(三)从jvm层面了解线程的启动和停止

摘要：最后我们重点关注与一下实际就是调用平台创建线程的方法来创建线程。线程的中断标识判断了解了方法的作用以后，再回过头来看中这段代码，就很好理解了。

这一篇主要围绕线程状态控制相关的操作分析线程的原理，比如线程的中断、线程的通信等，内容比较多，可能会分两篇文章

线程的启动的实现原理

线程停止的实现原理分析

为什么中断线程会抛出InterruptedException

前面我们简单分析过了线程的使用，通过调用线程的start方法来启动线程，线程启动后会调用run方法执行业务逻辑，run方法执行完毕后，线程的生命周期也就终止了。
很多同学最早学习线程的时候会比较疑惑，启动一个线程为什么是调用start方法，而不是run方法，这做一个简单的分析，先简单看一下start方法的定义

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(); //注意这里
            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();//注意这里
...

我们看到调用start方法实际上是调用一个native方法start0()来启动一个线程，首先start0()这个方法是在Thread的静态块中来注册的，代码如下

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

这个registerNatives的作用是注册一些本地方法提供给Thread类来使用，比如start0()、isAlive()、currentThread()、sleep()；这些都是大家很熟悉的方法。
registerNatives的本地方法的定义在文件 Thread.c,
Thread.c定义了各个操作系统平台要用的关于线程的公共数据和操作，以下是Thread.c的全部内容

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));
}

从这段代码可以看出，start0()，实际会执行 JVM_StartThread方法，这个方法是干嘛的呢？ 从名字上来看，似乎是在JVM层面去启动一个线程，如果真的是这样，那么在JVM层面，一定会调用Java中定义的run方法。那接下来继续去找找答案。我们找到 jvm.cpp这个文件；这个文件需要下载hotspot的源码才能找到.

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

JVM_ENTRY是用来定义 JVM_StartThread函数的，在这个函数里面创建了一个真正和平台有关的本地线程. 本着打破砂锅查到底的原则，继续看看 newJavaThread做了什么事情,继续寻找JavaThread的定义
在hotspot的源码中 thread.cpp文件中1558行的位置可以找到如下代码

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)
}

这个方法有两个参数，第一个是函数名称，线程创建成功之后会根据这个函数名称调用对应的函数；第二个是当前进程内已经有的线程数量。最后我们重点关注与一下 os::create_thread,实际就是调用平台创建线程的方法来创建线程。
接下来就是线程的启动，会调用Thread.cpp文件中的Thread::start(Thread* thread)方法，代码如下

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方法中有一个函数调用： os::start_thread(thread);，调用平台启动线程的方法，最终会调用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!
}

这个方法中主要是做一系列的初始化操作，最后有一个方法 thread_main_inner, 接下来看看这个方法的逻辑是什么样的

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;
}

和主流程无关的代码咱们先不去看，直接找到最核心的代码块 this->entry_point()(this,this);, 这个entrypoint应该比较熟悉了，因为我们在前面提到了，在::JavaThread这个方法中传递的第一个参数，代表函数名称，线程启动的时候会调用这个函数。
如果大家还没有晕车的话，应该记得我们在jvm.cpp文件中看到的代码，在创建 native_thread=newJavaThread(&thread_entry,sz); 的时候传递了一个threadentry函数，所以我们在jvm.cpp中找到这个函数的定义如下

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(), //注意这里
                          vmSymbols::void_method_signature(),
                          THREAD);
}

可以看到 vmSymbols::run_method_name()这个调用，其实就是通过回调方法调用Java线程中定义的run方法， run_method_name是一个宏定义，在vmSymbols.hpp文件中可以找到如下代码

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

所以结论就是，Java里面创建线程之后必须要调用start方法才能真正的创建一个线程，该方法会调用虚拟机启动一个本地线程，本地线程的创建会调用当前系统创建线程的方法进行创建，并且线程被执行的时候会回调 run方法进行业务逻辑的处理

线程的终止有主动和被动之分，被动表示线程出现异常退出或者run方法执行完毕，线程会自动终止。主动的方式是 Thread.stop()来实现线程的终止，但是stop()方法是一个过期的方法，官方是不建议使用，理由很简单，stop()方法在中介一个线程时不会保证线程的资源正常释放，也就是不会给线程完成资源释放工作的机会，相当于我们在linux上通过kill -9强制结束一个进程。

那么如何安全的终止一个线程呢?

