聊聊jstack的工作原理

實現一個jstack

在聊Jstack得工作原理前呢，不如讓我們先寫一個簡單的jstack玩玩。不用怕，很簡單的,就幾行代碼的事，看：

public class MyJstack {

    public static void main(String[] args)throws Exception {
        VirtualMachine virtualMachine = VirtualMachine.attach("6361");
        HotSpotVirtualMachine hotSpotVirtualMachine = (HotSpotVirtualMachine)virtualMachine;
        InputStream inputStream = hotSpotVirtualMachine.remoteDataDump(new String[]{});

        byte[] buff = new byte[256];
        int len;
        do {
            len = inputStream.read(buff);
            if (len > 0) {
                String respone = new String(buff, 0, len, "UTF-8");
                System.out.print(respone);
            }
        } while(len > 0);

        inputStream.close();
        virtualMachine.detach();
    }
}

 

很簡單吧，貼到你的開發環境里，運行就好了，別忘了把6361這個進程號換成你自己的Java進程號哦。

實現原理

jstack有兩種實現方式，一種是基於attach api,其實現可以在tools.jar里找到；另一種是基於SA的實現，它被放在了sa-jdi.jar里。如果你通過idea搜索Jstack類，你會看到tools.jar和sa-jdi.jar各有一個Jstack類。

本文呢，就通過分析attch api的源碼，來了解jstack的工作原理。

jstack本地源碼實現

我們來看一下HotSpotVirtualMachine的remoteDataDump方法：

public InputStream remoteDataDump(Object... var1) throws IOException {
        return this.executeCommand("threaddump", var1);
}

 

他是在執行一個叫threaddump的命令。沿着這個executeCommand方法繼續往里追，會發現他是調用了如下方法：

InputStream execute(String var1, Object... var2) throws AgentLoadException, IOException {
        assert var2.length <= 3;

        String var3;
        synchronized(this) {
            if (this.path == null) {
                throw new IOException("Detached from target VM");
            }

            var3 = this.path;
        }

        int var4 = socket();

        try {
            connect(var4, var3);
        } catch (IOException var9) {
            close(var4);
            throw var9;
        }

        IOException var5 = null;

        try {
            this.writeString(var4, "1");
            this.writeString(var4, var1);

 

var1參數就是我們的threaddump指令，不難看出，這個方法是建立了一個socket連接，然后將threaddump指令發送給另一端，即我們要檢查的jvm進程。

注意：限於篇幅我並沒有貼整個方法代碼。execute是HotSpotVirtualMachine的抽象方法，不同平台的jdk有不同的execute方法的實現，我這里的代碼是mac下的execute實現，位於BsdVirtualMachine類中。

通過jtack本地源代碼，我們大致可以粗略的認為：jstack就是通過與指定的jvm進程建立socket連接，然后發送指令，最后將jvm進程返回的內容打印出來。

JVM的源碼實現

了解了jstack的本地源碼，我們在看看jvm進程是如何處理的。

當我們使用Java命令啟動jvm進程時，Java命令會加載虛擬機共享庫，然后執行共享庫里的JNI_CreateJavaVM方法完成虛擬機的創建，在JNI_CreateJavaVM方法里會調用如下代碼，完成具體的一個創建過程：

result = Threads::create_vm((JavaVMInitArgs*) args, &can_try_again);

 

如果你有心，或許會留意到，在你啟動一個jvm進程時，即便你什么線程也沒創建，你用jstack查看還是有很多的線程，如：Signal Dispatcher，VM Thread，Attach Listener等等。當過閱讀本文，你會了解到這三個線程的作用。

01 VM Thread線程

Threads::create_vm這個方法很長，接下來咱們跳出一些重要的段落，來分析分析。

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

    if (!os::create_thread(vmthread, os::vm_thread))//調用操作系統api創建線程
      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);//啟動線程
      while (vmthread->active_handles() == NULL) {
        Notify_lock->wait();
      }
    }
  }

 

通過注釋，你也知道，這一段代碼是從來創建VM Thread線程的。VMThread::create()完成了對現成的命名工作，代碼如下：

void VMThread::create() {
  assert(vm_thread() == NULL, "we can only allocate one VMThread");
  _vm_thread = new VMThread();

