JVM启动创建虚拟机及初始化流程

经过上一篇JVM启动流程文章中，知道创建虚拟机并初始化的逻辑是在Thread::create_vm方法中,那么本文就朱分析其实现的主流程.

1. is_supported_jni_version检查是否支持JNI的版本是否是JNI_VERSION_1_1,JNI_VERSION_1_2,JNI_VERSION_1_4,JNI_VERSION_1_6,JNI_VERSION_1_8。
2. ostream_init初始化输出流模块。73. process_sun_java_launcher_properties处理java启动的配置。处理-Dsun.java.launcher和-Dsun.java.launcher.pid=参数
3. os::init初始化os的模块，依赖对应的操作系统(例如:Liunx系统调用的是(os_liunx.cpp中init函数)。
4. init_system_properties首先初始化系统配置参数,例如：java.class.path、java.home等参数。
5. JDK_Version_init初始化jdk的版本,主要有主版本号，次版本号等.
6. Arguments::parse 解析启动参数。
7. os::init_before_ergo() 主要large page的初始化，Liunx系统初始华large_page_size大小是2M。
8. Arguments::apply_ergo() 应用参数，主要设置堆的物理大小以及初始化MetaSpace。
9. PauseAtStartup参数为true,则调用os::pause(),os系统一直通过poll轮询等待JVM启动完成.这个参数默认是false,
10. HOTSPOT_VM_INIT_BEGIN JVM启动
11. 启动追踪JVM记录时间统计对象TraceVmCreationTime的start方法.
12. os::init_2() 解析完JVM参数后调用操作系统的第二阶段初始化,。
13. Arguments::adjust_after_os()进行os参数初始化后，进行参数的调整。
14. ThreadLocalStorage::init() 对ThreadLocalStorage初始化。
15. MemTracker::bootstrap_single_thread()单线程启动。
16. ostream_init_log方法主要初始化 -Xloggc:参数中日志文件的创建。
17. init_agents_at_startup方法是判断有没有-Xrun参数，将其转成-agentLibraryList中，将其转成-agentlib参数一样处理，主要是兼容之前jdk版本。
18. create_vm_init_agents 创建虚拟机的包含参数-agentlib: -agentpath参数执行的agent的库以及-Xrun转换来的,并调用agent动态链接库中Agent_OnLoad方法进行初始化。
19. 初始化线程的状态，_thread_list置空，_number_of_threads线程数量为0, _number_of_non_daemon_threads非守护线程数量为0.
20. vm_init_globals 初始化全局的数据结构,并且在堆中创建系统的class。
21. 创建JavaThread主线程对象,并设置状态为_thread_in_vm。并且record_stack_base_and_size记录栈基址和大小,initialize_thread_local_storage初始化线程本地存储,set_active_handles设置active的JNIHandler.set_as_starting_thread将自己作为启动main线程,create_stack_guard_pages受保护的栈页。
22. ObjectMonitor::Initialize() 初始化JVM的对象监控,主要之后同步系统使用.
23. MemTracker::bootstrap_multi_thread() JVM此时进入多线程启动阶段,
24. init_globals方法是初始化全局模块，主要包括字节码、classloader、stubroutines、os、codeCache、interpreter、jnihandles、javaClasses等模块的初始化.
25. cache_global_variables缓存全局变量，jdk1.8中是空的实现.
26. 加一个线程的MutexLocker锁，然后执行thrads::add方法将主线程加入线程list列表中，主要是统计线程数量。
27. JvmtiExport::transition_pending_onload_raw_monitors()是JVMTI的接口回调.
28. MemTracker::start() 启动native的内容记录。
29. VMThread::create() 创建虚拟机线程
30. os::create_thread(vmthread, os::vm_thread)创建OSThread对象.
31. 加MutexLocker锁，启动VM的线程，直到启动完成
32. VerifyDuringStartup判断是否启动中验证。默认是false.
33. DumpSharedSpaces是否导出共享空间状态,默认是fallse，
34. 执行JVMTI的两个回调函数，enter_start_phase和post_vm_start可以监听到JVM启动事件。
35. 加载并初始化java.lang.String和java.lang.System。java.lang.OutOfMemoryError、java.lang.NullPointerException等系统类.
36. 创建线程组，并创建初始化线程并设置主线程的Thread的InstanceClass对象。
37. 设置标志位_init_completed=true, 记录hotspot启动结束.
38. SystemDictionary::compute_java_system_loader(THREAD);调用Classloader类中getSystemClassLoader获取系统类加载器。
39. 执行JVMTI回调enter_live_phase函数
40. AttachListener::init()是attacheListener的初始化.
41. os::signal_init()将线程加入线程组中，并初始化java处理操作系统的信号的数据结构.
42. BiasedLocking::init() 偏向锁的初始化
43. JvmtiExport::post_vm_initialized执行JVM初始化事件的回调.
44. 判断EnableInvokeDynamic是否开启invokeDynamic指令,默认是开启的,同时初始化java.lang.invoke包的MethodHandle、MemberName、MethodHandleNatives类。
45. WatcherThread::start() 启动观察线程.
46. os::init_3() JVM启动结束后操作系统的初始化.启动 MemNotifyThread线程监控低内存的
47. create_vm_timer.end() JVM启动记录结束。
48. 设置_vm_complete等于true，并返回JNI_OK。

jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {

  extern void JDK_Version_init();

  // Check version
  if (!is_supported_jni_version(args->version)) return JNI_EVERSION;

  // Initialize the output stream module
  ostream_init();

  // Process java launcher properties.
  Arguments::process_sun_java_launcher_properties(args);

  // Initialize the os module before using TLS
  os::init();

  // Initialize system properties.
  Arguments::init_system_properties();

  // So that JDK version can be used as a discrimintor when parsing arguments
  JDK_Version_init();

  // Update/Initialize System properties after JDK version number is known
  Arguments::init_version_specific_system_properties();

  // Parse arguments
  jint parse_result = Arguments::parse(args);
  if (parse_result != JNI_OK) return parse_result;

  os::init_before_ergo();

  jint ergo_result = Arguments::apply_ergo();
  if (ergo_result != JNI_OK) return ergo_result;

  if (PauseAtStartup) {
    os::pause();
  }

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__begin);
#else /* USDT2 */
  HOTSPOT_VM_INIT_BEGIN();
#endif /* USDT2 */

  // Record VM creation timing statistics
  TraceVmCreationTime create_vm_timer;
  create_vm_timer.start();

  // Timing (must come after argument parsing)
  TraceTime timer("Create VM", TraceStartupTime);

  // Initialize the os module after parsing the args
  jint os_init_2_result = os::init_2();
  if (os_init_2_result != JNI_OK) return os_init_2_result;

  jint adjust_after_os_result = Arguments::adjust_after_os();
  if (adjust_after_os_result != JNI_OK) return adjust_after_os_result;

  // intialize TLS
  ThreadLocalStorage::init();

  // Bootstrap native memory tracking, so it can start recording memory
  // activities before worker thread is started. This is the first phase
  // of bootstrapping, VM is currently running in single-thread mode.
  MemTracker::bootstrap_single_thread();

  // Initialize output stream logging
  ostream_init_log();

  // Convert -Xrun to -agentlib: if there is no JVM_OnLoad
  // Must be before create_vm_init_agents()
  if (Arguments::init_libraries_at_startup()) {
    convert_vm_init_libraries_to_agents();
  }

  // Launch -agentlib/-agentpath and converted -Xrun agents
  if (Arguments::init_agents_at_startup()) {
    create_vm_init_agents();
  }

  // Initialize Threads state
  _thread_list = NULL;
  _number_of_threads = 0;
  _number_of_non_daemon_threads = 0;

