Dex2Oat源码流程(1)——Android6.0

当framework层的PackageManagerService需要执行安装请求时，需要对目标APK文件进行处理，而这个过程归根到底是对dex文件的处理，Android将dex文件转化为虚拟机可以直接执行的oat文件，这个转化过程是在art/dex2oat/dex2oat.cc(http://androidxref.com/6.0.0_r1/xref/art/dex2oat/dex2oat.cc)中完成的。

main()

从dex2oat.cc的执行入口main函数开始跟踪，可以很容易看出dex2oat的逻辑：

int main(int argc, char** argv) {
  int result = art::dex2oat(argc, argv);
  // Everything was done, do an explicit exit here to avoid running Runtime destructors that take
  // time (bug 10645725) unless we're a debug build or running on valgrind. Note: The Dex2Oat class
  // should not destruct the runtime in this case.
  if (!art::kIsDebugBuild && (RUNNING_ON_VALGRIND == 0)) {
    exit(result);
  }
  return result;
}

dex2oat()

main函数中，调用了art命名空间下的dex2oat函数，在dex2oat()函数中，

1. arm上的workaround由b13564922()完成，这个与dex2oat的主要线程无关，可以暂时忽略。
2. 构造Dex2oat对象——Dex2Oat dex2oat(&timings);
3. 处理命令行参数，将dex文件名文件路径等参数传入函数中——dex2oat.ParseArgs(argc, argv);
4. 判断对于dex文件是否有写的权限——dex2oat.OpenFile()
5. 打印命令行参数——LOG(INFO) << CommandLine();
6. 判断dex2oat编译环境是否启动成功——dex2oat.Setup()
7. 根据是否image分别调用CompileImage(dex2oat)或CompileApp(dex2oat)的处理

static int dex2oat(int argc, char** argv) {
  b13564922();

  TimingLogger timings("compiler", false, false);

  Dex2Oat dex2oat(&timings);

  // Parse arguments. Argument mistakes will lead to exit(EXIT_FAILURE) in UsageError.
  dex2oat.ParseArgs(argc, argv);

  // Check early that the result of compilation can be written
  if (!dex2oat.OpenFile()) {
    return EXIT_FAILURE;
  }

  // Print the complete line when any of the following is true:
  //   1) Debug build
  //   2) Compiling an image
  //   3) Compiling with --host
  //   4) Compiling on the host (not a target build)
  // Otherwise, print a stripped command line.
  if (kIsDebugBuild || dex2oat.IsImage() || dex2oat.IsHost() || !kIsTargetBuild) {
    LOG(INFO) << CommandLine();
  } else {
    LOG(INFO) << StrippedCommandLine();
  }

  if (!dex2oat.Setup()) {
    dex2oat.EraseOatFile();
    return EXIT_FAILURE;
  }

  if (dex2oat.IsImage()) {
    return CompileImage(dex2oat);
  } else {
    return CompileApp(dex2oat);
  }
}
}  // namespace art

CompileApp()&&CompileImage()

这两个函数可以一起分析，从他们的代码中可以看出，代码基本上一样。
dex2oat.Compile()的代码如下所示：

static int CompileImage(Dex2Oat& dex2oat) {
  dex2oat.Compile();

  // Create the boot.oat.
  if (!dex2oat.CreateOatFile()) {
    dex2oat.EraseOatFile();
    return EXIT_FAILURE;
  }

  // Flush and close the boot.oat. We always expect the output file by name, and it will be
  // re-opened from the unstripped name.
  if (!dex2oat.FlushCloseOatFile()) {
    return EXIT_FAILURE;
  }

  // Creates the boot.art and patches the boot.oat.
  if (!dex2oat.HandleImage()) {
    return EXIT_FAILURE;
  }

  // When given --host, finish early without stripping.
  if (dex2oat.IsHost()) {
    dex2oat.DumpTiming();
    return EXIT_SUCCESS;
  }