  // Create VM operation queue
  _vm_queue = new VMOperationQueue();
  guarantee(_vm_queue != NULL, "just checking");

  _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true);

  if (UsePerfData) {
    // jvmstat performance counters
    Thread* THREAD = Thread::current();
    _perf_accumulated_vm_operation_time =
                 PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
                                                 PerfData::U_Ticks, CHECK);
  }
}


VMThread::VMThread() : NamedThread() {
  set_name("VM Thread");
}

 

通過new VMThread()創建線程對象，在VMThread的構造方法里將線程命名成VM Thread，這就是我們jstack看到的VM Thread線程,同時還為這個線程創建了一個叫VMOperationQueue的隊列。

至於VM Thread線程的作用，我們留到最后再說。

02 Signal Dispatcher線程

繼續沿着 Threads::create_vm方法往下看，我們會看到如下代碼：

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

 

這一句代碼實現了Signal Dispatcher線程的創建，進入到signal_init()方法看看:

void os::signal_init() {
  if (!ReduceSignalUsage) {
    // Setup JavaThread for processing signals
    EXCEPTION_MARK;
    Klass* k = SystemDictionary::resolve_or_fail(vmSymbols::java_lang_Thread(), true, CHECK);
    instanceKlassHandle klass (THREAD, k);
    instanceHandle thread_oop = klass->allocate_instance_handle(CHECK);

    const char thread_name[] = "Signal Dispatcher";
    Handle string = java_lang_String::create_from_str(thread_name, CHECK);

    // Initialize thread_oop to put it into the system threadGroup
    Handle thread_group (THREAD, Universe::system_thread_group());
    JavaValue result(T_VOID);
    JavaCalls::call_special(&result, thread_oop,
                           klass,
                           vmSymbols::object_initializer_name(),
                           vmSymbols::threadgroup_string_void_signature(),
                           thread_group,
                           string,
                           CHECK);

    KlassHandle group(THREAD, SystemDictionary::ThreadGroup_klass());
    JavaCalls::call_special(&result,
                            thread_group,
                            group,
                            vmSymbols::add_method_name(),
                            vmSymbols::thread_void_signature(),
                            thread_oop,         // ARG 1
                            CHECK);

    os::signal_init_pd();

    { MutexLocker mu(Threads_lock);
      JavaThread* signal_thread = new JavaThread(&signal_thread_entry);

      // At this point it may be possible that no osthread was created for the
      // JavaThread due to lack of memory. We would have to throw an exception
      // in that case. However, since this must work and we do not allow
      // exceptions anyway, check and abort if this fails.
      if (signal_thread == NULL || signal_thread->osthread() == NULL) {
        vm_exit_during_initialization("java.lang.OutOfMemoryError",
                                      "unable to create new native thread");
      }

      java_lang_Thread::set_thread(thread_oop(), signal_thread);
      java_lang_Thread::set_priority(thread_oop(), NearMaxPriority);
      java_lang_Thread::set_daemon(thread_oop());

      signal_thread->set_threadObj(thread_oop());
      Threads::add(signal_thread);
      Thread::start(signal_thread);
    }
    // Handle ^BREAK
    os::signal(SIGBREAK, os::user_handler());
  }
}

 

在這個方法里，我們可以看到要創建的線程名字:Signal Dispatcher，以及線程啟動后調用的方法signal_thread_entry。（方法較長，看重點就好，沒必要每句話都扣清楚）。

有了對上邊代碼的分析，我們只需要看看signal_thread_entry方法，就知道Signal Dispatcher線程的作用了。

static void signal_thread_entry(JavaThread* thread, TRAPS) {
  os::set_priority(thread, NearMaxPriority);
  while (true) {
    int sig;
    {
      // FIXME : Currently we have not decieded what should be the status
      //         for this java thread blocked here. Once we decide about
      //         that we should fix this.
      sig = os::signal_wait();//等待獲取信號
    }
    if (sig == os::sigexitnum_pd()) {
       // Terminate the signal thread
       return;
    }

