PathClassLoader加载与查找类

简述

PathClassLoader加载的过程 :

• 通过DexPathList.addDexPath来加载Dex文件
• 通过makeDexElements来加载DexElement , 每一个Element就是一个Dex
• 在Native层 , 加载Oat/dex文件
• 根据加载到内存的基址来找到各个Section
• 进行DexFile文件的校验
• 校验成功后 , 通过ClassLinker->class_table添加class_table
• 最后返回dex_files数组所在的Cookie

PathClassLoader的查找过程 :

• 通过DexFile.loadClassBinaryName加载
• 调用到dalvik_system_DexFile.cc中的DexFile_defineClassNative函数
• 将Cookie也就是dex_files基址传给Native , 初始化oat_file以及dex_files
• 根据类名生成描述符descriptor
• 遍历dex_files , 从dex_file中查找Class

PathClassLoader

PathClassLoader继承自BaseDexClassLoader , 主要是把dexPath传递给BaseDexClassLoader

public class PathClassLoader extends BaseDexClassLoader {

    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }
    public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
        super(dexPath, null, librarySearchPath, parent);
    }
}

BaseDexClassLoader

主要是通过DexPathList来查找类 , 以及添加Dex路径 , 初始化DexFile的

public class BaseDexClassLoader extends ClassLoader {

    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
        Class c = pathList.findClass(name, suppressedExceptions);
        if (c == null) {
            ClassNotFoundException cnfe = new ClassNotFoundException(
                    "Didn't find class \"" + name + "\" on path: " + pathList);
            for (Throwable t : suppressedExceptions) {
                cnfe.addSuppressed(t);
            }
            throw cnfe;
        }
        return c;
    }

    public void addDexPath(String dexPath) {
        pathList.addDexPath(dexPath, null /*optimizedDirectory*/);
    }
}

DexPathList

1. DexPathList构造函数

• 通过makeDexElements加载Dex文件 , 生成Element对象
• 通过makePathElement加载动态链接库 , 生成Element对象

public DexPathList(ClassLoader definingContext, String dexPath,
            String librarySearchPath, File optimizedDirectory) {  
        ...
        // 通过makeDexElements来加载dexPath的路径
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions, definingContext);

        // 初始化so的路径
        this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
        // 初始化系统So路径
        this.systemNativeLibraryDirectories =
                splitPaths(System.getProperty("java.library.path"), true);
        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
        // 添加所有So路径集合
        allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
        // 加载动态链接库的Element
        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories);
        ...
    }

2. makeDexElements

• 遍历Dex文件 , 加载Dex文件 , 放入Elements数组中

private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
            List<IOException> suppressedExceptions, ClassLoader loader) {
      Element[] elements = new Element[files.size()];
      int elementsPos = 0;
      // 遍历所有的Dex文件路径
      for (File file : files) {
          if (file.isDirectory()) {
              // 将目录文件放到对应的数组中
              elements[elementsPos++] = new Element(file);
          } else if (file.isFile()) {
              String name = file.getName();
              // 判断是否以.dex结尾
              if (name.endsWith(DEX_SUFFIX)) {
                  try {
                      // 通过loadDexFile加载dexs文件
                      DexFile dex = loadDexFile(file, optimizedDirectory, loader, elements);
                      if (dex != null) {
                          // 将dex放到Elements数组中
                          elements[elementsPos++] = new Element(dex, null);
                      }
                  } catch (IOException suppressed) {
                  }
              } else {
                  DexFile dex = null;
                  try {
                      // 如果不以dex结尾也会通过`loadDexFile`尝试加载
                      dex = loadDexFile(file, optimizedDirectory, loader, elements);
                  } catch (IOException suppressed) {
                      suppressedExceptions.add(suppressed);
                  }
                  if (dex == null) {
                      // 如果加载失败 , 将文件放到elements中
                      elements[elementsPos++] = new Element(file);
                  } else {
                      elements[elementsPos++] = new Element(dex, file);
                  }
              }
          } else {
              System.logW("ClassLoader referenced unknown path: " + file);
          }
      }
      return elements;
    }

