从JVM层面深度了解线程的启动和停止原理

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文章简介

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

内容导航

1. 线程的启动的实现原理
2. 线程停止的实现原理分析
3. 为什么中断线程会抛出InterruptedException

线程的启动原理

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
...

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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 <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }

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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));
}

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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);
...

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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)
}

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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);
  }
}

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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!
}


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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;
}

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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);
}
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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")  
...

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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

线程的终止方法及原理

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
    }
}

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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。

线程中断的原理分析

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();
    }
...

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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

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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);
}

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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() ;
}

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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。

1. 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
2. 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
3. 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

1. 直接捕获异常不做任何处理
2. 将异常往外抛出
3. 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;
}

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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);
...

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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。

Java线程的中断标识判断

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。