时机

遇到某些指令时，如：new、getstatic、putstatic、invokestatic等

作用

执行类的初始化方法：<clint>

源码

代码很复杂，主要是为了支持多线程之间的同步

void instanceKlass::initialize_impl(instanceKlassHandle this_oop, TRAPS) {
  // Make sure klass is linked (verified) before initialization
  // A class could already be verified, since it has been reflected upon.
  this_oop->link_class(CHECK);

  DTRACE_CLASSINIT_PROBE(required, instanceKlass::cast(this_oop()), -1);

  bool wait = false;

  // refer to the JVM book page 47 for description of steps
  // Step 1
  { ObjectLocker ol(this_oop, THREAD);

    Thread *self = THREAD; // it's passed the current thread

    // Step 2
    // If we were to use wait() instead of waitInterruptibly() then
    // we might end up throwing IE from link/symbol resolution sites
    // that aren't expected to throw.  This would wreak havoc.  See 6320309.
    while(this_oop->is_being_initialized() && !this_oop->is_reentrant_initialization(self)) {
        wait = true;
      ol.waitUninterruptibly(CHECK);
    }

    // Step 3
    if (this_oop->is_being_initialized() && this_oop->is_reentrant_initialization(self)) {
      DTRACE_CLASSINIT_PROBE_WAIT(recursive, instanceKlass::cast(this_oop()), -1,wait);
      return;
    }

    // Step 4
    if (this_oop->is_initialized()) {
      DTRACE_CLASSINIT_PROBE_WAIT(concurrent, instanceKlass::cast(this_oop()), -1,wait);
      return;
    }

    // Step 5
    if (this_oop->is_in_error_state()) {
      DTRACE_CLASSINIT_PROBE_WAIT(erroneous, instanceKlass::cast(this_oop()), -1,wait);
      ResourceMark rm(THREAD);
      const char* desc = "Could not initialize class ";
      const char* className = this_oop->external_name();
      size_t msglen = strlen(desc) + strlen(className) + 1;
      char* message = NEW_RESOURCE_ARRAY(char, msglen);
      if (NULL == message) {
        // Out of memory: can't create detailed error message
        THROW_MSG(vmSymbols::java_lang_NoClassDefFoundError(), className);
      } else {
        jio_snprintf(message, msglen, "%s%s", desc, className);
        THROW_MSG(vmSymbols::java_lang_NoClassDefFoundError(), message);
      }
    }

    // Step 6
    this_oop->set_init_state(being_initialized);
    this_oop->set_init_thread(self);
  }

  // Step 7
  klassOop super_klass = this_oop->super();
  if (super_klass != NULL && !this_oop->is_interface() && Klass::cast(super_klass)->should_be_initialized()) {
    Klass::cast(super_klass)->initialize(THREAD);

    if (HAS_PENDING_EXCEPTION) {
      Handle e(THREAD, PENDING_EXCEPTION);
      CLEAR_PENDING_EXCEPTION;
      {
        EXCEPTION_MARK;
        this_oop->set_initialization_state_and_notify(initialization_error, THREAD); // Locks object, set state, and notify all waiting threads
        CLEAR_PENDING_EXCEPTION;   // ignore any exception thrown, superclass initialization error is thrown below
      }
      DTRACE_CLASSINIT_PROBE_WAIT(super__failed, instanceKlass::cast(this_oop()), -1,wait);
      THROW_OOP(e());
    }
  }

  // Step 8
  {
    assert(THREAD->is_Java_thread(), "non-JavaThread in initialize_impl");
    JavaThread* jt = (JavaThread*)THREAD;
    DTRACE_CLASSINIT_PROBE_WAIT(clinit, instanceKlass::cast(this_oop()), -1,wait);
    // Timer includes any side effects of class initialization (resolution,
    // etc), but not recursive entry into call_class_initializer().
    PerfClassTraceTime timer(ClassLoader::perf_class_init_time(),
                             ClassLoader::perf_class_init_selftime(),
                             ClassLoader::perf_classes_inited(),
                             jt->get_thread_stat()->perf_recursion_counts_addr(),
                             jt->get_thread_stat()->perf_timers_addr(),
                             PerfClassTraceTime::CLASS_CLINIT);
    this_oop->call_class_initializer(THREAD);
  }

  // Step 9
  if (!HAS_PENDING_EXCEPTION) {
    this_oop->set_initialization_state_and_notify(fully_initialized, CHECK);
    { ResourceMark rm(THREAD);
      debug_only(this_oop->vtable()->verify(tty, true);)
    }
  }
  else {
    // Step 10 and 11
    Handle e(THREAD, PENDING_EXCEPTION);
    CLEAR_PENDING_EXCEPTION;
    {
      EXCEPTION_MARK;
      this_oop->set_initialization_state_and_notify(initialization_error, THREAD);
      CLEAR_PENDING_EXCEPTION;   // ignore any exception thrown, class initialization error is thrown below
    }
    DTRACE_CLASSINIT_PROBE_WAIT(error, instanceKlass::cast(this_oop()), -1,wait);
    if (e->is_a(SystemDictionary::Error_klass())) {
      THROW_OOP(e());
    } else {
      JavaCallArguments args(e);
      THROW_ARG(vmSymbols::java_lang_ExceptionInInitializerError(),
                vmSymbols::throwable_void_signature(),
                &args);
    }
  }
  DTRACE_CLASSINIT_PROBE_WAIT(end, instanceKlass::cast(this_oop()), -1,wait);
}