  // Copy unstripped to stripped location, if necessary.
  if (!dex2oat.CopyUnstrippedToStripped()) {
    return EXIT_FAILURE;
  }

  // FlushClose again, as stripping might have re-opened the oat file.
  if (!dex2oat.FlushCloseOatFile()) {
    return EXIT_FAILURE;
  }

  dex2oat.DumpTiming();
  return EXIT_SUCCESS;
}

CompileApp()的代码如下所示：

static int CompileApp(Dex2Oat& dex2oat) {
  dex2oat.Compile();

  // Create the app oat.
  if (!dex2oat.CreateOatFile()) {
    dex2oat.EraseOatFile();
    return EXIT_FAILURE;
  }

  // Do not close the oat file here. We might haven gotten the output file by file descriptor,
  // which we would lose.
  if (!dex2oat.FlushOatFile()) {
    return EXIT_FAILURE;
  }

  // When given --host, finish early without stripping.
  if (dex2oat.IsHost()) {
    if (!dex2oat.FlushCloseOatFile()) {
      return EXIT_FAILURE;
    }

    dex2oat.DumpTiming();
    return EXIT_SUCCESS;
  }

  // Copy unstripped to stripped location, if necessary. This will implicitly flush & close the
  // unstripped version. If this is given, we expect to be able to open writable files by name.
  if (!dex2oat.CopyUnstrippedToStripped()) {
    return EXIT_FAILURE;
  }

  // Flush and close the file.
  if (!dex2oat.FlushCloseOatFile()) {
    return EXIT_FAILURE;
  }

  dex2oat.DumpTiming();
  return EXIT_SUCCESS;
}

可以看出他们有一个共同的特点，就是都先调用了dex2oat.Compile()函数。

Compile()

在这一步，创建compiler driver，编译所有的dex文件。dex2oat.Compile()的代码如下所示：

// Create and invoke the compiler driver. This will compile all the dex files.
  void Compile() {
    TimingLogger::ScopedTiming t("dex2oat Compile", timings_);
    compiler_phases_timings_.reset(new CumulativeLogger("compilation times"));

    // Handle and ClassLoader creation needs to come after Runtime::Create
    jobject class_loader = nullptr;
    Thread* self = Thread::Current();
    if (!boot_image_option_.empty()) {
      ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
      OpenClassPathFiles(runtime_->GetClassPathString(), dex_files_, &class_path_files_);
      ScopedObjectAccess soa(self);

      // Classpath: first the class-path given.
      std::vector<const DexFile*> class_path_files;
      for (auto& class_path_file : class_path_files_) {
        class_path_files.push_back(class_path_file.get());
      }

      // Store the classpath we have right now.
      key_value_store_->Put(OatHeader::kClassPathKey,
                            OatFile::EncodeDexFileDependencies(class_path_files));

      // Then the dex files we'll compile. Thus we'll resolve the class-path first.
      class_path_files.insert(class_path_files.end(), dex_files_.begin(), dex_files_.end());

      class_loader = class_linker->CreatePathClassLoader(self, class_path_files);
    }

    driver_.reset(new CompilerDriver(compiler_options_.get(),
                                     verification_results_.get(),
                                     &method_inliner_map_,
                                     compiler_kind_,
                                     instruction_set_,
                                     instruction_set_features_.get(),
                                     image_,
                                     image_classes_.release(),
                                     compiled_classes_.release(),
                                     nullptr,
                                     thread_count_,
                                     dump_stats_,
                                     dump_passes_,
                                     dump_cfg_file_name_,
                                     compiler_phases_timings_.get(),
                                     swap_fd_,
                                     profile_file_));

    driver_->CompileAll(class_loader, dex_files_, timings_);
  }

这一步的操作由driver_这个对象完成的，而这个对象在Dex2Oat类中由CompilerDriver 创建的——CompilerDriver* driver_;之后调用driver_的ComileAll()函数来完成编译过程。