    switch (sig) {
      case SIGBREAK: {
        // Check if the signal is a trigger to start the Attach Listener - in that
        // case don't print stack traces.
        if (!DisableAttachMechanism && AttachListener::is_init_trigger()) {
          continue;
        }
        // Print stack traces
        // Any SIGBREAK operations added here should make sure to flush
        // the output stream (e.g. tty->flush()) after output.  See 4803766.
        // Each module also prints an extra carriage return after its output.
        VM_PrintThreads op;
        VMThread::execute(&op);
        VM_PrintJNI jni_op;
        VMThread::execute(&jni_op);
        VM_FindDeadlocks op1(tty);
        VMThread::execute(&op1);
        Universe::print_heap_at_SIGBREAK();
        if (PrintClassHistogram) {
          VM_GC_HeapInspection op1(gclog_or_tty, true /* force full GC before heap inspection */);
          VMThread::execute(&op1);
        }
        if (JvmtiExport::should_post_data_dump()) {
          JvmtiExport::post_data_dump();
        }
        break;

 

這個方法里調用os::signal_wait()獲取傳給該jvm進程的信號，然后對信號進行處理。

說下case SIGBREAK里的處理邏輯，當接收到SIGBREAK信號時，會先判斷是否禁止Attach機制，如果沒有禁止，會調用AttachListener::is_init_trigger()方法觸發Attach Listener線程的初始化.如果attach機制被禁用，則會創建VM_PrintThreads、VM_PrintJNI、VM_FindDeadlocks等代表某一個操作的對象，通過VMThread::execute()方法扔到VM Thread線程的VMOperationQueue隊列。

03 Attach Listener線程

繼續沿着 Threads::create_vm方法往下看，在緊挨着啟動Signal Dispatcher線程的下邊，就是啟動Attach Listener線程的語句：

// 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();
    }
  }

 

重點就在AttachListener::init()方法里：

// Starts the Attach Listener thread
void AttachListener::init() {
  EXCEPTION_MARK;
  Klass* k = SystemDictionary::resolve_or_fail(vmSymbols::java_lang_Thread(), true, CHECK);
  instanceKlassHandle klass (THREAD, k);
  instanceHandle thread_oop = klass->allocate_instance_handle(CHECK);

  const char thread_name[] = "Attach Listener";
  Handle string = java_lang_String::create_from_str(thread_name, CHECK);

  // Initialize thread_oop to put it into the system threadGroup
  Handle thread_group (THREAD, Universe::system_thread_group());
  JavaValue result(T_VOID);
  JavaCalls::call_special(&result, thread_oop,
                       klass,
                       vmSymbols::object_initializer_name(),
                       vmSymbols::threadgroup_string_void_signature(),
                       thread_group,
                       string,
                       THREAD);

  if (HAS_PENDING_EXCEPTION) {
    tty->print_cr("Exception in VM (AttachListener::init) : ");
    java_lang_Throwable::print(PENDING_EXCEPTION, tty);
    tty->cr();

    CLEAR_PENDING_EXCEPTION;

    return;
  }

  KlassHandle group(THREAD, SystemDictionary::ThreadGroup_klass());
  JavaCalls::call_special(&result,
                        thread_group,
                        group,
                        vmSymbols::add_method_name(),
                        vmSymbols::thread_void_signature(),
                        thread_oop,             // ARG 1
                        THREAD);

  if (HAS_PENDING_EXCEPTION) {
    tty->print_cr("Exception in VM (AttachListener::init) : ");
    java_lang_Throwable::print(PENDING_EXCEPTION, tty);
    tty->cr();

    CLEAR_PENDING_EXCEPTION;

    return;
  }

  { MutexLocker mu(Threads_lock);
    JavaThread* listener_thread = new JavaThread(&attach_listener_thread_entry);

    // Check that thread and osthread were created
    if (listener_thread == NULL || listener_thread->osthread() == NULL) {
      vm_exit_during_initialization("java.lang.OutOfMemoryError",
                                    "unable to create new native thread");
    }

    java_lang_Thread::set_thread(thread_oop(), listener_thread);
    java_lang_Thread::set_daemon(thread_oop());

    listener_thread->set_threadObj(thread_oop());
    Threads::add(listener_thread);
    Thread::start(listener_thread);
  }
}