  // Initialize global data structures and create system classes in heap
  vm_init_globals();

  // Attach the main thread to this os thread
  JavaThread* main_thread = new JavaThread();
  main_thread->set_thread_state(_thread_in_vm);
  // must do this before set_active_handles and initialize_thread_local_storage
  // Note: on solaris initialize_thread_local_storage() will (indirectly)
  // change the stack size recorded here to one based on the java thread
  // stacksize. This adjusted size is what is used to figure the placement
  // of the guard pages.
  main_thread->record_stack_base_and_size();
  main_thread->initialize_thread_local_storage();

  main_thread->set_active_handles(JNIHandleBlock::allocate_block());

  if (!main_thread->set_as_starting_thread()) {
    vm_shutdown_during_initialization(
      "Failed necessary internal allocation. Out of swap space");
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return JNI_ENOMEM;
  }

  // Enable guard page *after* os::create_main_thread(), otherwise it would
  // crash Linux VM, see notes in os_linux.cpp.
  main_thread->create_stack_guard_pages();

  // Initialize Java-Level synchronization subsystem
  ObjectMonitor::Initialize() ;

  // Second phase of bootstrapping, VM is about entering multi-thread mode
  MemTracker::bootstrap_multi_thread();

  // Initialize global modules
  jint status = init_globals();
  if (status != JNI_OK) {
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return status;
  }

  // Should be done after the heap is fully created
  main_thread->cache_global_variables();

  HandleMark hm;

  { MutexLocker mu(Threads_lock);
    Threads::add(main_thread);
  }

  // Any JVMTI raw monitors entered in onload will transition into
  // real raw monitor. VM is setup enough here for raw monitor enter.
  JvmtiExport::transition_pending_onload_raw_monitors();

  // Fully start NMT
  MemTracker::start();

  // Create the VMThread
  { TraceTime timer("Start VMThread", TraceStartupTime);
    VMThread::create();
    Thread* vmthread = VMThread::vm_thread();

    if (!os::create_thread(vmthread, os::vm_thread))
      vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");

    // Wait for the VM thread to become ready, and VMThread::run to initialize
    // Monitors can have spurious returns, must always check another state flag
    {
      MutexLocker ml(Notify_lock);
      os::start_thread(vmthread);
      while (vmthread->active_handles() == NULL) {
        Notify_lock->wait();
      }
    }
  }

  assert (Universe::is_fully_initialized(), "not initialized");
  if (VerifyDuringStartup) {
    // Make sure we're starting with a clean slate.
    VM_Verify verify_op;
    VMThread::execute(&verify_op);
  }

  EXCEPTION_MARK;

  // At this point, the Universe is initialized, but we have not executed
  // any byte code.  Now is a good time (the only time) to dump out the
  // internal state of the JVM for sharing.
  if (DumpSharedSpaces) {
    MetaspaceShared::preload_and_dump(CHECK_0);
    ShouldNotReachHere();
  }

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_start_phase();

  // Notify JVMTI agents that VM has started (JNI is up) - nop if no agents.
  JvmtiExport::post_vm_start();

  {
    TraceTime timer("Initialize java.lang classes", TraceStartupTime);

    if (EagerXrunInit && Arguments::init_libraries_at_startup()) {
      create_vm_init_libraries();
    }

    initialize_class(vmSymbols::java_lang_String(), CHECK_0);

    // Initialize java_lang.System (needed before creating the thread)
    initialize_class(vmSymbols::java_lang_System(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ThreadGroup(), CHECK_0);
    Handle thread_group = create_initial_thread_group(CHECK_0);
    Universe::set_main_thread_group(thread_group());
    initialize_class(vmSymbols::java_lang_Thread(), CHECK_0);
    oop thread_object = create_initial_thread(thread_group, main_thread, CHECK_0);
    main_thread->set_threadObj(thread_object);
    // Set thread status to running since main thread has
    // been started and running.
    java_lang_Thread::set_thread_status(thread_object,
                                        java_lang_Thread::RUNNABLE);

