摘要：最后我們重點關注與一下實際就是調用平臺創建線程的方法來創建線程。線程的中斷標識判斷了解了方法的作用以后，再回過頭來看中這段代碼，就很好理解了。

這一篇主要圍繞線程狀態控制相關的操作分析線程的原理，比如線程的中斷、線程的通信等，內容比較多，可能會分兩篇文章

線程的啟動的實現原理

線程停止的實現原理分析

為什么中斷線程會拋出InterruptedException

前面我們簡單分析過了線程的使用，通過調用線程的start方法來啟動線程，線程啟動后會調用run方法執行業務邏輯，run方法執行完畢后，線程的生命周期也就終止了。
很多同學最早學習線程的時候會比較疑惑，啟動一個線程為什么是調用start方法，而不是run方法，這做一個簡單的分析，先簡單看一下start方法的定義

public class Thread implements Runnable {
...
public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */
        if (threadStatus != 0)
            throw new IllegalThreadStateException();
        /* Notify the group that this thread is about to be started
         * so that it can be added to the group"s list of threads
         * and the group"s unstarted count can be decremented. */
        group.add(this);
        boolean started = false;
        try {
            start0(); //注意這里
            started = true;
        } finally {
            try {
                if (!started) {
                    group.threadStartFailed(this);
                }
            } catch (Throwable ignore) {
                /* do nothing. If start0 threw a Throwable then
                  it will be passed up the call stack */
            }
        }
    }
    private native void start0();//注意這里
...

我們看到調用start方法實際上是調用一個native方法start0()來啟動一個線程，首先start0()這個方法是在Thread的靜態塊中來注冊的，代碼如下

public class Thread implements Runnable {
    /* Make sure registerNatives is the first thing  does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }

這個registerNatives的作用是注冊一些本地方法提供給Thread類來使用，比如start0()、isAlive()、currentThread()、sleep()；這些都是大家很熟悉的方法。
registerNatives的本地方法的定義在文件 Thread.c,
Thread.c定義了各個操作系統平臺要用的關于線程的公共數據和操作，以下是Thread.c的全部內容

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},
};
#undef THD
#undef OBJ
#undef STE
#undef STR
JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
    (*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}

從這段代碼可以看出，start0()，實際會執行 JVM_StartThread方法，這個方法是干嘛的呢？ 從名字上來看，似乎是在JVM層面去啟動一個線程，如果真的是這樣，那么在JVM層面，一定會調用Java中定義的run方法。那接下來繼續去找找答案。我們找到 jvm.cpp這個文件；這個文件需要下載hotspot的源碼才能找到.

JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
  JVMWrapper("JVM_StartThread");
...
native_thread = new JavaThread(&thread_entry, sz);
...

JVM_ENTRY是用來定義 JVM_StartThread函數的，在這個函數里面創建了一個真正和平臺有關的本地線程. 本著打破砂鍋查到底的原則，繼續看看 newJavaThread做了什么事情,繼續尋找JavaThread的定義
在hotspot的源碼中 thread.cpp文件中1558行的位置可以找到如下代碼

JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) :
  Thread()
#if INCLUDE_ALL_GCS
  , _satb_mark_queue(&_satb_mark_queue_set),
  _dirty_card_queue(&_dirty_card_queue_set)
#endif // INCLUDE_ALL_GCS
{
  if (TraceThreadEvents) {
    tty->print_cr("creating thread %p", this);
  }
  initialize();
  _jni_attach_state = _not_attaching_via_jni;
  set_entry_point(entry_point);
  // Create the native thread itself.
  // %note runtime_23
  os::ThreadType thr_type = os::java_thread;
  thr_type = entry_point == &compiler_thread_entry ? os::compiler_thread :
                                                     os::java_thread;
  os::create_thread(this, thr_type, stack_sz);
  _safepoint_visible = false;
  // The _osthread may be NULL here because we ran out of memory (too many threads active).
  // We need to throw and OutOfMemoryError - however we cannot do this here because the caller
  // may hold a lock and all locks must be unlocked before throwing the exception (throwing
  // the exception consists of creating the exception object & initializing it, initialization
  // will leave the VM via a JavaCall and then all locks must be unlocked).
  //
  // The thread is still suspended when we reach here. Thread must be explicit started
  // by creator! Furthermore, the thread must also explicitly be added to the Threads list
  // by calling Threads:add. The reason why this is not done here, is because the thread
  // object must be fully initialized (take a look at JVM_Start)
}

