看了《深入理解java虚拟机》java与线程一章提到了linux提供的线程模型是一对一的。我也写过一段linux c，当时开辟多线程也就是调用了pthread_create的库函数。
linux c 线程函数
int pthread_create(pthread_t *tid,const pthread_attr_t *attr,
(void*)(*start_rtn)(void*),void *arg);
tid就是线程标识
attr是线程属性
start_rtn是函数指针，就是线程运行的函数
arg是函数的参数
这个函数的运用比较简单，就是把你线程要执行的函数的指针传递给第三个参数，把函数的参数用第四个参数传入。
我的问题也就来了，java的线程执行的内容是写在run方法的，那么c的线程是如何调用java的方法的。最开始我以为是jni做的，就看了看openjdk的源码来验证。
openjdk的实现
java线程是通过start的方法启动执行的，主要内容在native方法start0中。
openjdk的写jni一般是一一对应的，Thread.java对应的就是Thread.c。
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},
};
start0其实就是JVM_StartThread。此时通过search and replace工具查找JVM_StartThread关键字，在jvm.h中找到了声明，jvm.cpp中有实现。
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
JVMWrapper("JVM_StartThread");
JavaThread *native_thread = NULL;
bool throw_illegal_thread_state = false;
// We must release the Threads_lock before we can post a jvmti event
// in Thread::start.
{
// Ensure that the C++ Thread and OSThread structures aren't freed before
// we operate.
MutexLocker mu(Threads_lock);
if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
throw_illegal_thread_state = true;
} else {
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);
……
……
大概浏览上下文，native_thread的构造应该是在JavaThread的构造中完成的，此处代码就展示到构造JavaThread处。
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);
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;
}
在构造函数中做了几件事，设置entry_point，设置stack_size,create_thread。entry_point在后面讲，这里需要知道此处有这么一个点。我们继续看线程的实现。线程的实现基本是基于系统库函数的（c++11才有的统一线程库）。具体实现在os_linux.cpp中（这里我们只说linux的，其他系统的实现就不在这里了）。
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
……
pthread_t tid;
int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
……
｝
这里终于看到了系统库函数的调用。线程执行的方法是java_start，参数是thread。
// Thread start routine for all newly created threads
static void *java_start(Thread *thread) {
// Try to randomize the cache line index of hot stack frames.
// This helps when threads of the same stack traces evict each other's
// cache lines. The threads can be either from the same JVM instance, or
// from different JVM instances. The benefit is especially true for
// processors with hyperthreading technology.
static int counter = 0;
int pid = os::current_process_id();
alloca(((pid ^ counter++) & 7) * 128);
ThreadLocalStorage::set_thread(thread);
OSThread* osthread = thread->osthread();
Monitor* sync = osthread->startThread_lock();
// non floating stack LinuxThreads needs extra check, see above
if (!_thread_safety_check(thread)) {
// notify parent thread
MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);
osthread->set_state(ZOMBIE);
sync->notify_all();
return NULL;
}
// thread_id is kernel thread id (similar to Solaris LWP id)
osthread->set_thread_id(os::Linux::gettid());
if (UseNUMA) {
int lgrp_id = os::numa_get_group_id();
if (lgrp_id != -1) {
thread->set_lgrp_id(lgrp_id);
}
}
// initialize signal mask for this thread
os::Linux::hotspot_sigmask(thread);
// initialize floating point control register
os::Linux::init_thread_fpu_state();
// handshaking with parent thread
{
MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);
// notify parent thread
osthread->set_state(INITIALIZED);
sync->notify_all();
// wait until os::start_thread()
while (osthread->get_state() == INITIALIZED) {
sync->wait(Mutex::_no_safepoint_check_flag);
}
}
// call one more level start routine
thread->run();
return 0;
}
这个方法可以直接跳到最后一行，他执行的是传递过来对象的run方法，大家还记得前面构造的JavaThread对象吧。
// 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();
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();
}
thread_main_inner();
}
这里的代码需要大概过一下，我们看到方法的开头初始化很多信息，例如TLAB，TLS 等。真正执行的代码在thread_main_inner中。
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;
}
这个方法看到最后都发现没有执行run方法吧，这也是我第一次跟踪代码走完很奇特的地方，完全没说run方法的事情。 this->entry_point()(this, this);其实就是调用run方法的。上面我特意说记着entry_point这个东西。这个在构造JavaThread传入的，是一个名字叫thread_entry的函数。
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);
}
里面有JavaCalls的过程，传入的就是 vmSymbols::run_method_name(),就是run方法。具体call_virtual怎么调用的。请参考https://www.zhihu.com/question/64677339/answer/223307974我也是参考了这里回答才明白的。
总结
jvm在linux中线程的实现就是调用的pthread库，但是线程执行的内容，却是解释执行的（就是上面说的call_virtual的过程），并不是jni的反调。我的理解就是每个c的线程都在解释执行java的run方法，从而达到了java的多线程效果。