在前面的博客中（http://blog.csdn.net/raintungli/article/details/7034005）所提到的信号转发线程，Attach Listener 线程都只是操作socket文件，并没有去执行比如stack 分析，或者heap的分析，真正的工作线程其实是vm thread.
（一）启动vm thread
jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
...
// Create the VMThread
{ TraceTime timer("Start VMThread", TraceStartupTime);
VMThread::create();
Thread* vmthread = VMThread::vm_thread();
if (!os::create_thread(vmthread, os::vm_thread))
vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");
// Wait for the VM thread to become ready, and VMThread::run to initialize
// Monitors can have spurious returns, must always check another state flag
{
MutexLocker ml(Notify_lock);
os::start_thread(vmthread);
while (vmthread->active_handles() == NULL) {
Notify_lock->wait();
}
}
}
...
}我们可以看到，在thread.cpp里启动了线程vm thread，在这里我们同时也稍微的略带的讲一下jvm在linux里如何启动线程的。
通常在linux中启动线程，是调用
int pthread_create((pthread_t *__thread, __const pthread_attr_t *__attr,void *(*__start_routine) (void *), void *__arg));
而在java里却增加了os:create_thread --初始化线程 和os:start_thread--启动线程
我们去看一下jvm里面是如何在linux里做到的
在os_linux.cpp中来看create_thread的方法
bool os::create_thread(Thread* thread, ThreadType thr_type, size_t stack_size) {
....
int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
....
}继续看java_start方法
static void *java_start(Thread *thread) {
....
// 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;
}
首先jvm先设置了当前线程的状态是Initialized, 然后notify所有的线程，
while (osthread->get_state() == INITIALIZED) {
sync->wait(Mutex::_no_safepoint_check_flag);
}
不停的查看线程的当前状态是不是Initialized, 如果是的话，调用了sync->wait()的方法等待。
来看os:start_thread的方法 os.cpp
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);
}这时候设置了线程的状态为runnable,但没有notify线程
在 pd_start_thread(thread)中, os_linux.cpp中
void os::pd_start_thread(Thread* thread) {
OSThread * osthread = thread->osthread();
assert(osthread->get_state() != INITIALIZED, "just checking");
Monitor* sync_with_child = osthread->startThread_lock();
MutexLockerEx ml(sync_with_child, Mutex::_no_safepoint_check_flag);
sync_with_child->notify();
}
这时候我们看到了notify 线程的操作
也就是这时候notify了线程，因为这时候的线程的状态是RUNNABLE, 方法java_start继续往下执行，于是调用了thread->run()的方法
对于线程vm Thread 也就是调用了vmthread::run方法
vmThread.cpp
void VMThread::run() {
...
this->loop();
...
}调用了loop函数，处理了VM_Operation 的queue 关于queue的级别和优先级处理算法：可以参考 另一篇博客：http://blog.csdn.net/raintungli/article/details/6553337
（二）Jstack 运行在vm thread里的VM_Operation
jstack 处理也就是在前面博客所提到的attach Listener 线程所做的 operation
static jint thread_dump(AttachOperation* op, outputStream* out) {
bool print_concurrent_locks = false;
if (op->arg(0) != NULL && strcmp(op->arg(0), "-l") == 0) {
print_concurrent_locks = true;
}
// thread stacks
VM_PrintThreads op1(out, print_concurrent_locks);
VMThread::execute(&op1);
// JNI global handles
VM_PrintJNI op2(out);
VMThread::execute(&op2);
// Deadlock detection
VM_FindDeadlocks op3(out);
VMThread::execute(&op3);
return JNI_OK;
}简单看一下类VM_PrintThreads 它 继承了VM_Operation
class VM_PrintThreads: public VM_Operation {
private:
outputStream* _out;
bool _print_concurrent_locks;
public:
VM_PrintThreads() { _out = tty; _print_concurrent_locks = PrintConcurrentLocks; }
VM_PrintThreads(outputStream* out, bool print_concurrent_locks) { _out = out; _print_concurrent_locks = print_concurrent_locks; }
VMOp_Type type() const { return VMOp_PrintThreads; }
void doit();
bool doit_prologue();
void doit_epilogue();
};当调用VMThread::execute()也就是将VM_PrintThreads 放入了_vm_queue中，交给vm thread 处理，对vm thread来说取出queue里的VM_Operation,并且调用doit方法。
在jstack里，attach listener 的线程产生了VM_PrintThreads，VM_PrintJNI，VM_FindDeadlocks 3个operations，交给了vm thread 的线程处理。