Java 工具（jmap,jstack）在linux上的源码分析（三）执行的线程vm thread

在前面的博客中（http://blog.youkuaiyun.com/raintungli/article/details/7034005）所提到的信号转发线程，Attach Listener 线程都只是操作socket文件，并没有去执行比如stack 分析，或者heap的分析，真正的工作线程其实是vm thread.

（一）启动vm thread

jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
...
  // 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();
      }
    }
  }
...


}

我们可以看到，在thread.cpp里启动了线程vm thread，在这里我们同时也稍微的略带的讲一下jvm在linux里如何启动线程的。

通常在linux中启动线程，是调用

int pthread_create((pthread_t *__thread, __const pthread_attr_t *__attr,void *(*__start_routine) (void *), void *__arg));

而在java里却增加了os:create_thread --初始化线程 和os:start_thread--启动线程

我们去看一下jvm里面是如何在linux里做到的

在os_linux.cpp中来看create_thread的方法

bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
....
int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
....
}

继续看java_start方法

static void *java_start(Thread *thread) {
....
  // handshaking with parent thread
  {
    MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);

    // notify parent thread
    osthread->set_state(INITIALIZED);
    sync->notify_all();

    // wait until os::start_thread()
    while (osthread->get_state() == INITIALIZED) {
      sync->wait(Mutex::_no_safepoint_check_flag);
    }
  }

  // call one more level start routine
  thread->run();

  return 0;
}

首先jvm先设置了当前线程的状态是Initialized, 然后notify所有的线程，

 while (osthread->get_state() == INITIALIZED) {
      sync->wait(Mutex::_no_safepoint_check_flag);
    }

不停的查看线程的当前状态是不是Initialized, 如果是的话，调用了sync->wait()的方法等待。

来看os:start_thread的方法 os.cpp

void os::start_thread(Thread* thread) {
  // guard suspend/resume
  MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag);
  OSThread* osthread = thread->osthread();
  osthread->set_state(RUNNABLE);
  pd_start_thread(thread);
}

这时候设置了线程的状态为runnable,但没有notify线程

在 pd_start_thread(thread)中, os_linux.cpp中

void os::pd_start_thread(Thread* thread) {
  OSThread * osthread = thread->osthread();
  assert(osthread->get_state() != INITIALIZED, "just checking");
  Monitor* sync_with_child = osthread->startThread_lock();
  MutexLockerEx ml(sync_with_child, Mutex::_no_safepoint_check_flag);
  sync_with_child->notify();
}

这时候我们看到了notify 线程的操作

也就是这时候notify了线程，因为这时候的线程的状态是RUNNABLE, 方法java_start继续往下执行，于是调用了thread->run()的方法

对于线程vm Thread 也就是调用了vmthread::run方法

vmThread.cpp

void VMThread::run() {
...
this->loop();
...
}

调用了loop函数，处理了VM_Operation 的queue 关于queue的级别和优先级处理算法：可以参考 另一篇博客：http://blog.youkuaiyun.com/raintungli/article/details/6553337

 

（二）Jstack 运行在vm thread里的VM_Operation

jstack 处理也就是在前面博客所提到的attach Listener 线程所做的 operation

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;
}

简单看一下类VM_PrintThreads 它 继承了VM_Operation

class VM_PrintThreads: public VM_Operation {
 private:
  outputStream* _out;
  bool _print_concurrent_locks;
 public:
  VM_PrintThreads()                                                { _out = tty; _print_concurrent_locks = PrintConcurrentLocks; }
  VM_PrintThreads(outputStream* out, bool print_concurrent_locks)  { _out = out; _print_concurrent_locks = print_concurrent_locks; }
  VMOp_Type type() const                                           {  return VMOp_PrintThreads; }
  void doit();
  bool doit_prologue();
  void doit_epilogue();
};

当调用VMThread::execute()也就是将VM_PrintThreads 放入了_vm_queue中，交给vm thread 处理，对vm thread来说取出queue里的VM_Operation,并且调用doit方法。

在jstack里，attach listener 的线程产生了VM_PrintThreads，VM_PrintJNI，VM_FindDeadlocks 3个operations，交给了vm thread  的线程处理。