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
- 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
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... 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 STRJNIEXPORT void JNICALLJava_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 threadvoid 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 }} 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(); }... 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。
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。