摘要:Java 的线程创建和启动非常简单,但如果问一个线程是怎么启动起来的往往并不清楚,甚至不知道为什么启动时是调用start(),而不是调用run()方法呢?
本文分享自华为云社区《Thread.start() ,工作这么久,还不知道它是怎么让线程跑起来的!》,作者:小傅哥。
面试官:我考你个题,看看你进大厂的几率大不。嗯... Java 线程如何启动的?
谢飞机:如何启动的?start 启动的!
面试官:还有吗?
谢飞机:嗯…,没了!
面试官:嗯,可能会与不会这一个题并不会让你代码有多牛、有多好,但是你的技术栈深度和广度,决定你的编程职业生涯是否有一条康庄大道。还是要多努力!
new Thread(() -> { // todo }).start();
咳咳,Java 的线程创建和启动非常简单,但如果问一个线程是怎么启动起来的往往并不清楚,甚至不知道为什么启动时是调用start(),而不是调用run()方法呢?
那么,为了让大家有一个更直观的认知,我们先站在上帝视角。把这段 Java 的线程代码,到 JDK 方法使用,以及 JVM 的相应处理过程,展示给大家,以方便我们后续逐步分析。
以上,就是一个线程启动的整体过程分析,会涉及到如下知识点:
接下来,我们就开始逐步分析每一步源码的执行内容,从而了解线程启动过程。
如图 19-2 是线程的启动过程时序图,整体的链路较长,会涉及到 JVM 的操作。核心源码如下:
new Thread(() -> { // todo }).start(); // JDK 源码 public synchronized void start() { if (threadStatus != 0) throw new IllegalThreadStateException(); group.add(this); boolean started = false; try { start0(); started = true; } finally { try { if (!started) { group.threadStartFailed(this); } } catch (Throwable ignore) {} } }
// 本地方法 start0 private native void start0(); // 注册本地方法 public class Thread implements Runnable { /* Make sure registerNatives is the first thing <clinit> does. */ private static native void registerNatives(); static { registerNatives(); } // ... }
registerNatives,本地方法定义在 Thread.c 中,以下是定义的核心源码:
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}, {"interrupt0", "()V", (void *)&JVM_Interrupt}, {"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock}, {"getThreads", "()[" THD, (void *)&JVM_GetAllThreads}, {"dumpThreads", "([" THD ")[[" STE, (void *)&JVM_DumpThreads}, {"setNativeName", "(" STR ")V", (void *)&JVM_SetNativeThreadName}, };
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) JVMWrapper("JVM_StartThread"); JavaThread *native_thread = NULL; // 创建线程 native_thread = new JavaThread(&thread_entry, sz); // 启动线程 Thread::start(native_thread); JVM_END
thread_entry,线程入口
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(), vmSymbols::void_method_signature(), THREAD); }
重点,在创建线程引入这个线程入口的方法时,thread_entry 中包括了 Java 的回调函数 JavaCalls::call_virtual。这个回调函数会由 JVM 调用。
vmSymbols::run_method_name(),就是那个被回调的方法,源码如下:
#define VM_SYMBOLS_DO(template, do_alias) template(run_method_name, "run")
native_thread = new JavaThread(&thread_entry, sz);
接下来,我们继续看 JavaThread 的源码执行内容。
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); }
源码:
众所周知,JVM 是个啥!,所以它的 OS 服务实现,Liunx 还有 Windows 等,都会实现线程的创建逻辑。这有点像适配器模式
os_linux -> os::create_thread
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) { assert(thread->osthread() == NULL, "caller responsible"); // Allocate the OSThread object OSThread* osthread = new OSThread(NULL, NULL); // Initial state is ALLOCATED but not INITIALIZED osthread->set_state(ALLOCATED); pthread_t tid; int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread); return true; }
static void *java_start(Thread *thread) { // 线程ID int pid = os::current_process_id(); // 设置线程 ThreadLocalStorage::set_thread(thread); // 设置线程状态:INITIALIZED 初始化完成 osthread->set_state(INITIALIZED); // 唤醒所有线程 sync->notify_all(); // 循环,初始化状态,则一致等待 wait while (osthread->get_state() == INITIALIZED) { sync->wait(Mutex::_no_safepoint_check_flag); } // 等待唤醒后,执行 run 方法 thread->run(); return 0; }
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread)) JVMWrapper("JVM_StartThread"); JavaThread *native_thread = NULL; // 创建线程 native_thread = new JavaThread(&thread_entry, sz); // 启动线程 Thread::start(native_thread); JVM_END
void Thread::start(Thread* thread) { trace("start", thread); if (!DisableStartThread) { if (thread->is_Java_thread()) { java_lang_Thread::set_thread_status(((JavaThread*)thread)->threadObj(), java_lang_Thread::RUNNABLE); } // 不同的 OS 会有不同的启动代码逻辑 os::start_thread(thread); } }
void os::start_thread(Thread* thread) { // guard suspend/resume MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag); OSThread* osthread = thread->osthread(); osthread->set_state(RUNNABLE); pd_start_thread(thread); }
void os::pd_start_thread(Thread* thread) { OSThread * osthread = thread->osthread(); assert(osthread->get_state() != INITIALIZED, "just checking"); Monitor* sync_with_child = osthread->startThread_lock(); MutexLockerEx ml(sync_with_child, Mutex::_no_safepoint_check_flag); sync_with_child->notify(); }
// The first routine called by a new Java thread void JavaThread::run() { // ... 初始化线程操作 thread_main_inner(); }
void JavaThread::thread_main_inner() { 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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