Java 工具（jmap,jstack）在Linux上的源碼分析（三）執行的線程vm thread

在前面的文章中（http://www.linuxidc.com/Linux/2012-01/51213.htm）所提到的信號轉發線程，Attach Listener 線程都只是操作socket文件，並沒有去執行比如stack 分析，或者heap的分析，真正的工作線程其實是vm thread.

（一）啟動vm thread

1. jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
2. ...
3. // Create the VMThread
4. { TraceTime timer("Start VMThread", TraceStartupTime);
5. VMThread::create();
6. Thread* vmthread = VMThread::vm_thread();
8. if (!os::create_thread(vmthread, os::vm_thread))
9. vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");
11. // Wait for the VM thread to become ready, and VMThread::run to initialize
12. // Monitors can have spurious returns, must always check another state flag
13. {
14. MutexLocker ml(Notify_lock);
15. os::start_thread(vmthread);
16. while (vmthread->active_handles() == NULL) {
17. Notify_lock->wait();
18. }
19. }
20. }
21. ...
24. }

我們可以看到，在thread.cpp裡啟動了線程vm thread，在這裡我們同時也稍微的略帶的講一下jvm在linux裡如何啟動線程的。

通常在linux中啟動線程，是調用

1. int pthread_create((pthread_t *__thread, __const pthread_attr_t *__attr,void *(*__start_routine) (void *), void *__arg));

而在java裡卻增加了os:create_thread --初始化線程 和os:start_thread--啟動線程

我們去看一下jvm裡面是如何在linux裡做到的

在os_linux.cpp中來看create_thread的方法

1. bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
2. ....
3. int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
4. ....
5. }

繼續看java_start方法

1. static void *java_start(Thread *thread) {
2. ....
3. // handshaking with parent thread
4. {
5. MutexLockerEx ml(sync, Mutex::_no_safepoint_check_flag);
7. // notify parent thread
8. osthread->set_state(INITIALIZED);
9. sync->notify_all();
11. // wait until os::start_thread()
12. while (osthread->get_state() == INITIALIZED) {
13. sync->wait(Mutex::_no_safepoint_check_flag);
14. }
15. }
17. // call one more level start routine
18. thread->run();
20. return 0;
21. }

首先jvm先設置了當前線程的狀態是Initialized, 然後notify所有的線程，

while (osthread->get_state() == INITIALIZED) {
sync->wait(Mutex::_no_safepoint_check_flag);
}

不停的查看線程的當前狀態是不是Initialized, 如果是的話，調用了sync->wait()的方法等待。

來看os:start_thread的方法 os.cpp

1. void os::start_thread(Thread* thread) {
2. // guard suspend/resume
3. MutexLockerEx ml(thread->SR_lock(), Mutex::_no_safepoint_check_flag);
4. OSThread* osthread = thread->osthread();
5. osthread->set_state(RUNNABLE);
6. pd_start_thread(thread);
7. }