跟着sleep看jvm线程变化

jvm线程是维护了线程的状态。new，running，waiting，timed waiting，blocked，terminated。我们通过jstack等工具查看的时候，线程状态就是上面的一种。jvm本身是做了一种抽象，我们现在从一个典型的方法，来跟踪查看一下jvm内部又是怎么做状态变化的。

sleep方法入手

   public static native void sleep(long millis) throws InterruptedException;

sleep是一个native 方法，我们通过jvm原来来进行跟着（源码来自openjdk11)。
根据jni的规范，我们通过包名或者是jni的注册方式找到了对应的声明。

static JNINativeMethod methods[] = {
    {"start0",           "()V",        (void *)&JVM_StartThread},
    {"stop0",            "(" OBJ ")V", (void *)&JVM_StopThread},
    {"isAlive",          "()Z",        (void *)&JVM_IsThreadAlive},
    {"suspend0",         "()V",        (void *)&JVM_SuspendThread},
    {"resume0",          "()V",        (void *)&JVM_ResumeThread},
    {"setPriority0",     "(I)V",       (void *)&JVM_SetThreadPriority},
    {"yield",            "()V",        (void *)&JVM_Yield},
    {"sleep",            "(J)V",       (void *)&JVM_Sleep},
    {"currentThread",    "()" THD,     (void *)&JVM_CurrentThread},
    {"countStackFrames", "()I",        (void *)&JVM_CountStackFrames},
    {"interrupt0",       "()V",        (void *)&JVM_Interrupt},
    {"isInterrupted",    "(Z)Z",       (void *)&JVM_IsInterrupted},
    {"holdsLock",        "(" OBJ ")Z", (void *)&JVM_HoldsLock},
    {"getThreads",        "()[" THD,   (void *)&JVM_GetAllThreads},
    {"dumpThreads",      "([" THD ")[[" STE, (void *)&JVM_DumpThreads},
    {"setNativeName",    "(" STR ")V", (void *)&JVM_SetNativeThreadName},
};

接下来我们要跟踪的就是JVM_Sleep了。
我们一点一点来解析这个方法。
首先这里有宏定义。

JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))

我们展开JVM_ENTRY。

#define JVM_ENTRY(result_type, header)                               \
extern "C" {                                                         \
  result_type JNICALL header {                                       \
    JavaThread* thread=JavaThread::thread_from_jni_environment(env); \
    ThreadInVMfromNative __tiv(thread);                              \
    debug_only(VMNativeEntryWrapper __vew;)                          \
    VM_ENTRY_BASE(result_type, header, thread)

在ThreadInVMfromNative中，发生了一次线程状态的变更。

class ThreadInVMfromNative : public ThreadStateTransition {
 public:
  ThreadInVMfromNative(JavaThread* thread) : ThreadStateTransition(thread) {
    trans_from_native(_thread_in_vm);
  }
  ~ThreadInVMfromNative() {
    trans_and_fence(_thread_in_vm, _thread_in_native);
  }
};

在构造方法中把JavaThread的**thread->thread_state()**状态变为了_thread_in_vm。在析构函数中把状态改成了_thread_in_native。这里的__tiv是一个本地对象，只有在栈销毁的时候才会触发析构，也就是说这里的转为_thread_in_native只不过是一瞬间的事情。
展开头结束后，我们再继续往后观察。

  JavaThreadSleepState jtss(thread);

在这个构造方法中。把**java_thread->threadObj()**的状态变为了java_lang_Thread::SLEEPING

  static void set_thread_status(JavaThread* java_thread,
                                java_lang_Thread::ThreadStatus state) {
    java_lang_Thread::set_thread_status(java_thread->threadObj(), state);
  }

这里对应的就是java的线程的状态了。
在往下走就直接设置**thread->osthread()**的状态为sleep。

    ThreadState old_state = thread->osthread()->get_state();
    thread->osthread()->set_state(SLEEPING);

状态到这里全部设置完成。

状态梳理

通过上面的代码，我们可以发现最核心的就是JavaThread的这个对象，他本身代表的jvm中的线程状态。会标识线程是在vm还是线程是在java或者在native。具体的状态如下

enum JavaThreadState {
  _thread_uninitialized     =  0, // should never happen (missing initialization)
  _thread_new               =  2, // just starting up, i.e., in process of being initialized
  _thread_new_trans         =  3, // corresponding transition state (not used, included for completness)
  _thread_in_native         =  4, // running in native code
  _thread_in_native_trans   =  5, // corresponding transition state
  _thread_in_vm             =  6, // running in VM
  _thread_in_vm_trans       =  7, // corresponding transition state
  _thread_in_Java           =  8, // running in Java or in stub code
  _thread_in_Java_trans     =  9, // corresponding transition state (not used, included for completness)
  _thread_blocked           = 10, // blocked in vm
  _thread_blocked_trans     = 11, // corresponding transition state
  _thread_max_state         = 12  // maximum thread state+1 - used for statistics allocation
};

这个类同时那种java线程状态的引用。就是java_thread->threadObj()。这个对应的是java的线程状态，这个也是我们jstack看到的状态。

"main" #1 prio=5 os_prio=31 tid=0x00007fee9b809000 nid=0xe03 waiting on condition [0x0000700008d65000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
	at java.lang.Thread.sleep(Native Method)
	at com.company.Sleep.main(Sleep.java:7)

为什么输出的是timed_waiting(sleep)，这个主要是因为格式化输出的原因。

    if(status == THREAD_STATUS_NEW){
      return "NEW";
    }else if(status == THREAD_STATUS_RUNNABLE){
      return "RUNNABLE";
    }else if(status == THREAD_STATUS_SLEEPING){
      return "TIMED_WAITING (sleeping)";
    }else if(status == THREAD_STATUS_IN_OBJECT_WAIT){
      return "WAITING (on object monitor)";
    }else if(status == THREAD_STATUS_IN_OBJECT_WAIT_TIMED){
      return "TIMED_WAITING (on object monitor)";
    }else if(status == THREAD_STATUS_PARKED){
      return "WAITING (parking)";
    }else if(status == THREAD_STATUS_PARKED_TIMED){
      return "TIMED_WAITING (parking)";
    }else if(status == THREAD_STATUS_BLOCKED_ON_MONITOR_ENTER){
      return "BLOCKED (on object monitor)";
    }else if(status == THREAD_STATUS_TERMINATED){
      return "TERMINATED";
    }
    return "UNKNOWN";
  }

在状态格式化的时候，把sleeping的归类成为TIMED_WAITING (sleeping)。
于此同时Java Thread还拿着系统的线程thread->osthread()。

小结

jvm中起码有三种状态的变化，一种是代表的java规范中的线程状态（java_thread->threadObj()），一种是表示的jvm代理的系统线程的状态（thread->osthread()），同时还有jvm线程在做转换的一种状态，是vm自己的状态表示。