3. loadDexFile

• 通过两种方式加载DexFile

private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader,
                                       Element[] elements)
            throws IOException {
        if (optimizedDirectory == null) {
            // 如果odex文件路径为空的话 , 则会直接创建DexFile
            return new DexFile(file, loader, elements);
        } else {
            // 如果odex文件路径不为空的话 , 则会通过DexFile.loadDex从odex中直接读取
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
            return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
        }
    }

4. DexFile

• 校验uid
• 通过openDexFileNative加载Dex文件
• 返回Cookie代表JNI层dex_file的指针

private DexFile(String sourceName, String outputName, int flags, ClassLoader loader,
            DexPathList.Element[] elements) throws IOException {
        if (outputName != null) {
            try {
                String parent = new File(outputName).getParent();
                // 检查文件uid是否相同 , 如果不同的话 , 则抛异常
                if (Libcore.os.getuid() != Libcore.os.stat(parent).st_uid) {
                    throw new IllegalArgumentException("Optimized data directory " + parent
                            + " is not owned by the current user. Shared storage cannot protect"
                            + " your application from code injection attacks.");
                }
            } catch (ErrnoException ignored) {
                // assume we'll fail with a more contextual error later
            }
        }
        // 调用openDexFileNative函数 , 加载Dex文件
        mCookie = openDexFile(sourceName, outputName, flags, loader, elements);
        mInternalCookie = mCookie;
        mFileName = sourceName;
        //System.out.println("DEX FILE cookie is " + mCookie + " sourceName=" + sourceName + " outputName=" + outputName);
    }

5. openDexFileNative

• 获取ClassLinker对象
• 通过OatFilemanager.OpenDexFilesFromOat从oat文件中生成dex的数组(Vector)
• 将·dex_files·数组的地址返回给Java层

static jobject DexFile_openDexFileNative(JNIEnv* env,
                                         jclass,
                                         jstring javaSourceName,
                                         jstring javaOutputName ATTRIBUTE_UNUSED,
                                         jint flags ATTRIBUTE_UNUSED,
                                         jobject class_loader,
                                         jobjectArray dex_elements) {
  // 获取Runtime
  Runtime* const runtime = Runtime::Current();
  // 获取ClassLinker
  ClassLinker* linker = runtime->GetClassLinker();
  std::vector<std::unique_ptr<const DexFile>> dex_files;
  std::vector<std::string> error_msgs;
  const OatFile* oat_file = nullptr;
  // 通过OpenDexFilesFromOat函数加载文件
  dex_files = runtime->GetOatFileManager().OpenDexFilesFromOat(sourceName.c_str(),
                                                               class_loader,
                                                               dex_elements,
                                                               /*out*/ &oat_file,
                                                               /*out*/ &error_msgs);
  // 如果dex_files不为空的话
  if (!dex_files.empty()) {
    // 将DexFile转成地址数组
    jlongArray array = ConvertDexFilesToJavaArray(env, oat_file, dex_files);
    if (array == nullptr) {
      ScopedObjectAccess soa(env);
      // 如果array为空 , 则释放defFile
      for (auto& dex_file : dex_files) {
        if (linker->IsDexFileRegistered(soa.Self(), *dex_file)) {
          dex_file.release();
        }
      }
    }
    // 返回array数组作为Cookie返回
    return array;
  } else {
    ScopedObjectAccess soa(env);
    CHECK(!error_msgs.empty());
   // 抛异常
    auto it = error_msgs.begin();
    auto itEnd = error_msgs.end();
    for ( ; it != itEnd; ++it) {
      ThrowWrappedIOException("%s", it->c_str());
    }
    return nullptr;
  }
}

6. 在art/runtime/oat_file_manager.cc中

• 初始化oat_file_assistant对象 , 主要用于保存odex、oat路径
• oat_file类型为ElfOatFile
• 通过oat_file_assistant.OpenImageSpace初始化ImageSpace, 其中会读取oat文件的header、data , 然后校验oat文件的checksum
• 接着校验Section、Map , 最后得到ImageSpace
• 将ImageSpace添加到Heap
• 主要通过ClassLinker->AddImageSpace将oat/dex文件添加到JNIEnv中
• 添加成功后 , 释放imageSpace
• 如果没有添加成功 , 则通过LoadDexFiles重新尝试加载一次