我们先看一下下面的代码，代码演示了一个正确终止线程的方法，至于它的实现原理，稍后我们再分析

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
    }
}

代码中有两处需要注意，在main线程中，调用了线程的interrupt()方法、在run方法中，while循环中通过 Thread.currentThread().isInterrupted()来判断线程中断的标识。所以我们在这里猜想一下，应该是在线程中维护了一个中断标识，通过 thread.interrupt()方法去改变了中断标识的值使得run方法中while循环的判断不成立而跳出循环，因此run方法执行完毕以后线程就终止了。

我们来看一下 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();
    }
...

这个方法里面，调用了interrupt0()，这个方法在前面分析start方法的时候见过，是一个native方法，这里就不再重复贴代码了，同样，我们找到jvm.cpp文件，找到JVM_Interrupt的定义

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

这个方法比较简单，直接调用了 Thread::interrupt(thr)这个方法，这个方法的定义在Thread.cpp文件中，代码如下

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

Thread::interrupt方法调用了os::interrupt方法，这个是调用平台的interrupt方法，这个方法的实现是在 os_*.cpp文件中，其中星号代表的是不同平台，因为jvm是跨平台的，所以对于不同的操作平台，线程的调度方式都是不一样的。我们以os_linux.cpp文件为例

void os::interrupt(Thread* thread) {
  assert(Thread::current() == thread || Threads_lock->owned_by_self(),
    "possibility of dangling Thread pointer");
  //获取本地线程对象
  OSThread* osthread = thread->osthread();
  if (!osthread->interrupted()) {//判断本地线程对象是否为中断
    osthread->set_interrupted(true);//设置中断状态为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().
    //这里是内存屏障，这块在后续的文章中会剖析；内存屏障的目的是使得interrupted状态对其他线程立即可见
    OrderAccess::fence();
    //_SleepEvent相当于Thread.sleep，表示如果线程调用了sleep方法，则通过unpark唤醒
    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同步块和Object.wait()，这里相当于也是通过unpark进行唤醒
  ParkEvent * ev = thread->_ParkEvent ;
  if (ev != NULL) ev->unpark() ;
}

通过上面的代码分析可以知道，thread.interrupt()方法实际就是设置一个interrupted状态标识为true、并且通过ParkEvent的unpark方法来唤醒线程。

对于synchronized阻塞的线程，被唤醒以后会继续尝试获取锁，如果失败仍然可能被park

在调用ParkEvent的park方法之前，会先判断线程的中断状态，如果为true，会清除当前线程的中断标识

Object.wait、Thread.sleep、Thread.join会抛出InterruptedException

这里给大家普及一个知识点，为什么Object.wait、Thread.sleep和Thread.join都会抛出InterruptedException?首先，这个异常的意思是表示一个阻塞被其他线程中断了。然后，由于线程调用了interrupt()中断方法，那么Object.wait、Thread.sleep等被阻塞的线程被唤醒以后会通过is_interrupted方法判断中断标识的状态变化，如果发现中断标识为true，则先清除中断标识，然后抛出InterruptedException

需要注意的是，InterruptedException异常的抛出并不意味着线程必须终止，而是提醒当前线程有中断的操作发生，至于接下来怎么处理取决于线程本身，比如

直接捕获异常不做任何处理

将异常往外抛出

停止当前线程，并打印异常信息

为了让大家能够更好的理解上面这段话，我们以Thread.sleep为例直接从jdk的源码中找到中断标识的清除以及异常抛出的方法代码

找到 is_interrupted()方法，linux平台中的实现在os_linux.cpp文件中，代码如下

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(); //获取线程的中断标识
  if (interrupted && clear_interrupted) {//如果中断标识为true
    osthread->set_interrupted(false);//设置中断标识为false
    // consider thread->_SleepEvent->reset() ... optional optimization
  }
  return interrupted;
}

找到Thread.sleep这个操作在jdk中的源码体现，怎么找?相信如果前面大家有认真看的话，应该能很快找到，代码在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");
  }
  //判断并清除线程中断状态，如果中断状态为true,抛出中断异常
  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释的地方的代码，先判断is_interrupted的状态，然后抛出一个InterruptedException异常。到此为止，我们就已经分析清楚了中断的整个流程。

了解了thread.interrupt方法的作用以后，再回过头来看Java中 Thread.currentThread().isInterrupted()这段代码，就很好理解了。由于前者先设置了一个中断标识为true，所以 isInterrupted()这个方法的返回值为true，故而不满足while循环的判断条件导致退出循环。
这里有必要再提一句，就是这个线程中断标识有两种方式复位，第一种是前面提到过的InterruptedException；另一种是通过Thread.interrupted()对当前线程的中断标识进行复位。