 

我們可以通過代碼看出其創建了一個叫Attach Listener的線程，線程執行的邏輯封裝在了attach_listener_thread_entry方法里。

Attach Listener線程的作用，我們看看attach_listener_thread_entry方法便知:

static void attach_listener_thread_entry(JavaThread* thread, TRAPS) {
  os::set_priority(thread, NearMaxPriority);

  thread->record_stack_base_and_size();

  if (AttachListener::pd_init() != 0) {
    return;
  }
  AttachListener::set_initialized();

  for (;;) {
    AttachOperation* op = AttachListener::dequeue();//從隊列里獲取操作對象
    if (op == NULL) {
      return;   // dequeue failed or shutdown
    }

    ResourceMark rm;
    bufferedStream st;
    jint res = JNI_OK;

    // handle special detachall operation
    if (strcmp(op->name(), AttachOperation::detachall_operation_name()) == 0) {
      AttachListener::detachall();
    } else {
      // find the function to dispatch too
      AttachOperationFunctionInfo* info = NULL;
      for (int i=0; funcs[i].name != NULL; i++) {
        const char* name = funcs[i].name;
        assert(strlen(name) <= AttachOperation::name_length_max, "operation <= name_length_max");
        if (strcmp(op->name(), name) == 0) {
          info = &(funcs[i]);
          break;
        }
      }

      // check for platform dependent attach operation
      if (info == NULL) {
        info = AttachListener::pd_find_operation(op->name());
      }

      if (info != NULL) {
        // dispatch to the function that implements this operation
        res = (info->func)(op, &st);//執行操作對象
      } else {
        st.print("Operation %s not recognized!", op->name());
        res = JNI_ERR;
      }
    }

    // operation complete - send result and output to client
    op->complete(res, &st);
  }
}

 

方法很長，我把重點挑出來分析。

首先我們看看調用AttachListener::pd_init()完了什么：

int AttachListener::pd_init() {
  JavaThread* thread = JavaThread::current();
  ThreadBlockInVM tbivm(thread);

  thread->set_suspend_equivalent();
  // cleared by handle_special_suspend_equivalent_condition() or
  // java_suspend_self() via check_and_wait_while_suspended()

  int ret_code = LinuxAttachListener::init();

  // were we externally suspended while we were waiting?
  thread->check_and_wait_while_suspended();

  return ret_code;
}

int LinuxAttachListener::init() {
  char path[UNIX_PATH_MAX];          // socket file
  char initial_path[UNIX_PATH_MAX];  // socket file during setup
  int listener;                      // listener socket (file descriptor)

  // register function to cleanup
  ::atexit(listener_cleanup);

  int n = snprintf(path, UNIX_PATH_MAX, "%s/.java_pid%d",
                   os::get_temp_directory(), os::current_process_id());
  if (n < (int)UNIX_PATH_MAX) {
    n = snprintf(initial_path, UNIX_PATH_MAX, "%s.tmp", path);
  }
  if (n >= (int)UNIX_PATH_MAX) {
    return -1;
  }

  // create the listener socket
  listener = ::socket(PF_UNIX, SOCK_STREAM, 0);//創建套接字
  if (listener == -1) {
    return -1;
  }

  // bind socket
  struct sockaddr_un addr;
  addr.sun_family = AF_UNIX;
  strcpy(addr.sun_path, initial_path);
  ::unlink(initial_path);
  int res = ::bind(listener, (struct sockaddr*)&addr, sizeof(addr));//綁定地址
  if (res == -1) {
    ::close(listener);
    return -1;
  }

  // put in listen mode, set permissions, and rename into place
  res = ::listen(listener, 5);//發起監聽
  if (res == 0) {
      RESTARTABLE(::chmod(initial_path, S_IREAD|S_IWRITE), res);
      if (res == 0) {
          res = ::rename(initial_path, path);
      }
  }
  if (res == -1) {
    ::close(listener);
    ::unlink(initial_path);
    return -1;
  }
  set_path(path);
  set_listener(listener);

  return 0;
}

 

