「阿里面试系列」面试加分项，从jvm层面了解线程的启动和中止

 

1. 线程的启动的实现原理
2. 线程中止的实现原理分析
3. 为何中断线程会抛出InterruptedException

线程的启动原理

前面咱们简单分析过了线程的使用，经过调用线程的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。

不少同窗最先学习线程的时候会比较疑惑，启动一个线程为何是调用start方法，而不是run方法，这作一个简单的分析，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

咱们看到调用start方法其实是调用一个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，代码以下

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

这个registerNatives的做用是注册一些本地方法提供给Thread类来使用，好比start0()、isAlive()、currentThread()、sleep()；这些都是你们很熟悉的方法。

registerNatives的本地方法的定义在文件 Thread.c,

Thread.c定义了各个操做系统平台要用的关于线程的公共数据和操做，如下是Thread.c的所有内容

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));
}

从这段代码能够看出，start0()，实际会执行 JVM_StartThread方法，这个方法是干吗的呢？ 从名字上来看，彷佛是在JVM层面去启动一个线程，若是真的是这样，那么在JVM层面，必定会调用Java中定义的run方法。那接下来继续去找找答案。咱们找到 jvm.cpp这个文件；这个文件须要下载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，在这个函数里面建立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作了什么事情,继续寻找JavaThread的定义

在hotspot的源码中 thread.cpp文件中1558行的位置能够找到以下代码

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，线程建立成功以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。最后咱们重点关注与一下 os::create_thread,实际就是调用平台建立线程的方法来建立线程。

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，代码以下

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!
}

这个方法中主要是作一系列的初始化操做，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我们先不去看，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，由于咱们在前面提到了，在::JavaThread这个方法中传递的第一个参数，表明函数名称，线程启动的时候会调用这个函数。

若是你们尚未晕车的话，应该记得咱们在jvm.cpp文件中看到的代码，在建立 native_thread=newJavaThread(&thread_entry,sz); 的时候传递了一个threadentry函数，因此咱们在jvm.cpp中找到这个函数的定义以下

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);
}

能够看到 vmSymbols::run_method_name()这个调用，其实就是经过回调方法调用Java线程中定义的run方法， run_method_name是一个宏定义，在vmSymbols.hpp文件中能够找到以下代码

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

因此结论就是，Java里面建立线程以后必需要调用start方法才能真正的建立一个线程，该方法会调用虚拟机启动一个本地线程，本地线程的建立会调用当前系统建立线程的方法进行建立，而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，可是stop()方法是一个过时的方法，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做的机会，至关于咱们在linux上经过kill -9强制结束一个进程。

那么如何安全的终止一个线程呢?

咱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，稍后咱们再分析

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
 }
}

代码中有两处须要注意，在main线程中，调用了线程的interrupt()方法、在run方法中，while循环中经过 Thread.currentThread().isInterrupted()来判断线程中断的标识。因此咱们在这里猜测一下，应该是在线程中维护了一个中断标识，经过 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，所以run方法执行完毕之后线程就终止了。

线程中断的原理分析

咱们来看一下 thread.interrupt()方法作了什么事情

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，这里就再也不重复贴代码了，一样，咱们找到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，代码以下

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，其中星号表明的是不一样平台，由于jvm是跨平台的，因此对于不一样的操做平台，线程的调度方式都是不同的。咱们以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状态对其余线程当即可见
 OrderAccess::fence();
 //_SleepEvent至关于Thread.sleep，表示若是线程调用了sleep方法，则经过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()，这里至关于也是经过unpark进行唤醒
 ParkEvent * ev = thread->_ParkEvent ;
 if (ev != NULL) ev->unpark() ;
}

经过上面的代码分析能够知道，thread.interrupt()方法实际就是设置一个interrupted状态标识为true、而且经过ParkEvent的unpark方法来唤醒线程。

1. 对于synchronized阻塞的线程，被唤醒之后会继续尝试获取锁，若是失败仍然可能被park
2. 在调用ParkEvent的park方法以前，会先判断线程的中断状态，若是为true，会清除当前线程的中断标识
3. Object.wait、Thread.sleep、Thread.join会抛出InterruptedException

这里给你们普及一个知识点，为何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余线程中断了。而后，因为线程调用了interrupt()中断方法，那么Object.wait、Thread.sleep等被阻塞的线程被唤醒之后会经过is_interrupted方法判断中断标识的状态变化，若是发现中断标识为true，则先清除中断标识，而后抛出InterruptedException

须要注意的是，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做发生，至于接下来怎么处理取决于线程自己，好比

1. 直接捕获异常不作任何处理
2. 将异常往外抛出
3. 中止当前线程，并打印异常信息
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为了让你们可以更好的理解上面这段话，咱们以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平台中的实如今os_linux.cpp文件中，代码以下

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) {//若是中断标识为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这个操做在jdk中的源码体现，怎么找?相信若是前面你们有认真看的话，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，若是中断状态为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，而后抛出一个InterruptedException异常。到此为止，咱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方法的做用之后，再回过头来看Java中 Thread.currentThread().isInterrupted()这段代码，就很好理解了。因为前者先设置了一个中断标识为true，因此 isInterrupted()这个方法的返回值为true，故而不知足while循环的判断条件致使退出循环。

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；另外一种是经过Thread.interrupted()对当前线程的中断标识进行复位。