td::vector<std::unique_ptr<const DexFile>> OatFileManager::OpenDexFilesFromOat(
    const char* dex_location,
    jobject class_loader,
    jobjectArray dex_elements,
    const OatFile** out_oat_file,
    std::vector<std::string>* error_msgs) {
  // 此处不用Mutex进行并发 , 因为分配oat/dex文件会导致大量GC
  Thread* const self = Thread::Current();
  // 验证没有锁
  Locks::mutator_lock_->AssertNotHeld(self);
  // 获取Rumtine
  Runtime* const runtime = Runtime::Current();
  // 创建OatFileAssistant对象 , 内部会判断当前的指令集与SOC架构是否匹配
  // 同时会找到odex以及oat文件的路径
  OatFileAssistant oat_file_assistant(dex_location,
                                      kRuntimeISA,
                                      !runtime->IsAotCompiler());
  ... 
  const OatFile* source_oat_file = nullptr;
  // 从路径中读取oat_file , 类型为ElfOatFile
  std::unique_ptr<const OatFile> oat_file(oat_file_assistant.GetBestOatFile().release());
 
    if (accept_oat_file) {
      VLOG(class_linker) << "Registering " << oat_file->GetLocation();
      // 将oat_file插入oat_files_队列中
      source_oat_file = RegisterOatFile(std::move(oat_file));
      *out_oat_file = source_oat_file;
    }
  }
  // 声明oat文件中的dex_file
  std::vector<std::unique_ptr<const DexFile>> dex_files;

  // 开始从oat文件中加载dex
  if (source_oat_file != nullptr) {
    bool added_image_space = false;
    // 判断是否为可执行文件 , 因为是ELF文件 , 所以为true
    if (source_oat_file->IsExecutable()) {
      // 调用oat_file_assistant.OpenImageSpace函数将oat文件加载到内存中
      // 并且初始化class_,method_等数组对象
      std::unique_ptr<gc::space::ImageSpace> image_space =
          kEnableAppImage ? oat_file_assistant.OpenImageSpace(source_oat_file) : nullptr;
      // 如果加载成功
      if (image_space != nullptr) {
          {
            ...
            // 将oat等镜像空间添加到Heap中
            runtime->GetHeap()->AddSpace(image_space.get());
          }
          {
            // 将dex_files添加到ClassLinker中
            added_image_space = runtime->GetClassLinker()->AddImageSpace(image_space.get(),h_loader,dex_elements, dex_location,
                                                                         /*out*/&dex_files,
                                                                         /*out*/&temp_error_msg);
          }
          // 如果添加成功 , 则释放image_space
          if (added_image_space) {
            image_space.release();
          } else {
            LOG(INFO) << "Failed to add image file " << temp_error_msg;
            // 否则清理dex_files
            dex_files.clear();
            {
              // 清理Space
              runtime->GetHeap()->RemoveSpace(image_space.get());
            }
          }
        }
      }
    }
    if (!added_image_space) {
      DCHECK(dex_files.empty());
      // 如果classLinker添加失败 , 则重新加载一次
      dex_files = oat_file_assistant.LoadDexFiles(*source_oat_file, dex_location);
    }
     ...
  return dex_files;
}

7. 在ClassLinker. AddImageSpace中

• 从ImageSpace中获取OatFile指针
• 遍历dex_caches, 通过OpenOatDexFile生成dex_file对象
• 将dex_file添加到out_dex_files数组中 , 在函数结束后返回
• 校验ClassLoader与BootClassLoader
• 将dex_file_name加入loader_dex_file_names中
• 将classLoader的ClassTable添加到Class

