通过跟踪源码证明在Java中通过执行Start()方法创建线程

/***
 * 线程创建跟踪
 */
public class ThreadCreate {
    
    
    public static void main(String[] args) {
    
    
        new Thread(()->{
    
    
            System.out.println(1);
        }).start();
    }
}

在以上代码中，线程并非在new的时候就被创建，而是执行了start()方法之后才被创建。
继续往下跟踪，进入start()方法所在类查看源码，找到关键的一行代码，
在这里插入图片描述 start0()，这个方法是创建线程的方法，继续往下点。
会发现start0是native方法，简单科普一下，被native修饰的方法代表此方法由其他语言的接口提供，比如C或C++，此时我们再点，发现点不进去了。
再往下我们需要下载OpenJDK来查看源代码，点此下载最新版本，
找到和Thread相关的Thread.c文件后，用任意方法查看此文件，找到start0()方法，具体如下
在这里插入图片描述在上图可以看出，这里对start0()方法做了一个1对1的映射，映射到了JVM_StartThread()这个方法，那么我们继续在OpenJDK中寻找这个方法。
到这里之后可以看到这里又调用了一个C++的方法，我们继续往下找这个JavaThread(&thread_entry, sz)

在这里插入图片描述一如既往，继续找os::create_thread(this, thr_type, stack_sz)

bool os::create_thread(Thread* thread, ThreadType thr_type,
                       size_t req_stack_size) {
    
    
  assert(thread->osthread() == NULL, "caller responsible");

  // Allocate the OSThread object
  OSThread* osthread = new OSThread(NULL, NULL);
  if (osthread == NULL) {
    
    
    return false;
  }

  // set the correct thread state
  osthread->set_thread_type(thr_type);

  // Initial state is ALLOCATED but not INITIALIZED
  osthread->set_state(ALLOCATED);

  thread->set_osthread(osthread);

  // init thread attributes
  pthread_attr_t attr;
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);

  // Calculate stack size if it's not specified by caller.
  size_t stack_size = os::Posix::get_initial_stack_size(thr_type, req_stack_size);
  // In glibc versions prior to 2.7 the guard size mechanism
  // is not implemented properly. The posix standard requires adding
  // the size of the guard pages to the stack size, instead Linux
  // takes the space out of 'stacksize'. Thus we adapt the requested
  // stack_size by the size of the guard pages to mimick proper
  // behaviour. However, be careful not to end up with a size
  // of zero due to overflow. Don't add the guard page in that case.
  size_t guard_size = os::Linux::default_guard_size(thr_type);
  // Configure glibc guard page. Must happen before calling
  // get_static_tls_area_size(), which uses the guard_size.
  pthread_attr_setguardsize(&attr, guard_size);

  size_t stack_adjust_size = 0;
  if (AdjustStackSizeForTLS) {
    
    
    // Adjust the stack_size for on-stack TLS - see get_static_tls_area_size().
    stack_adjust_size += get_static_tls_area_size(&attr);
  } else {
    
    
    stack_adjust_size += guard_size;
  }

  stack_adjust_size = align_up(stack_adjust_size, os::vm_page_size());
  if (stack_size <= SIZE_MAX - stack_adjust_size) {
    
    
    stack_size += stack_adjust_size;
  }
  assert(is_aligned(stack_size, os::vm_page_size()), "stack_size not aligned");

  int status = pthread_attr_setstacksize(&attr, stack_size);
  assert_status(status == 0, status, "pthread_attr_setstacksize");

  ThreadState state;

  {
    
    
    pthread_t tid;
    int ret = pthread_create(&tid, &attr, (void* (*)(void*)) thread_native_entry, thread);

    char buf[64];
    if (ret == 0) {
    
    
      log_info(os, thread)("Thread started (pthread id: " UINTX_FORMAT ", attributes: %s). ",
        (uintx) tid, os::Posix::describe_pthread_attr(buf, sizeof(buf), &attr));
    } else {
    
    
      log_warning(os, thread)("Failed to start thread - pthread_create failed (%s) for attributes: %s.",
        os::errno_name(ret), os::Posix::describe_pthread_attr(buf, sizeof(buf), &attr));
      // Log some OS information which might explain why creating the thread failed.
      log_info(os, thread)("Number of threads approx. running in the VM: %d", Threads::number_of_threads());
      LogStream st(Log(os, thread)::info());
      os::Posix::print_rlimit_info(&st);
      os::print_memory_info(&st);
      os::Linux::print_proc_sys_info(&st);
      os::Linux::print_container_info(&st);
    }

    pthread_attr_destroy(&attr);

    if (ret != 0) {
    
    
      // Need to clean up stuff we've allocated so far
      thread->set_osthread(NULL);
      delete osthread;
      return false;
    }

    // Store pthread info into the OSThread
    osthread->set_pthread_id(tid);

    // Wait until child thread is either initialized or aborted
    {
    
    
      Monitor* sync_with_child = osthread->startThread_lock();
      MutexLocker ml(sync_with_child, Mutex::_no_safepoint_check_flag);
      while ((state = osthread->get_state()) == ALLOCATED) {
    
    
        sync_with_child->wait_without_safepoint_check();
      }
    }
  }

  // Aborted due to thread limit being reached
  if (state == ZOMBIE) {
    
    
    thread->set_osthread(NULL);
    delete osthread;
    return false;
  }

  // The thread is returned suspended (in state INITIALIZED),
  // and is started higher up in the call chain
  assert(state == INITIALIZED, "race condition");
  return true;
}

int ret = pthread_create(&tid, &attr, (void* (*)(void*)) thread_native_entry, thread);

关键代码：int ret = pthread_create(&tid, &attr, (void* (*)(void*)) thread_native_entry, thread);
最终得证，执行start0方法才会调用系统内核的pthread_create()方法创建一个系统级别的线程。

最后更新于2021年1月5日早上5点
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通过跟踪源码证明在Java中通过执行Start()方法创建线程

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