聊聊jstack的工作原理

实现一个jstack

在聊Jstack得工作原理前呢，不如让我们先写一个简单的jstack玩玩。不用怕，很简单的,就几行代码的事，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();
    }
}

很简单吧，贴到你的开发环境里，运行就好了，别忘了把6361这个进程号换成你自己的Java进程号哦。

实现原理

jstack有两种实现方式，一种是基于attach api,其实现可以在tools.jar里找到；另一种是基于SA的实现，它被放在了sa-jdi.jar里。如果你通过idea搜索Jstack类，你会看到tools.jar和sa-jdi.jar各有一个Jstack类。

本文呢，就通过分析attch api的源码，来了解jstack的工作原理。

jstack本地源码实现

我们来看一下HotSpotVirtualMachine的remoteDataDump方法：

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

他是在执行一个叫threaddump的命令。沿着这个executeCommand方法继续往里追，会发现他是调用了如下方法：

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);

var1参数就是我们的threaddump指令，不难看出，这个方法是建立了一个socket连接，然后将threaddump指令发送给另一端，即我们要检查的jvm进程。

注意：限于篇幅我并没有贴整个方法代码。execute是HotSpotVirtualMachine的抽象方法，不同平台的jdk有不同的execute方法的实现，我这里的代码是mac下的execute实现，位于BsdVirtualMachine类中。

通过jtack本地源代码，我们大致可以粗略的认为：jstack就是通过与指定的jvm进程建立socket连接，然后发送指令，最后将jvm进程返回的内容打印出来。

JVM的源码实现

了解了jstack的本地源码，我们在看看jvm进程是如何处理的。

当我们使用Java命令启动jvm进程时，Java命令会加载虚拟机共享库，然后执行共享库里的JNI_CreateJavaVM方法完成虚拟机的创建，在JNI_CreateJavaVM方法里会调用如下代码，完成具体的一个创建过程：

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

如果你有心，或许会留意到，在你启动一个jvm进程时，即便你什么线程也没创建，你用jstack查看还是有很多的线程，如：Signal Dispatcher，VM Thread，Attach Listener等等。当过阅读本文，你会了解到这三个线程的作用。

01 VM Thread线程

Threads::create_vm这个方法很长，接下来咱们跳出一些重要的段落，来分析分析。

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

    if (!os::create_thread(vmthread, os::vm_thread))//调用操作系统api创建线程
      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);//启动线程
      while (vmthread->active_handles() == NULL) {
        Notify_lock->wait();
      }
    }
  }

通过注释，你也知道，这一段代码是从来创建VM Thread线程的。VMThread::create()完成了对现成的命名工作，代码如下：

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");
}

通过new VMThread()创建线程对象，在VMThread的构造方法里将线程命名成VM Thread，这就是我们jstack看到的VM Thread线程,同时还为这个线程创建了一个叫VMOperationQueue的队列。

至于VM Thread线程的作用，我们留到最后再说。

02 Signal Dispatcher线程

继续沿着 Threads::create_vm方法往下看，我们会看到如下代码：

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

这一句代码实现了Signal Dispatcher线程的创建，进入到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());
  }
}

在这个方法里，我们可以看到要创建的线程名字:Signal Dispatcher，以及线程启动后调用的方法signal_thread_entry。（方法较长，看重点就好，没必要每句话都扣清楚）。

有了对上边代码的分析，我们只需要看看signal_thread_entry方法，就知道Signal Dispatcher线程的作用了。

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();//等待获取信号
    }
    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;

这个方法里调用os::signal_wait()获取传给该jvm进程的信号，然后对信号进行处理。

说下case SIGBREAK里的处理逻辑，当接收到SIGBREAK信号时，会先判断是否禁止Attach机制，如果没有禁止，会调用AttachListener::is_init_trigger()方法触发Attach Listener线程的初始化.如果attach机制被禁用，则会创建VM_PrintThreads、VM_PrintJNI、VM_FindDeadlocks等代表某一个操作的对象，通过VMThread::execute()方法扔到VM Thread线程的VMOperationQueue队列。