bool ClassLinker::AddImageSpace(
    gc::space::ImageSpace* space,
    Handle<mirror::ClassLoader> class_loader,
    jobjectArray dex_elements,
    const char* dex_location,
    std::vector<std::unique_ptr<const DexFile>>* out_dex_files,
    std::string* error_msg) {
  // 初始化参数
  Runtime* const runtime = Runtime::Current();
  gc::Heap* const heap = runtime->GetHeap();
  Thread* const self = Thread::Current();
  ...
  // 从header中获取dex_Cache_object引用
  Handle<mirror::ObjectArray<mirror::DexCache>> dex_caches(
      hs.NewHandle(dex_caches_object->AsObjectArray<mirror::DexCache>()));
  // 从header中获取Class数组的日志
  Handle<mirror::ObjectArray<mirror::Class>> class_roots(hs.NewHandle(
      header.GetImageRoot(ImageHeader::kClassRoots)->AsObjectArray<mirror::Class>()));
  // 初始化 ClassLoader
  MutableHandle<mirror::ClassLoader> image_class_loader(hs.NewHandle(
      app_image ? header.GetImageRoot(ImageHeader::kClassLoader)->AsClassLoader() : nullptr));
  // 从ImageSpace获取OatFile
  const OatFile* oat_file = space->GetOatFile();
   ... 
  // 开始遍历dex_caches
  for (int32_t i = 0; i < dex_caches->GetLength(); i++) {
    // 获取DexCache指针
    ObjPtr<mirror::DexCache> dex_cache = dex_caches->Get(i);
    // 获取dex_cache的路径
    std::string dex_file_location(dex_cache->GetLocation()->ToModifiedUtf8());
    // 通过OpenOatFile打开Oat文件 , 获取DexFile
    std::unique_ptr<const DexFile> dex_file = OpenOatDexFile(oat_file,
                                                             dex_file_location.c_str(),
                                                             error_msg);

      ...
      //  设置dex_cache对象
      dex_cache->SetDexFile(dex_file.get());
     //  将dex_file加到out_dex_files数组中 , 后面返回
      out_dex_files->push_back(std::move(dex_file));
   }
   ...
  // 如果是oat文件话
  if (app_image) {
    ScopedObjectAccessUnchecked soa(Thread::Current());
    // 接下来要开始校验
    // 判断当前ClassLoader是否是BootClassLoader
    if (IsBootClassLoader(soa, image_class_loader.Get())) {
      *error_msg = "Unexpected BootClassLoader in app image";
      return false;
    }
    std::list<ObjPtr<mirror::String>> image_dex_file_names;
    std::string temp_error_msg;
    if (!FlattenPathClassLoader(image_class_loader.Get(), &image_dex_file_names, &temp_error_msg)) {
      *error_msg = StringPrintf("Failed to flatten image class loader hierarchy '%s'",
                                temp_error_msg.c_str());
      return false;
    }
    std::list<ObjPtr<mirror::String>> loader_dex_file_names;
    if (!FlattenPathClassLoader(class_loader.Get(), &loader_dex_file_names, &temp_error_msg)) {
      *error_msg = StringPrintf("Failed to flatten class loader hierarchy '%s'",
                                temp_error_msg.c_str());
      return false;
    }
    // 遍历elements , 将有效的dex添加到已加载到dexfile中
    auto elements = soa.Decode<mirror::ObjectArray<mirror::Object>>(dex_elements);
    for (size_t i = 0, num_elems = elements->GetLength(); i < num_elems; ++i) {
      ObjPtr<mirror::Object> element = elements->GetWithoutChecks(i);
      if (element != nullptr) {
        // If we are somewhere in the middle of the array, there may be nulls at the end.
        ObjPtr<mirror::String> name;
        // 在element中检查其中的dexfile以及name是否为空
        //  不为空则将dex的名称push到loader_dex_file_names中
        if (GetDexPathListElementName(element, &name) && name != nullptr) {
          loader_dex_file_names.push_back(name);
        }
      }
    }
  ...
   //  开始更新ClassLinker中的class_table
  ClassTable* class_table = nullptr;
  {
    WriterMutexLock mu(self, *Locks::classlinker_classes_lock_);
    // 创建ClassLoader中的ClassTable
    class_table = InsertClassTableForClassLoader(class_loader.Get());
  }
   // 创建临时的ClassTable
  ClassTable::ClassSet temp_set;
   // 从oat文件中读取ClassTable的Section
  const ImageSection& class_table_section = header.GetImageSection(ImageHeader::kSectionClassTable);
    // 判断classTable的Section大小是否大于0
  const bool added_class_table = class_table_section.Size() > 0;
  if (added_class_table) {
    const uint64_t start_time2 = NanoTime();
    size_t read_count = 0;
    // 如果存在ClassTableSection的话 , 则读取出来保存到temp_set中
    temp_set = ClassTable::ClassSet(space->Begin() + class_table_section.Offset(),
                                    /*make copy*/false,
                                    &read_count);
    VLOG(image) << "Adding class table classes took " << PrettyDuration(NanoTime() - start_time2);
  }
  if (app_image) {
    bool forward_dex_cache_arrays = false;
    // 校验ClassLoader以及DexCache
    if (!UpdateAppImageClassLoadersAndDexCaches(space,
                                                class_loader,
                                                dex_caches,
                                                &temp_set,
                                                /*out*/&forward_dex_cache_arrays,
                                                /*out*/error_msg)) {
      return false;
    }
  ...
  // 把oat文件插入到class_table中
  if (!oat_file->GetBssGcRoots().empty()) {
    // Insert oat file to class table for visiting .bss GC roots.
    class_table->InsertOatFile(oat_file);
  }
  if (added_class_table) {
    // 如果需要添加到classtable , 则添加到class_table中
    WriterMutexLock mu(self, *Locks::classlinker_classes_lock_);
    class_table->AddClassSet(std::move(temp_set));
  }
  ...
  return true;
}

