr file(OS::OpenFileForReading(image_filename));
if (file.get() == nullptr) {
*error_msg = StringPrintf("Failed to open '%s'", image_filename);
return nullptr;
}
ImageHeader image_header;
bool success = file->ReadFully(&image_header, sizeof(image_header));
if (!success || !image_header.IsValid()) {
*error_msg = StringPrintf("Invalid image header in '%s'", image_filename);
return nullptr;
}
// Check that the file is large enough.
uint64_t image_file_size = static_cast(file->GetLength());
if (image_header.GetImageSize() > image_file_size) {
*error_msg = StringPrintf("Image file too small for image heap: %" PRIu64 " vs. %zu.",
image_file_size, image_header.GetImageSize());
return nullptr;
}
if (kIsDebugBuild) {
LOG(INFO) << "Dumping image sections";
for (size_t i = 0; i < ImageHeader::kSectionCount; ++i) {
const auto section_idx = static_cast(i);
auto& section = image_header.GetImageSection(section_idx);
LOG(INFO) << section_idx << " start="
<< reinterpret_cast(image_header.GetImageBegin() + section.Offset()) << " "
<< section;
}
}
const auto& bitmap_section = image_header.GetImageSection(ImageHeader::kSectionImageBitmap);
auto end_of_bitmap = static_cast(bitmap_section.End());
if (end_of_bitmap != image_file_size) {
*error_msg = StringPrintf(
"Image file size does not equal end of bitmap: size=%" PRIu64 " vs. %zu.", image_file_size,
end_of_bitmap);
return nullptr;
}
// Note: The image header is part of the image due to mmap page alignment required of offset.
std::unique_ptr map(MemMap::MapFileAtAddress(
image_header.GetImageBegin(), image_header.GetImageSize(),
PROT_READ | PROT_WRITE, MAP_PRIVATE, file->Fd(), 0, false, image_filename, error_msg));
if (map.get() == nullptr) {
DCHECK(!error_msg->empty());
return nullptr;
}
CHECK_EQ(image_header.GetImageBegin(), map->Begin());
DCHECK_EQ(0, memcmp(&image_header, map->Begin(), sizeof(ImageHeader)));
std::unique_ptr image_map(MemMap::MapFileAtAddress(
nullptr, bitmap_section.Size(), PROT_READ, MAP_PRIVATE, file->Fd(),
bitmap_section.Offset(), false, image_filename, error_msg));
if (image_map.get() == nullptr) {
*error_msg = StringPrintf("Failed to map image bitmap: %s", error_msg->c_str());
return nullptr;
}
uint32_t bitmap_index = bitmap_index_.FetchAndAddSequentiallyConsistent(1);
std::string bitmap_name(StringPrintf("imagespace %s live-bitmap %u", image_filename,
bitmap_index));
std::unique_ptr bitmap(
accounting::ContinuousSpaceBitmap::CreateFromMemMap(
bitmap_name, image_map.release(), reinterpret_cast(map->Begin()),
accounting::ContinuousSpaceBitmap::ComputeHeapSize(bitmap_section.Size())));
if (bitmap.get() == nullptr) {
*error_msg = StringPrintf("Could not create bitmap '%s'", bitmap_name.c_str());
return nullptr;
}
// We only want the mirror object, not the ArtFields and ArtMethods.
uint8_t* const image_end =
map->Begin() + image_header.GetImageSection(ImageHeader::kSectionObjects).End();
std::unique_ptr space(new ImageSpace(image_filename, image_location,
map.release(), bitmap.release(), image_end));
// VerifyImageAllocations() will be called later in Runtime::Init()
// as some class roots like ArtMethod::java_lang_reflect_ArtMethod_
// and ArtField::java_lang_reflect_ArtField_, which are used from
// Object::SizeOf() which VerifyImageAllocations() calls, are not
// set yet at this point.
space->oat_file_.reset(space->OpenOatFile(image_filename, error_msg));
if (space->oat_file_.get() == nullptr) {
DCHECK(!error_msg->empty());
return nullptr;
}
space->oat_file_non_owned_ = space->oat_file_.get();
if (validate_oat_file && !space->ValidateOatFile(error_msg)) {
DCHECK(!error_msg->empty());
return nullptr;
}
Runtime* runtime = Runtime::Current();
runtime->SetInstructionSet(space->oat_file_->GetOatHeader().GetInstructionSet());
runtime->SetResolutionMethod(image_header.GetImageMethod(ImageHeader::kResolutionMethod));
runtime->SetImtConflictMethod(image_header.GetImageMethod(ImageHeader::kImtConflictMethod));
runtime->SetImtUnimplementedMethod(
image_header.GetImageMethod(ImageHeader::kImtUnimplementedMethod));
runtime->SetCalleeSaveMethod(
image_header.GetImageMethod(ImageHeader::kCalleeSaveMethod), Runtime::kSaveAll);
runtime->SetCalleeSaveMethod(
image_header.GetImageMethod(ImageHeader::kRefsOnlySaveMethod), Runtime::kRefsOnly);
runtime->SetCalleeSaveMethod(
image_header.GetImageMethod(ImageHeader::kRefsAndArgsSaveMethod), Runtime::kRefsAndArgs);
if (VLOG_IS_ON(heap) || VLOG_IS_ON(startup)) {
LOG(INFO) << "ImageSpace::Init exiting (" << PrettyDuration(NanoTime() - start_time)
<< ") " << *space.get();
}
return space.release();
}
分析Init函數，先是打開文件讀出文件的內容
std::unique_ptrfile(OS::OpenFileForReading(image_filename)); if (file.get() == nullptr) { *error_msg = StringPrintf("Failed to open '%s'", image_filename); return nullptr; }
boot.art的ImageHeader布局如下：
ImageHeader image_header;
bool success = file->ReadFully(&image_header, sizeof(image_header));
if (!success || !image_header.IsValid()) {
*error_msg = StringPrintf("Invalid image header in '%s'", image_filename);
return nullptr;
}
std::unique_ptrHeader的開頭是magic string，接著Image Begin和ImageSize，就是鏡像的開始和大小，再後面就是Bitmap的offset和size，bitmap的session被mmap之後會創建bitmap的SpaceBitmap，這個主要是作為ImageSpace的live_bitmap，GC的時候會用。map(MemMap::MapFileAtAddress( image_header.GetImageBegin(), image_header.GetImageSize(), PROT_READ | PROT_WRITE, MAP_PRIVATE, file->Fd(), 0, false, image_filename, error_msg)); if (map.get() == nullptr) { DCHECK(!error_msg->empty()); return nullptr; }