03 Attach Listener线程

继续沿着 Threads::create_vm方法往下看，在紧挨着启动Signal Dispatcher线程的下边，就是启动Attach Listener线程的语句：

// 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();
    }
  }

重点就在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);
  }
}

我们可以通过代码看出其创建了一个叫Attach Listener的线程，线程执行的逻辑封装在了attach_listener_thread_entry方法里。

Attach Listener线程的作用，我们看看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() != ) {
    return;
  }
  AttachListener::set_initialized();

  for (;;) {
    AttachOperation* op = AttachListener::dequeue();//从队列里获取操作对象
    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()) == ) {
      AttachListener::detachall();
    } else {
      // find the function to dispatch too
      AttachOperationFunctionInfo* info = NULL;
      for (int i=; 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) == ) {
          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);//执行操作对象
      } else {
        st.print("Operation %s not recognized!", op->name());
        res = JNI_ERR;
      }
    }

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

方法很长，我把重点挑出来分析。

首先我们看看调用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 -;
  }

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

  // 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));//绑定地址
  if (res == -) {
    ::close(listener);
    return -;
  }

  // put in listen mode, set permissions, and rename into place
  res = ::listen(listener, );//发起监听
  if (res == ) {
      RESTARTABLE(::chmod(initial_path, S_IREAD|S_IWRITE), res);
      if (res == ) {
          res = ::rename(initial_path, path);
      }
  }
  if (res == -) {
    ::close(listener);
    ::unlink(initial_path);
    return -;
  }
  set_path(path);
  set_listener(listener);

  return ;
}

不难发现，AttachListener::pd_init()方法又调用了LinuxAttachListener::init()方法，完成了对套接字的创建和监听。这与jstack本地代码建立socket连接发送命令，不谋而合。

再就是有一个for死循环，不停地调用AttachOperation* op = AttachListener::dequeue();获取操作对象。如果进入到AttachListener::dequeue()方法看一看，其实就是在读上边监听的套接字，我这里就不贴源码了。

在这个死循环里，我们重点看看如下代码:

 // find the function to dispatch too
      AttachOperationFunctionInfo* info = NULL;
      for (int i=; 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) == ) {
          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);//调动方法
      } else {
        st.print("Operation %s not recognized!", op->name());
        res = JNI_ERR;
      }
    }

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

这个for循环会遍历funcs数组，然后根据从队列里拿到的AttachOperation对象的name来找到一个匹配的AttachOperationFunctionInfo对象，然后调用其func方法。

看到这里你或许很多疑惑，当然看看funcs数组里的东西，就开朗了：

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 }
};

有没有看到上文中我们提到的threaddump命令。jstack通过与jvm进程建立socket连接，然后向jvm进程发送threaddump指令。上文说道调用AttachOperationFunctionInfo对象的func方法处理指令，其实就是调用了thread_dump方法，针对threaddump命令来说。

坚持，马上就要说完了。来看看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() != NULL && strcmp(op->arg(), "-l") == ) {
    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;
}

很简单，创建了VM_PrintThreads、VM_PrintJNI、VM_FindDeadlocks三个对象，扔给了VM Thread线程的队列。

说到这里，VM Thread线程的作用，应该真相大白了，就是读取队列，然后执行相应的操作。有兴趣你可以继续追进去看看源代码，我这里就不追下去了。

总结

看了这么多代码，确实很头疼，总结下吧。

jstack是通过与jvm进程建立socket连接，然后发送指令来实现相关操作。

jvm的Attach Listener线程监听套接字，读取jstack发来的指令，然后将相关的操作扔给VM Thread线程来执行，最后返回给jstack。

在jvm启动的时候，如果没有指定StartAttachListener，Attach Listener线程是不会启动的，在Signal Dispatcher线程收到SIGBREAK信号时，会调用 AttachListener::is_init_trigger（）通过调用用AttachListener::init（）启动了Attach Listener 线程。

---

加入知识星球，可以有更多的交流，更多的学习和更快的提高。