Java并发系列-深入Jvm理解Thread启动流程

前言

近期整理笔记想开个专题，准备从并发入手。并发这块又从哪里入手，一开始想的是AQS，偶然间看到自己之前编译调试openjdk的时候整理的一些笔记，又有了新的想法，决定先从最基础的开始，并发这块脱离不了线程，那么我们就结合jdk,hotspot探究一下线程的来龙去脉。
线程的定义：程序运行的最小单元，被包含在进程中。

Java中的线程

class Thread implements Runnable {
/* Make sure registerNatives is the first thing <clinit> does. */
private static native void registerNatives();
static {
registerNatives();
}
...//省略一大波代码，和一些英文注释
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) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}

通常我们使用线程都是调用的start()方法；
从上面的Thread的源码结构我们可以看到start()方法
先判断是否是NEW状态，如果不是，抛出线程状态异常；
接下来执行start0()；这是个本地方法

Jvm中定义的线程

下面我们打开openjdk查看一下jdk中本地方法的定义，打开Thread.c

#include "jni.h"
#include "jvm.h"
#include "java_lang_Thread.h"
#define THD "Ljava/lang/Thread;"
#define OBJ "Ljava/lang/Object;"
#define STE "Ljava/lang/StackTraceElement;"
#define STR "Ljava/lang/String;"
#define ARRAY_LENGTH(a) (sizeof(a)/sizeof(a[0]))
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},
};
#undef THD
#undef OBJ
#undef STE
#undef STR
JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
(*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}

以上代码是与JNI注册相关的，很简单，有必要的话后面出一篇介绍一下；
下面我们找到start0对应的jvm中的方法，JVM_StartThread
在jvm.cpp中

JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
JVMWrapper("JVM_StartThread");
JavaThread *native_thread = NULL;
// Heap_lock while we construct the exception.
bool throw_illegal_thread_state = false;
{
MutexLocker mu(Threads_lock);
if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
throw_illegal_thread_state = true;
} else {
// 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));
size_t sz = size > 0 ? (size_t) size : 0;
native_thread = new JavaThread(&thread_entry, sz);
if (native_thread->osthread() != NULL) {
// Note: the current thread is not being used within "prepare".
native_thread->prepare(jthread);
}
}
}
if (throw_illegal_thread_state) {
THROW(vmSymbols::java_lang_IllegalThreadStateException());
}
assert(native_thread != NULL, "Starting null thread?");
if (native_thread->osthread() == NULL) {
// No one should hold a reference to the 'native_thread'.
delete native_thread;
if (JvmtiExport::should_post_resource_exhausted()) {
JvmtiExport::post_resource_exhausted(
JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_THREADS,
"unable to create new native thread");
}
THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(),
"unable to create new native thread");
}
Thread::start(native_thread);
JVM_END

以上是主要逻辑，我们接下来一步步分析

首先是获取互斥锁

MutexLocker mu(Threads_lock);

检查线程状态，确保是未启动状态

 if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
throw_illegal_thread_state = true;

上一步检查通过，则创建与jvm中的JavaThread，主要是通过构造函数来实现

native_thread = new JavaThread(&thread_entry, sz);

看看这个构造函数的主要内容

JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) :
Thread()
...//省略
{
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);

我们看到最后一行 os::create_thread(this, thr_type, stack_sz);这个才是真正映射到系统层面，创建线程。

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);
......//省略
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);

创建os级别的线程，与JavaThread进行绑定，接下来划重点

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

java_start就是新创建的线程启动入口，它会等待一个信号来调用Java中的run()方法，
jvm中具体的实现逻辑代码如下

// 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()

接下来回到JVM_StartThread方法，线程创建好了，调用native_thread->prepare(jthread);将Java 中的Thread和Jvm中的Thread进行绑定，此前还有一步跟系统级Thread绑定。

最后真正启动线程

Thread::start(native_thread);

同样也是逐层调用到os级别线程，具体逻辑如下

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);
}

至此，我们真正的逻辑就被系统线程调用了，当然其中还有些细节。通过这种方式我们能整体上把握线程的在Java层面，Jvm层面，Os层面的关系，好了本章就算给后续并发系列文章做个引子，在后续的文章中还会继续聊并发这块的知识。

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