這個方法有兩個參數，第一個是函數名稱，線程創建成功之后會根據這個函數名稱調用對應的函數；第二個是當前進程內已經有的線程數量。最后我們重點關注與一下 os::create_thread,實際就是調用平臺創建線程的方法來創建線程。
接下來就是線程的啟動，會調用Thread.cpp文件中的Thread::start(Thread* thread)方法，代碼如下

void Thread::start(Thread* thread) {
  trace("start", thread);
  // Start is different from resume in that its safety is guaranteed by context or
  // being called from a Java method synchronized on the Thread object.
  if (!DisableStartThread) {
    if (thread->is_Java_thread()) {
      // Initialize the thread state to RUNNABLE before starting this thread.
      // Can not set it after the thread started because we do not know the
      // exact thread state at that time. It could be in MONITOR_WAIT or
      // in SLEEPING or some other state.
      java_lang_Thread::set_thread_status(((JavaThread*)thread)->threadObj(),
                                          java_lang_Thread::RUNNABLE);
    }
    os::start_thread(thread);
  }
}

start方法中有一個函數調用： os::start_thread(thread);，調用平臺啟動線程的方法，最終會調用Thread.cpp文件中的JavaThread::run()方法

// The first routine called by a new Java thread
void JavaThread::run() {
  // initialize thread-local alloc buffer related fields
  this->initialize_tlab();
  // used to test validitity of stack trace backs
  this->record_base_of_stack_pointer();
  // Record real stack base and size.
  this->record_stack_base_and_size();
  // Initialize thread local storage; set before calling MutexLocker
  this->initialize_thread_local_storage();
  this->create_stack_guard_pages();
  this->cache_global_variables();
  // Thread is now sufficient initialized to be handled by the safepoint code as being
  // in the VM. Change thread state from _thread_new to _thread_in_vm
  ThreadStateTransition::transition_and_fence(this, _thread_new, _thread_in_vm);
  assert(JavaThread::current() == this, "sanity check");
  assert(!Thread::current()->owns_locks(), "sanity check");
  DTRACE_THREAD_PROBE(start, this);
  // This operation might block. We call that after all safepoint checks for a new thread has
  // been completed.
  this->set_active_handles(JNIHandleBlock::allocate_block());
  if (JvmtiExport::should_post_thread_life()) {
    JvmtiExport::post_thread_start(this);
  }
  EventThreadStart event;
  if (event.should_commit()) {
     event.set_javalangthread(java_lang_Thread::thread_id(this->threadObj()));
     event.commit();
  }
  // We call another function to do the rest so we are sure that the stack addresses used
  // from there will be lower than the stack base just computed
  thread_main_inner();
  // Note, thread is no longer valid at this point!
}

這個方法中主要是做一系列的初始化操作，最后有一個方法 thread_main_inner, 接下來看看這個方法的邏輯是什么樣的

void JavaThread::thread_main_inner() {
  assert(JavaThread::current() == this, "sanity check");
  assert(this->threadObj() != NULL, "just checking");
  // Execute thread entry point unless this thread has a pending exception
  // or has been stopped before starting.
  // Note: Due to JVM_StopThread we can have pending exceptions already!
  if (!this->has_pending_exception() &&
      !java_lang_Thread::is_stillborn(this->threadObj())) {
    {
      ResourceMark rm(this);
      this->set_native_thread_name(this->get_thread_name());
    }
    HandleMark hm(this);
    this->entry_point()(this, this);
  }
  DTRACE_THREAD_PROBE(stop, this);
  this->exit(false);
  delete this;
}

和主流程無關的代碼咱們先不去看，直接找到最核心的代碼塊 this->entry_point()(this,this);, 這個entrypoint應該比較熟悉了，因為我們在前面提到了，在::JavaThread這個方法中傳遞的第一個參數，代表函數名稱，線程啟動的時候會調用這個函數。
如果大家還沒有暈車的話，應該記得我們在jvm.cpp文件中看到的代碼，在創建 native_thread=newJavaThread(&thread_entry,sz); 的時候傳遞了一個threadentry函數，所以我們在jvm.cpp中找到這個函數的定義如下

static void thread_entry(JavaThread* thread, TRAPS) {
{
  HandleMark hm(THREAD);
  Handle obj(THREAD, thread->threadObj());
  JavaValue result(T_VOID);
  JavaCalls::call_virtual(&result,
                          obj,
                          KlassHandle(THREAD, SystemDictionary::Thread_klass()),
                          vmSymbols::run_method_name(), //注意這里
                          vmSymbols::void_method_signature(),
                          THREAD);
}

可以看到 vmSymbols::run_method_name()這個調用，其實就是通過回調方法調用Java線程中定義的run方法， run_method_name是一個宏定義，在vmSymbols.hpp文件中可以找到如下代碼

#define VM_SYMBOLS_DO(template, do_alias)    
...
template(run_method_name, "run")  
...

