【深入Java开发】JVM源码分析之一个Java进程究竟能创建多少线程

概述
虽然这篇文章的标题打着JVM源码分析的旗号，不过本文不仅仅从JVM源码角度来分析，更多的来自于Linux Kernel的源码分析，今天要说的是JVM里比较常见的一个问题

这个问题可能有几种表述

一个Java进程到底能创建多少线程？
到底有哪些因素决定了能创建多少线程？
java.lang.OutOfMemoryError: unable to create new native thread的异常究竟是怎么回事
不过我这里先声明下可能不能完全百分百将各种因素都理出来，因为毕竟我不是做Linux Kernel开发的，还有不少细节没有注意到的，我将我能分析到的因素和大家分享一下，如果大家在平时工作中还碰到别的因素，欢迎在文章下面留言，让更多人参与进来讨论

从JVM说起
线程大家都熟悉，new Thread().start()即会创建一个线程，这里我首先指出一点new Thread()其实并不会创建一个真正的线程，只有在调用了start方法之后才会创建一个线程，这个大家分析下Java代码就知道了，Thread的构造函数是纯Java代码，start方法会调到一个native方法start0里，而start0其实就是JVM_StartThread这个方法

JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))

  ...

      // We could also check the stillborn flag to see if this thread was already stopped, but
      // for historical reasons we let the thread detect that itself when it starts running

      jlong size =
             java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));
      // Allocate the C++ Thread structure and create the native thread.  The
      // stack size retrieved from java is signed, but the constructor takes
      // size_t (an unsigned type), so avoid passing negative values which would
      // result in really large stacks.
      size_t sz = size > 0 ? (size_t) size : 0;
      native_thread = new JavaThread(&thread_entry, sz);

  ...  

  if (native_thread->osthread() == NULL) {
    ...
    THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(),
              "unable to create new native thread");
  }

  Thread::start(native_thread);

JVM_END
从上面代码里首先要大家关注下最后的那个if判断if (native_thread->osthread() == NULL)，如果osthread为空，那将会抛出大家比较熟悉的unable to create new native thread OOM异常，因此osthread为空非常关键，后面会看到什么情况下osthread会为空

另外大家应该注意到了native_thread = new JavaThread(&thread_entry, sz)，在这里才会真正创建一个线程

JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) :
  Thread()
#ifndef SERIALGC
  , _satb_mark_queue(&_satb_mark_queue_set),
  _dirty_card_queue(&_dirty_card_queue_set)
#endif // !SERIALGC
{
  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);

}
上面代码里的os::create_thread(this, thr_type, stack_sz)会通过pthread_create来创建线程，而Linux下对应的实现如下：

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);
  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);

  // stack size
  if (os::Linux::supports_variable_stack_size()) {
    // calculate stack size if it's not specified by caller
    if (stack_size == 0) {
      stack_size = os::Linux::default_stack_size(thr_type);

      switch (thr_type) {
      case os::java_thread:
        // Java threads use ThreadStackSize which default value can be
        // changed with the flag -Xss
        assert (JavaThread::stack_size_at_create() > 0, "this should be set");
        stack_size = JavaThread::stack_size_at_create();
        break;
      case os::compiler_thread:
        if (CompilerThreadStackSize > 0) {
          stack_size = (size_t)(CompilerThreadStackSize * K);
          break;
        } // else fall through:
          // use VMThreadStackSize if CompilerThreadStackSize is not defined
      case os::vm_thread:
      case os::pgc_thread:
      case os::cgc_thread:
      case os::watcher_thread:
        if (VMThreadStackSize > 0) stack_size = (size_t)(VMThreadStackSize * K);
        break;
      }
    }

    stack_size = MAX2(stack_size, os::Linux::min_stack_allowed);
    pthread_attr_setstacksize(&attr, stack_size);
  } else {
    // let pthread_create() pick the default value.
  }

  // glibc guard page
  pthread_attr_setguardsize(&attr, os::Linux::default_guard_size(thr_type));

  ThreadState state;

  {
    // Serialize thread creation if we are running with fixed stack LinuxThreads
    bool lock = os::Linux::is_LinuxThreads() && !os::Linux::is_floating_stack();
    if (lock) {
      os::Linux::createThread_lock()->lock_without_safepoint_check();
    }

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

    pthread_attr_destroy(&attr);

    if (ret != 0) {
      if (PrintMiscellaneous && (Verbose || WizardMode)) {
        perror("pthread_create()");
      }
      // Need to clean up stuff we've allocated so far
      thread->set_osthread(NULL);
      delete osthread;
      if (lock) os::Linux::createThread_lock()->unlock();
      return false;
    }

    // Store pthread info into the OSThread
    osthread->set_pthread_id(tid);
     ...
  }
   ...
  return true;
}
如果在new OSThread的过程中就失败了，那显然osthread为NULL，那再回到上面第一段代码，此时会抛出java.lang.OutOfMemoryError: unable to create new native thread的异常，而什么情况下new OSThread会失败，比如说内存不够了，而这里的内存其实是C Heap，而非Java Heap，由此可见从JVM的角度来说，影响线程创建的因素包括了Xmx，MaxPermSize，MaxDirectMemorySize，ReservedCodeCacheSize等，因为这些参数会影响剩余的内存
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