    // The VM creates & returns objects of this class. Make sure it's initialized.
    initialize_class(vmSymbols::java_lang_Class(), CHECK_0);

    // The VM preresolves methods to these classes. Make sure that they get initialized
    initialize_class(vmSymbols::java_lang_reflect_Method(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ref_Finalizer(),  CHECK_0);
    call_initializeSystemClass(CHECK_0);

    // get the Java runtime name after java.lang.System is initialized
    JDK_Version::set_runtime_name(get_java_runtime_name(THREAD));
    JDK_Version::set_runtime_version(get_java_runtime_version(THREAD));

    // an instance of OutOfMemory exception has been allocated earlier
    initialize_class(vmSymbols::java_lang_OutOfMemoryError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_NullPointerException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ClassCastException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArrayStoreException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArithmeticException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_StackOverflowError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalMonitorStateException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalArgumentException(), CHECK_0);
  }

  // See        : bugid 4211085.
  // Background : the static initializer of java.lang.Compiler tries to read
  //              property"java.compiler" and read & write property "java.vm.info".
  //              When a security manager is installed through the command line
  //              option "-Djava.security.manager", the above properties are not
  //              readable and the static initializer for java.lang.Compiler fails
  //              resulting in a NoClassDefFoundError.  This can happen in any
  //              user code which calls methods in java.lang.Compiler.
  // Hack :       the hack is to pre-load and initialize this class, so that only
  //              system domains are on the stack when the properties are read.
  //              Currently even the AWT code has calls to methods in java.lang.Compiler.
  //              On the classic VM, java.lang.Compiler is loaded very early to load the JIT.
  // Future Fix : the best fix is to grant everyone permissions to read "java.compiler" and
  //              read and write"java.vm.info" in the default policy file. See bugid 4211383
  //              Once that is done, we should remove this hack.
  initialize_class(vmSymbols::java_lang_Compiler(), CHECK_0);

  // More hackery - the static initializer of java.lang.Compiler adds the string "nojit" to
  // the java.vm.info property if no jit gets loaded through java.lang.Compiler (the hotspot
  // compiler does not get loaded through java.lang.Compiler).  "java -version" with the
  // hotspot vm says "nojit" all the time which is confusing.  So, we reset it here.
  // This should also be taken out as soon as 4211383 gets fixed.
  reset_vm_info_property(CHECK_0);

  quicken_jni_functions();

  // Must be run after init_ft which initializes ft_enabled
  if (TRACE_INITIALIZE() != JNI_OK) {
    vm_exit_during_initialization("Failed to initialize tracing backend");
  }

  // Set flag that basic initialization has completed. Used by exceptions and various
  // debug stuff, that does not work until all basic classes have been initialized.
  set_init_completed();

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__end);
#else /* USDT2 */
  HOTSPOT_VM_INIT_END();
#endif /* USDT2 */

  // record VM initialization completion time
#if INCLUDE_MANAGEMENT
  Management::record_vm_init_completed();
#endif // INCLUDE_MANAGEMENT

  // Compute system loader. Note that this has to occur after set_init_completed, since
  // valid exceptions may be thrown in the process.
  // Note that we do not use CHECK_0 here since we are inside an EXCEPTION_MARK and
  // set_init_completed has just been called, causing exceptions not to be shortcut
  // anymore. We call vm_exit_during_initialization directly instead.
  SystemDictionary::compute_java_system_loader(THREAD);
  if (HAS_PENDING_EXCEPTION) {
    vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
  }

