Android ART VM启动流程（六）

冷不防 2022-05-30 20:48 299阅读 0赞

接着前一篇博文　[ART VM的启动流程，继续分析][ART VM]

启动虚拟机的核心函数是startVM,[  
][ART VM]

frameworks/base/core/jni/AndroidRuntime.cpp

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
    {
        /*
         * Initialize the VM.
         *
         * The JavaVM* is essentially per-process, and the JNIEnv* is per-thread.
         * If this call succeeds, the VM is ready, and we can start issuing
         * JNI calls.
         */
        if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
            ALOGE("JNI_CreateJavaVM failed\n");
            return -1;
        }
    }

startVM的前半部分是在处理虚拟机的启动参数，处理完配置参数后，会调用libart.so提供的一个接口：JNI\_CreateJavaVM函数。

在art虚拟机下，这个函数的实现在java\_vm\_ext.cc中

art/runtime/java\_vm\_ext.cc

// JNI Invocation interface.
    
    extern "C" jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
    //解析虚拟机选项
      RuntimeOptions options;
      for (int i = 0; i < args->nOptions; ++i) {
        JavaVMOption* option = &args->options[i];
        options.push_back(std::make_pair(std::string(option->optionString), option->extraInfo));
      }
      bool ignore_unrecognized = args->ignoreUnrecognized;
    //创建一个Ｒｕｎｔｉｍｅ运行时对象
    　if (!Runtime::Create(options, ignore_unrecognized)) {
        return JNI_ERR;
      }
    
      // Initialize native loader. This step makes sure we have
      // everything set up before we start using JNI.
      android::InitializeNativeLoader();
    
      Runtime* runtime = Runtime::Current();
    //通过Ｒｕｎｔｉｍｅ启动其管理的虚拟机
    　bool started = runtime->Start();
      if (!started) {
        delete Thread::Current()->GetJniEnv();
        delete runtime->GetJavaVM();
        LOG(WARNING) << "CreateJavaVM failed";
        return JNI_ERR;
      }
    
      *p_env = Thread::Current()->GetJniEnv();
    //获取已经启动完成的虚拟机实例，并通过ｐ_ｖｍ这个指向指针的指针变量，把虚拟机实例传给AndroidRuntime中的变量JavaVM* AndroidRuntime::mJavaVM。
     *p_vm = runtime->GetJavaVM();
      return JNI_OK;
    }

首先通过Runtime的create方法创建单例的Runtime对象，runtime负责提供art虚拟机的运行时环境，然后调用其init方法来初始化虚拟机。

art/runtime/runtime.cc

bool Runtime::Init(RuntimeArgumentMap&& runtime_options_in) {
      MemMap::Init();
      oat_file_manager_ = new OatFileManager;
    
      Thread::SetSensitiveThreadHook(runtime_options.GetOrDefault(Opt::HookIsSensitiveThread));
      Monitor::Init(runtime_options.GetOrDefault(Opt::LockProfThreshold));
      XGcOption xgc_option = runtime_options.GetOrDefault(Opt::GcOption);
    //ｓｔｅｐ１，创建堆管理对象。
      heap_ = new gc::Heap(runtime_options.GetOrDefault(Opt::MemoryInitialSize),
                           runtime_options.GetOrDefault(Opt::HeapGrowthLimit),
                           runtime_options.GetOrDefault(Opt::HeapMinFree),
                           runtime_options.GetOrDefault(Opt::HeapMaxFree),
                           runtime_options.GetOrDefault(Opt::HeapTargetUtilization),
                           runtime_options.GetOrDefault(Opt::ForegroundHeapGrowthMultiplier),
                           runtime_options.GetOrDefault(Opt::MemoryMaximumSize),
                           runtime_options.GetOrDefault(Opt::NonMovingSpaceCapacity),
                           runtime_options.GetOrDefault(Opt::Image),
                           runtime_options.GetOrDefault(Opt::ImageInstructionSet),
                           xgc_option.collector_type_,
                           runtime_options.GetOrDefault(Opt::BackgroundGc),
                           runtime_options.GetOrDefault(Opt::LargeObjectSpace),
                           runtime_options.GetOrDefault(Opt::LargeObjectThreshold),
                           runtime_options.GetOrDefault(Opt::ParallelGCThreads),
                           runtime_options.GetOrDefault(Opt::ConcGCThreads),
                           runtime_options.Exists(Opt::LowMemoryMode),
                           runtime_options.GetOrDefault(Opt::LongPauseLogThreshold),
                           runtime_options.GetOrDefault(Opt::LongGCLogThreshold),
                           runtime_options.Exists(Opt::IgnoreMaxFootprint),
                           runtime_options.GetOrDefault(Opt::UseTLAB),
                           xgc_option.verify_pre_gc_heap_,
                           xgc_option.verify_pre_sweeping_heap_,
                           xgc_option.verify_post_gc_heap_,
                           xgc_option.verify_pre_gc_rosalloc_,
                           xgc_option.verify_pre_sweeping_rosalloc_,
                           xgc_option.verify_post_gc_rosalloc_,
                           xgc_option.gcstress_,
                           runtime_options.GetOrDefault(Opt::EnableHSpaceCompactForOOM),
                           runtime_options.GetOrDefault(Opt::HSpaceCompactForOOMMinIntervalsMs));
    