8. OpenOatDexFile

static std::unique_ptr<const DexFile> OpenOatDexFile(const OatFile* oat_file,
                                                     const char* location,
                                                     std::string* error_msg)
    REQUIRES_SHARED(Locks::mutator_lock_) {
  DCHECK(error_msg != nullptr);
  std::unique_ptr<const DexFile> dex_file;
  // 根据路径得到OatDexFile对象
  const OatFile::OatDexFile* oat_dex_file = oat_file->GetOatDexFile(location, nullptr, error_msg);
  if (oat_dex_file == nullptr) {
    return std::unique_ptr<const DexFile>();
  }
  std::string inner_error_msg;
  // 根据地址找到dex_file结构 , 并且填充信息
  dex_file = oat_dex_file->OpenDexFile(&inner_error_msg);
  // 如果打开失败
  if (dex_file == nullptr) {
    *error_msg = StringPrintf("Failed to open dex file %s from within oat file %s error '%s'",
                              location,
                              oat_file->GetLocation().c_str(),
                              inner_error_msg.c_str());
    return std::unique_ptr<const DexFile>();
  }
  // 校验失败
  if (dex_file->GetLocationChecksum() != oat_dex_file->GetDexFileLocationChecksum()) {
    *error_msg = StringPrintf("Checksums do not match for %s: %x vs %x",
                              location,
                              dex_file->GetLocationChecksum(),
                              oat_dex_file->GetDexFileLocationChecksum());
    return std::unique_ptr<const DexFile>();
  }
  return dex_file;
}

9. OpenDexFile

std::unique_ptr<const DexFile> OatFile::OatDexFile::OpenDexFile(std::string* error_msg) const {
  ScopedTrace trace(__PRETTY_FUNCTION__);
  static constexpr bool kVerify = false;
  static constexpr bool kVerifyChecksum = false;
  // oat文件中dex_file的指针
  return DexFile::Open(dex_file_pointer_,
                        // 整个文件大小
                       FileSize(),
                       // dex文件路径
                       dex_file_location_,
                       // dex文件路径的校验码
                       dex_file_location_checksum_,
                       this,
                       kVerify,
                       kVerifyChecksum,
                       error_msg);
}

10. OpenCommon

std::unique_ptr<DexFile> DexFile::OpenCommon(const uint8_t* base,
                                             size_t size,
                                             const std::string& location,
                                             uint32_t location_checksum,
                                             const OatDexFile* oat_dex_file,
                                             bool verify,
                                             bool verify_checksum,
                                             std::string* error_msg,
                                             VerifyResult* verify_result) {
 // 通过dex_file基址初始化Dex_File  
  std::unique_ptr<DexFile> dex_file(new DexFile(base,
                                                size,
                                                location,
                                                location_checksum,
                                                oat_dex_file));
  ... 
  // 开始校验Dex
  if (verify && !DexFileVerifier::Verify(dex_file.get(),
                                         dex_file->Begin(),
                                         dex_file->Size(),
                                         location.c_str(),
                                         verify_checksum,
                                         error_msg)) {
    if (verify_result != nullptr) {
      *verify_result = VerifyResult::kVerifyFailed;
    }
    return nullptr;
  }
  if (verify_result != nullptr) {
    *verify_result = VerifyResult::kVerifySucceeded;
  }
  return dex_file;
}