所以結論就是，Java里面創建線程之后必須要調用start方法才能真正的創建一個線程，該方法會調用虛擬機啟動一個本地線程，本地線程的創建會調用當前系統創建線程的方法進行創建，并且線程被執行的時候會回調 run方法進行業務邏輯的處理

線程的終止有主動和被動之分，被動表示線程出現異常退出或者run方法執行完畢，線程會自動終止。主動的方式是 Thread.stop()來實現線程的終止，但是stop()方法是一個過期的方法，官方是不建議使用，理由很簡單，stop()方法在中介一個線程時不會保證線程的資源正常釋放，也就是不會給線程完成資源釋放工作的機會，相當于我們在linux上通過kill -9強制結束一個進程。

那么如何安全的終止一個線程呢?

我們先看一下下面的代碼，代碼演示了一個正確終止線程的方法，至于它的實現原理，稍后我們再分析

public class InterruptedDemo implements Runnable{
    @Override
    public void run() {
        long i=0l;
        while(!Thread.currentThread().isInterrupted()){//notice here
            i++;
        }
        System.out.println("result:"+i);
    }
    public static void main(String[] args) throws InterruptedException {
        InterruptedDemo interruptedDemo=new InterruptedDemo();
        Thread thread=new Thread(interruptedDemo);
        thread.start();
        Thread.sleep(1000);//睡眠一秒
        thread.interrupt();//notice here
    }
}

代碼中有兩處需要注意，在main線程中，調用了線程的interrupt()方法、在run方法中，while循環中通過 Thread.currentThread().isInterrupted()來判斷線程中斷的標識。所以我們在這里猜想一下，應該是在線程中維護了一個中斷標識，通過 thread.interrupt()方法去改變了中斷標識的值使得run方法中while循環的判斷不成立而跳出循環，因此run方法執行完畢以后線程就終止了。

我們來看一下 thread.interrupt()方法做了什么事情

public class Thread implements Runnable {
...
    public void interrupt() {
        if (this != Thread.currentThread())
            checkAccess();
        synchronized (blockerLock) {
            Interruptible b = blocker;
            if (b != null) {
                interrupt0();           // Just to set the interrupt flag
                b.interrupt(this);
                return;
            }
        }
        interrupt0();
    }
...

這個方法里面，調用了interrupt0()，這個方法在前面分析start方法的時候見過，是一個native方法，這里就不再重復貼代碼了，同樣，我們找到jvm.cpp文件，找到JVM_Interrupt的定義

JVM_ENTRY(void, JVM_Interrupt(JNIEnv* env, jobject jthread))
  JVMWrapper("JVM_Interrupt");
  // Ensure that the C++ Thread and OSThread structures aren"t freed before we operate
  oop java_thread = JNIHandles::resolve_non_null(jthread);
  MutexLockerEx ml(thread->threadObj() == java_thread ? NULL : Threads_lock);
  // We need to re-resolve the java_thread, since a GC might have happened during the
  // acquire of the lock
  JavaThread* thr = java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread));
  if (thr != NULL) {
    Thread::interrupt(thr);
  }
JVM_END

這個方法比較簡單，直接調用了 Thread::interrupt(thr)這個方法，這個方法的定義在Thread.cpp文件中，代碼如下

void Thread::interrupt(Thread* thread) {
  trace("interrupt", thread);
  debug_only(check_for_dangling_thread_pointer(thread);)
  os::interrupt(thread);
}

