在ACE 6.0.0 里提供了一个Reactor示例程序，对其分析一下（在代码的注释里）。

Reactors_Test.cpp文件（在 \ACE-6.0.0\ACE_wrappers\tests\ 目录下）：

// $Id: Reactors_Test.cpp 91671 2010-09-08 18:39:23Z johnnyw $

// ============================================================================
//
// = LIBRARY
//    tests
//
// = FILENAME
//    Reactors_Test.cpp
//
// = DESCRIPTION
//      This is a test that performs a torture test of multiple
//      <ACE_Reactors> and <ACE_Tasks> in the same process.
//
// = AUTHOR
//    Prashant Jain <pjain@cs.wustl.edu>,//    Detlef Becker <Detlef.Becker@med.siemens.de>,and
//    Douglas C. Schmidt <schmidt@cs.wustl.edu>
//
// ============================================================================

#include "test_config.h"
#include "ace/Task.h"
#include "ace/Reactor.h"
#include "ace/Atomic_Op.h"
#include "ace/Recursive_Thread_Mutex.h"



#if defined (ACE_HAS_THREADS)

ACE_Thread_Manager *thr_mgr;

static const int MAX_TASKS = 20;

class Test_Task : public ACE_Task<ACE_MT_SYNCH>
  // = TITLE
  //    Exercise the tasks.
{
public:
  // = Initialization and termination methods.
  Test_Task (void);
  ~Test_Task (void);

  //FUZZ: disable check_for_lack_ACE_OS
  // = Task hooks.
  virtual int open (void *args = 0);
  virtual int close (u_long flags = 0);
  virtual int svc (void);
  //FUZZ: enable check_for_lack_ACE_OS

  // = Event Handler hooks.
  virtual int handle_input (ACE_HANDLE handle);
  virtual int handle_close (ACE_HANDLE fd,ACE_Reactor_Mask close_mask);
private:
  size_t handled_;
  // Number of iterations handled.

  static int task_count_;
  // Number of tasks running.
};

// Static data member initialization.
int Test_Task::task_count_ = 0;

static ACE_Atomic_Op<ACE_Thread_Mutex,int> done_count = MAX_TASKS * 2;



static ACE_Recursive_Thread_Mutex recursive_lock;

Test_Task::Test_Task (void)
  : handled_ (0)
{
  ACE_GUARD (ACE_Recursive_Thread_Mutex,ace_mon,recursive_lock);

  Test_Task::task_count_++;

  ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) TT+ Test_Task::task_count_ = %d\n"),Test_Task::task_count_));
}

Test_Task::~Test_Task (void)
{
  ACE_GUARD (ACE_Recursive_Thread_Mutex,recursive_lock);

  ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) TT- Test_Task::task_count_ = %d\n"),Test_Task::task_count_));

  ACE_ASSERT (Test_Task::task_count_ == 0);
}

int
Test_Task::open (void *args)
{
  this->reactor (reinterpret_cast<ACE_Reactor *> (args));
  return this->activate (THR_NEW_LWP);  // 只有调用了activate方法后，才能自动执行后面的svc方法。
}

int
Test_Task::close (u_long)
{
  ACE_GUARD_RETURN (ACE_Recursive_Thread_Mutex,recursive_lock,-1);

  Test_Task::task_count_--;

  ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) close Test_Task::task_count_ = %d\n"),Test_Task::task_count_));

  ACE_ASSERT (Test_Task::task_count_ >= 0);

  return 0;
}

int
Test_Task::svc (void) // 在上面调用了activate方法后，任务自动执行svc方法。
{
  ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) svc\n")));

  for (size_t i = 0; i < ACE_MAX_ITERATIONS; i++)
    {
      ACE_OS::thr_yield ();

      // Only wait up to 10 milliseconds to notify the Reactor.
      ACE_Time_Value timeout (0,10 * 1000);