11. DexFile构造函数

DexFile::DexFile(const uint8_t* base,
                 size_t size,
                 const std::string& location,
                 uint32_t location_checksum,
                 const OatDexFile* oat_dex_file)
    : begin_(base),
      size_(size),
      location_(location),
      location_checksum_(location_checksum),
      // 初始化Header
      header_(reinterpret_cast<const Header*>(base)),
      // 初始化Stringid
      string_ids_(reinterpret_cast<const StringId*>(base + header_->string_ids_off_)),
      // 初始化type id
      type_ids_(reinterpret_cast<const TypeId*>(base + header_->type_ids_off_)),
      // 初始化filed_id
      field_ids_(reinterpret_cast<const FieldId*>(base + header_->field_ids_off_)),
      // 初始化method_id
      method_ids_(reinterpret_cast<const MethodId*>(base + header_->method_ids_off_)),
      proto_ids_(reinterpret_cast<const ProtoId*>(base + header_->proto_ids_off_)),
      // 初始化class_def
      class_defs_(reinterpret_cast<const ClassDef*>(base + header_->class_defs_off_)),
      method_handles_(nullptr),
      num_method_handles_(0),
      call_site_ids_(nullptr),
      num_call_site_ids_(0),
      oat_dex_file_(oat_dex_file) {
  // 初始化各个Section
  InitializeSectionsFromMapList();
}

  
void DexFile::InitializeSectionsFromMapList() {
  const MapList* map_list = reinterpret_cast<const MapList*>(begin_ + header_->map_off_);
  if (header_->map_off_ == 0 || header_->map_off_ > size_) {
    // Bad offset. The dex file verifier runs after this method and will reject the file.
    return;
  }
  const size_t count = map_list->size_;
  // 得到Map大小
  size_t map_limit = header_->map_off_ + count * sizeof(MapItem);
  if (header_->map_off_ >= map_limit || map_limit > size_) {
    // Overflow or out out of bounds. The dex file verifier runs after
    // this method and will reject the file as it is malformed.
    return;
  }

  for (size_t i = 0; i < count; ++i) {
    // 遍历所有的Section
    const MapItem& map_item = map_list->list_[i];
    if (map_item.type_ == kDexTypeMethodHandleItem) {
      // 如果maptype为Method句柄的话
      // 其中begin_也就是base , 基址
      method_handles_ = reinterpret_cast<const MethodHandleItem*>(begin_ + map_item.offset_);
      num_method_handles_ = map_item.size_;
    } else if (map_item.type_ == kDexTypeCallSiteIdItem) {
      call_site_ids_ = reinterpret_cast<const CallSiteIdItem*>(begin_ + map_item.offset_);
      num_call_site_ids_ = map_item.size_;
    }
  }
}

}

12.DexFileVerifier::Verify校验Dex文件

bool DexFileVerifier::Verify() {
  // Check the header.
  if (!CheckHeader()) {
    return false;
  }

  // Check the map section.
  if (!CheckMap()) {
    return false;
  }

  // Check structure within remaining sections.
  if (!CheckIntraSection()) {
    return false;
  }

  // Check references from one section to another.
  if (!CheckInterSection()) {
    return false;
  }

  return true;
}

13. ElfOatFile , 由于在Linux平台上 , 所以对应的oat_file类型为ElfOatFile

ElfOatFile* ElfOatFile::OpenElfFile(File* file,
                                    const std::string& location,
                                    uint8_t* requested_base,
                                    uint8_t* oat_file_begin,  // Override base if not null
                                    bool writable,
                                    bool executable,
                                    bool low_4gb,
                                    const char* abs_dex_location,
                                    std::string* error_msg) {
  ScopedTrace trace("Open elf file " + location);
  std::unique_ptr<ElfOatFile> oat_file(new ElfOatFile(location, executable));
  bool success = oat_file->ElfFileOpen(file,
                                       oat_file_begin,
                                       writable,
                                       low_4gb,
                                       executable,
                                       error_msg);
  if (!success) {
    CHECK(!error_msg->empty());
    return nullptr;
  }