start0() 会调用 JVM_StartThread 这个方法，JVM层面去启动一个线程，我们需要下载hotspot的源码才能找到.我们找到： jvm.cpp这个文件 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 JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) JVMWrapper("JVM_StartThread"); ... native_thread = new JavaThread(&thread_entry, sz); hotspot的源码中 thread.cpp文件中1558行的位置 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; }
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。
6.其中start方法中会有一个函数调用，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
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); } } 在其中JavaThread会进行一系列的初始化操作，最后有一个方法 thread_main_inner 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! } thread_main_inner 中，找到核心的代码块 this->entry_point()(this,this), 在第四部中native_thread=newJavaThread(&thread_entry,sz); 的时候传递了一个threadentry函数 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; } 我们找到thread_entry函数，找到vmSymbols::run_method_name()这个调用，通过回调方法调用Java线程中定义的run方法，让我们来看看， run_method_name是一个宏定义，在vmSymbols.hpp文件中可以找到 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(), //《---看看看------这里 vmSymbols::void_method_signature(), THREAD); } 找到VM_SYMBOLS_DO 函数，我们可以看到，其对应的就是一个，方法， #define VM_SYMBOLS_DO(template, do_alias) ... template(run_method_name, "run") ...其大概意思就是，Java里面创建线程之后，必须要调用start方法才能创建一个线程，该方法会通过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去创建线程的方法进行创建线程，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，大概意思就是，系统给你留了一个接口，你要去用这个接口调用你的run 方法，该线程提供给你一系列的操作