      if (!heap_->HasBootImageSpace() && !allow_dex_file_fallback_) {
        LOG(ERROR) << "Dex file fallback disabled, cannot continue without image.";
        return false;
      }
    //ｓｔｅｐ２，创建ｊａｖａ虚拟机对象
      java_vm_ = new JavaVMExt(this, runtime_options);
    
      Thread::Startup();
    //ｓｔｅｐ３，连接主线程
      // ClassLinker needs an attached thread, but we can't fully attach a thread without creating
      // objects. We can't supply a thread group yet; it will be fixed later. Since we are the main
      // thread, we do not get a java peer.
      Thread* self = Thread::Attach("main", false, nullptr, false);
      CHECK_EQ(self->GetThreadId(), ThreadList::kMainThreadId);
      CHECK(self != nullptr);
    
      // Set us to runnable so tools using a runtime can allocate and GC by default
      self->TransitionFromSuspendedToRunnable();
    
      // Now we're attached, we can take the heap locks and validate the heap.
      GetHeap()->EnableObjectValidation();
    //ｓｔｅｐ４，创建类连接器
      class_linker_ = new ClassLinker(intern_table_);
      if (GetHeap()->HasBootImageSpace()) {
        std::string error_msg;
    //ｓｔｅｐ５，初始化类连接器
    　  bool result = class_linker_->InitFromBootImage(&error_msg);
        {
          ScopedTrace trace2("MoveImageClassesToClassTable");
          GetClassLinker()->AddBootImageClassesToClassTable();
        }
      } else {
        std::vector<std::unique_ptr<const DexFile>> boot_class_path;
        if (runtime_options.Exists(Opt::BootClassPathDexList)) {
          boot_class_path.swap(*runtime_options.GetOrDefault(Opt::BootClassPathDexList));
        } else {
          OpenDexFiles(dex_filenames,
                       dex_locations,
                       runtime_options.GetOrDefault(Opt::Image),
                       &boot_class_path);
        }
        instruction_set_ = runtime_options.GetOrDefault(Opt::ImageInstructionSet);
        std::string error_msg;
        if (!class_linker_->InitWithoutImage(std::move(boot_class_path), &error_msg)) {
          LOG(ERROR) << "Could not initialize without image: " << error_msg;
          return false;
        }
      verifier::MethodVerifier::Init();
      return true;
    }

step1,new gc::heap()，创建Heap对象，这是虚拟机管理对内存的起点。

step2,new JavaVmExt(),创建Java虚拟机实例。

step3,Thread::attach()，attach主线程，看下具体代码：

art/runtime/Thread.cc

Thread* Thread::Attach(const char* thread_name, bool as_daemon, jobject thread_group,
                           bool create_peer) {
    //获取进程中单实例的runtime
      Runtime* runtime = Runtime::Current();
      Thread* self;
      {
        MutexLock mu(nullptr, *Locks::runtime_shutdown_lock_);
        if (runtime->IsShuttingDownLocked()) {
    //如果ｒｕｎｔｉｍｅ是关闭状态，线程就不能attach
          LOG(WARNING) << "Thread attaching while runtime is shutting down: " << thread_name;
          return nullptr;
        } else {
          Runtime::Current()->StartThreadBirth();
    //新建一个线程类，执行其初始化
    　　　 self = new Thread(as_daemon);
          bool init_success = self->Init(runtime->GetThreadList(), runtime->GetJavaVM());
          Runtime::Current()->EndThreadBirth();
        }
      }
      self->InitStringEntryPoints();
    //将线程的状态设置为knative，表示运行在ｎａｔｉｖｅ环境中
      CHECK_NE(self->GetState(), kRunnable);
      self->SetState(kNative);
      return self;
    }

线程初始化的过程中，会为新线程分配一个自己的JNIEnvExt实例，因为虚拟机的每个线程都有自己独立的JNI环境，然后把它注册到Runtime的线程列表thread\_list。

step4,new ClassLinker()，成功attach到runtime环境后，创建ClassLinker实例负责管理java class。

到这里，虚拟机的创建和初始化就完成了，接下来就是通过JNI\_CreateJavaVm的start函数启动已经成功创建的虚拟机。

[ART VM]: http://blog.csdn.net/lin20044140410/article/details/79325635