#if INCLUDE_ALL_GCS
  // Support for ConcurrentMarkSweep. This should be cleaned up
  // and better encapsulated. The ugly nested if test would go away
  // once things are properly refactored. XXX YSR
  if (UseConcMarkSweepGC || UseG1GC) {
    if (UseConcMarkSweepGC) {
      ConcurrentMarkSweepThread::makeSurrogateLockerThread(THREAD);
    } else {
      ConcurrentMarkThread::makeSurrogateLockerThread(THREAD);
    }
    if (HAS_PENDING_EXCEPTION) {
      vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
    }
  }
#endif // INCLUDE_ALL_GCS

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_live_phase();

  // Signal Dispatcher needs to be started before VMInit event is posted
  os::signal_init();

  // Start Attach Listener if +StartAttachListener or it can't be started lazily
  if (!DisableAttachMechanism) {
    AttachListener::vm_start();
    if (StartAttachListener || AttachListener::init_at_startup()) {
      AttachListener::init();
    }
  }

  // Launch -Xrun agents
  // Must be done in the JVMTI live phase so that for backward compatibility the JDWP
  // back-end can launch with -Xdebug -Xrunjdwp.
  if (!EagerXrunInit && Arguments::init_libraries_at_startup()) {
    create_vm_init_libraries();
  }

  // Notify JVMTI agents that VM initialization is complete - nop if no agents.
  JvmtiExport::post_vm_initialized();

  if (TRACE_START() != JNI_OK) {
    vm_exit_during_initialization("Failed to start tracing backend.");
  }

  if (CleanChunkPoolAsync) {
    Chunk::start_chunk_pool_cleaner_task();
  }

  // initialize compiler(s)
#if defined(COMPILER1) || defined(COMPILER2) || defined(SHARK)
  CompileBroker::compilation_init();
#endif

  if (EnableInvokeDynamic) {
    // Pre-initialize some JSR292 core classes to avoid deadlock during class loading.
    // It is done after compilers are initialized, because otherwise compilations of
    // signature polymorphic MH intrinsics can be missed
    // (see SystemDictionary::find_method_handle_intrinsic).
    initialize_class(vmSymbols::java_lang_invoke_MethodHandle(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MemberName(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MethodHandleNatives(), CHECK_0);
  }

#if INCLUDE_MANAGEMENT
  Management::initialize(THREAD);
#endif // INCLUDE_MANAGEMENT

  if (HAS_PENDING_EXCEPTION) {
    // management agent fails to start possibly due to
    // configuration problem and is responsible for printing
    // stack trace if appropriate. Simply exit VM.
    vm_exit(1);
  }

  if (Arguments::has_profile())       FlatProfiler::engage(main_thread, true);
  if (MemProfiling)                   MemProfiler::engage();
  StatSampler::engage();
  if (CheckJNICalls)                  JniPeriodicChecker::engage();

  BiasedLocking::init();

  if (JDK_Version::current().post_vm_init_hook_enabled()) {
    call_postVMInitHook(THREAD);
    // The Java side of PostVMInitHook.run must deal with all
    // exceptions and provide means of diagnosis.
    if (HAS_PENDING_EXCEPTION) {
      CLEAR_PENDING_EXCEPTION;
    }
  }

  {
      MutexLockerEx ml(PeriodicTask_lock, Mutex::_no_safepoint_check_flag);
      // Make sure the watcher thread can be started by WatcherThread::start()
      // or by dynamic enrollment.
      WatcherThread::make_startable();
      // Start up the WatcherThread if there are any periodic tasks
      // NOTE:  All PeriodicTasks should be registered by now. If they
      //   aren't, late joiners might appear to start slowly (we might
      //   take a while to process their first tick).
      if (PeriodicTask::num_tasks() > 0) {
          WatcherThread::start();
      }
  }

  // Give os specific code one last chance to start
  os::init_3();

  create_vm_timer.end();
#ifdef ASSERT
  _vm_complete = true;
#endif
  return JNI_OK;
}

总结
本文主要就JVM启动的主流程，主要是thread.cpp的create_vm函数中逻辑分析，其中每一步详细的执行逻辑，由于篇幅原因没有全部分析，后续再单独意义分析。