	  // 调用 Reactor 的 notify 方法向 Reactor 发送通知，后面的 handle_input 和 handle_close 方法在 Reactor 处理通知（在下面）时被回调。
      if (this->reactor ()->notify (this,ACE_Event_Handler::READ_MASK,&timeout) == -1)
        {
          if (errno == ETIME)
            ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) %p\n"),ACE_TEXT ("notify() timed out")));
          else
            ACE_ERROR_RETURN ((LM_ERROR,ACE_TEXT ("notify")),-1);
        }
  }

  return 0;
}

int
Test_Task::handle_close (ACE_HANDLE,ACE_Reactor_Mask)
{
  return 0;
}

int
Test_Task::handle_input (ACE_HANDLE) 
{
  this->handled_++;

  if (this->handled_ == ACE_MAX_ITERATIONS)
    {
      done_count--;
      ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) handle_input,handled_ = %d,done_count = %d\n"),this->handled_,done_count.value ()));
    }

  ACE_OS::thr_yield ();
  return -1;
}

static void *
worker (void *args)
{
  ACE_Reactor *reactor = reinterpret_cast<ACE_Reactor *> (args);

  // Make this thread the owner of the Reactor's event loop.
  reactor->owner (ACE_Thread::self ());

  // Use a timeout to inform the Reactor when to shutdown.
  ACE_Time_Value timeout (4);

  for (;;)
    switch (reactor->handle_events (timeout))  // 调用 Reactor 的 handle_events 方法，循环处理任务发来的通知（在上面），回调任务的 handle_input 和 handle_close 方法。
      {
      case -1:
        ACE_ERROR_RETURN ((LM_ERROR,ACE_TEXT ("reactor")),0);
        /* NOTREACHED */
      case 0:
        ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) Reactor shutdown\n")));
        return 0;
      }

  ACE_NOTREACHED (return 0);
}

#endif /* ACE_HAS_THREADS */

int
run_main (int,ACE_TCHAR *[])
{
  ACE_START_TEST (ACE_TEXT ("Reactors_Test"));

#if defined (ACE_HAS_THREADS)
  ACE_ASSERT (ACE_LOG_MSG->op_status () != -1);

  thr_mgr = ACE_Thread_Manager::instance ();

  ACE_Reactor reactor;
  ACE_ASSERT (ACE_LOG_MSG->op_status () != -1);

  Test_Task tt1[MAX_TASKS];
  Test_Task tt2[MAX_TASKS];

  // Activate all of the Tasks.

  for (int i = 0; i < MAX_TASKS; i++)
    {
      tt1[i].open (ACE_Reactor::instance ());  // 给任务关联Reactor。
      tt2[i].open (&reactor); // 给任务关联Reactor。
    }

  //创建两线程，分别监视两个Reactor。
  // Spawn two threads each running a different reactor.

  if (ACE_Thread_Manager::instance ()->spawn
      (ACE_THR_FUNC (worker),(void *) ACE_Reactor::instance (),THR_BOUND | THR_DETACHED) == -1)
    ACE_ERROR_RETURN ((LM_ERROR,ACE_TEXT ("%p\n"),ACE_TEXT ("spawn")),-1);

  else if (ACE_Thread_Manager::instance ()->spawn
      (ACE_THR_FUNC (worker),(void *) &reactor,-1);

  // 等待所有线程结束。
  if (ACE_Thread_Manager::instance ()->wait () == -1)
    ACE_ERROR_RETURN ((LM_ERROR,ACE_TEXT ("wait")),-1);

  ACE_DEBUG ((LM_DEBUG,ACE_TEXT ("(%t) all threads are finished\n")));

#else
  ACE_ERROR ((LM_INFO,ACE_TEXT ("threads not supported on this platform\n")));
#endif /* ACE_HAS_THREADS */
  ACE_END_TEST;
  return 0;
}

在run_main函数里写上下面输出语句，看看 ACE_Reactor::instance() 和 &reactor 是否相等。

printf("ACE_Reactor::instance() = %d,&reactor = %d,reactor.instance() = %d\n",ACE_Reactor::instance(),&reactor,reactor.instance());

运行发现：ACE_Reactor::instance() 和 reactor.instance() 相等，但与 &reactor 不相等。

查看ACE源代码，得知应该是下面原因。

ACE_Reactor 类里有 reactor_ 静态成员：

/// Pointer to a process-wide ACE_Reactor singleton.（指向一个进程内的ACE_Reactor单例）
static ACE_Reactor *reactor_;

但ACE_Reactor的构造函数里并没有对 reactor_ 如下赋值：
reactor_ = this;

所以 ACE_Reactor reactor; 语句没有把 &reactor 传入类中。