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。
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