Thread::interrupt方法調用了os::interrupt方法，這個是調用平臺的interrupt方法，這個方法的實現是在 os_*.cpp文件中，其中星號代表的是不同平臺，因為jvm是跨平臺的，所以對于不同的操作平臺，線程的調度方式都是不一樣的。我們以os_linux.cpp文件為例

void os::interrupt(Thread* thread) {
  assert(Thread::current() == thread || Threads_lock->owned_by_self(),
    "possibility of dangling Thread pointer");
  //獲取本地線程對象
  OSThread* osthread = thread->osthread();
  if (!osthread->interrupted()) {//判斷本地線程對象是否為中斷
    osthread->set_interrupted(true);//設置中斷狀態為true
    // More than one thread can get here with the same value of osthread,
    // resulting in multiple notifications.  We do, however, want the store
    // to interrupted() to be visible to other threads before we execute unpark().
    //這里是內存屏障，這塊在后續的文章中會剖析；內存屏障的目的是使得interrupted狀態對其他線程立即可見
    OrderAccess::fence();
    //_SleepEvent相當于Thread.sleep，表示如果線程調用了sleep方法，則通過unpark喚醒
    ParkEvent * const slp = thread->_SleepEvent ;
    if (slp != NULL) slp->unpark() ;
  }
  // For JSR166. Unpark even if interrupt status already was set
  if (thread->is_Java_thread())
    ((JavaThread*)thread)->parker()->unpark();
  //_ParkEvent用于synchronized同步塊和Object.wait()，這里相當于也是通過unpark進行喚醒
  ParkEvent * ev = thread->_ParkEvent ;
  if (ev != NULL) ev->unpark() ;
}

通過上面的代碼分析可以知道，thread.interrupt()方法實際就是設置一個interrupted狀態標識為true、并且通過ParkEvent的unpark方法來喚醒線程。

對于synchronized阻塞的線程，被喚醒以后會繼續嘗試獲取鎖，如果失敗仍然可能被park

在調用ParkEvent的park方法之前，會先判斷線程的中斷狀態，如果為true，會清除當前線程的中斷標識

Object.wait、Thread.sleep、Thread.join會拋出InterruptedException

這里給大家普及一個知識點，為什么Object.wait、Thread.sleep和Thread.join都會拋出InterruptedException?首先，這個異常的意思是表示一個阻塞被其他線程中斷了。然后，由于線程調用了interrupt()中斷方法，那么Object.wait、Thread.sleep等被阻塞的線程被喚醒以后會通過is_interrupted方法判斷中斷標識的狀態變化，如果發現中斷標識為true，則先清除中斷標識，然后拋出InterruptedException

需要注意的是，InterruptedException異常的拋出并不意味著線程必須終止，而是提醒當前線程有中斷的操作發生，至于接下來怎么處理取決于線程本身，比如

直接捕獲異常不做任何處理

將異常往外拋出

停止當前線程，并打印異常信息

為了讓大家能夠更好的理解上面這段話，我們以Thread.sleep為例直接從jdk的源碼中找到中斷標識的清除以及異常拋出的方法代碼

找到 is_interrupted()方法，linux平臺中的實現在os_linux.cpp文件中，代碼如下

bool os::is_interrupted(Thread* thread, bool clear_interrupted) {
  assert(Thread::current() == thread || Threads_lock->owned_by_self(),
    "possibility of dangling Thread pointer");
  OSThread* osthread = thread->osthread();
  bool interrupted = osthread->interrupted(); //獲取線程的中斷標識
  if (interrupted && clear_interrupted) {//如果中斷標識為true
    osthread->set_interrupted(false);//設置中斷標識為false
    // consider thread->_SleepEvent->reset() ... optional optimization
  }
  return interrupted;
}

找到Thread.sleep這個操作在jdk中的源碼體現，怎么找?相信如果前面大家有認真看的話，應該能很快找到，代碼在jvm.cpp文件中

JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))
  JVMWrapper("JVM_Sleep");
  if (millis < 0) {
    THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
  }
  //判斷并清除線程中斷狀態，如果中斷狀態為true,拋出中斷異常
  if (Thread::is_interrupted (THREAD, true) && !HAS_PENDING_EXCEPTION) {
    THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
  }
  // Save current thread state and restore it at the end of this block.
  // And set new thread state to SLEEPING.
  JavaThreadSleepState jtss(thread);
...

注意上面加了中文注釋的地方的代碼，先判斷is_interrupted的狀態，然后拋出一個InterruptedException異常。到此為止，我們就已經分析清楚了中斷的整個流程。

了解了thread.interrupt方法的作用以后，再回過頭來看Java中 Thread.currentThread().isInterrupted()這段代碼，就很好理解了。由于前者先設置了一個中斷標識為true，所以 isInterrupted()這個方法的返回值為true，故而不滿足while循環的判斷條件導致退出循環。
這里有必要再提一句，就是這個線程中斷標識有兩種方式復位，第一種是前面提到過的InterruptedException；另一種是通過Thread.interrupted()對當前線程的中斷標識進行復位。