不難發現，AttachListener::pd_init()方法又調用了LinuxAttachListener::init()方法，完成了對套接字的創建和監聽。這與jstack本地代碼建立socket連接發送命令，不謀而合。

再就是有一個for死循環，不停地調用AttachOperation* op = AttachListener::dequeue();獲取操作對象。如果進入到AttachListener::dequeue()方法看一看，其實就是在讀上邊監聽的套接字，我這里就不貼源碼了。

在這個死循環里，我們重點看看如下代碼:

 // find the function to dispatch too
      AttachOperationFunctionInfo* info = NULL;
      for (int i=0; funcs[i].name != NULL; i++) {
        const char* name = funcs[i].name;
        assert(strlen(name) <= AttachOperation::name_length_max, "operation <= name_length_max");
        if (strcmp(op->name(), name) == 0) {
          info = &(funcs[i]);
          break;
        }
      }

      // check for platform dependent attach operation
      if (info == NULL) {
        info = AttachListener::pd_find_operation(op->name());
      }

      if (info != NULL) {
        // dispatch to the function that implements this operation
        res = (info->func)(op, &st);//調動方法
      } else {
        st.print("Operation %s not recognized!", op->name());
        res = JNI_ERR;
      }
    }

    // operation complete - send result and output to client
    op->complete(res, &st);

 

這個for循環會遍歷funcs數組，然后根據從隊列里拿到的AttachOperation對象的name來找到一個匹配的AttachOperationFunctionInfo對象，然后調用其func方法。

看到這里你或許很多疑惑，當然看看funcs數組里的東西，就開朗了：

static AttachOperationFunctionInfo funcs[] = {
  { "agentProperties",  get_agent_properties },
  { "datadump",         data_dump },
  { "dumpheap",         dump_heap },
  { "load",             JvmtiExport::load_agent_library },
  { "properties",       get_system_properties },
  { "threaddump",       thread_dump },
  { "inspectheap",      heap_inspection },
  { "setflag",          set_flag },
  { "printflag",        print_flag },
  { "jcmd",             jcmd },
  { NULL,               NULL }
};

 

有沒有看到上文中我們提到的threaddump命令。jstack通過與jvm進程建立socket連接，然后向jvm進程發送threaddump指令。上文說道調用AttachOperationFunctionInfo對象的func方法處理指令，其實就是調用了thread_dump方法，針對threaddump命令來說。

堅持，馬上就要說完了。來看看thread_dump方法干了些啥吧:

// Implementation of "threaddump" command - essentially a remote ctrl-break
// See also: ThreadDumpDCmd class
//
static jint thread_dump(AttachOperation* op, outputStream* out) {
  bool print_concurrent_locks = false;
  if (op->arg(0) != NULL && strcmp(op->arg(0), "-l") == 0) {
    print_concurrent_locks = true;
  }

  // thread stacks
  VM_PrintThreads op1(out, print_concurrent_locks);
  VMThread::execute(&op1);

  // JNI global handles
  VM_PrintJNI op2(out);
  VMThread::execute(&op2);

  // Deadlock detection
  VM_FindDeadlocks op3(out);
  VMThread::execute(&op3);

  return JNI_OK;
}

 

很簡單，創建了VM_PrintThreads、VM_PrintJNI、VM_FindDeadlocks三個對象，扔給了VM Thread線程的隊列。

說到這里，VM Thread線程的作用，應該真相大白了，就是讀取隊列，然后執行相應的操作。有興趣你可以繼續追進去看看源代碼，我這里就不追下去了。

總結

看了這么多代碼，確實很頭疼，總結下吧。

jstack是通過與jvm進程建立socket連接，然后發送指令來實現相關操作。

jvm的Attach Listener線程監聽套接字，讀取jstack發來的指令，然后將相關的操作扔給VM Thread線程來執行，最后返回給jstack。

在jvm啟動的時候，如果沒有指定StartAttachListener，Attach Listener線程是不會啟動的，在Signal Dispatcher線程收到SIGBREAK信號時，會調用 AttachListener::is_init_trigger（）通過調用用AttachListener::init（）啟動了Attach Listener 線程。

 

 

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