当java虚拟机启动的时候，会启动很多内部的线程，这些线程主要在thread.cpp里的create_vm方法体里实现
而在thread.cpp里主要起了2个线程来处理信号相关的
JvmtiExport::enter_live_phase();// Signal Dispatcher needs to be started before VMInit event is postedos::signal_init();// Start Attach Listener if +StartAttachListener or it can't be started lazilyif (!DisableAttachMechanism) {if (StartAttachListener || AttachListener::init_at_startup()) {AttachListener::init();}}
1. Signal Dispatcher 线程
在os.cpp中的signal_init()函数中，启动了signal dispatcher 线程，对signal dispather 线程主要是用于处理信号，等待信号并且分发处理，可以详细看signal_thread_entry的方法
static void signal_thread_entry(JavaThread* thread, TRAPS) {os::set_priority(thread, NearMaxPriority);while (true) {int sig;{// FIXME : Currently we have not decieded what should be the status// for this java thread blocked here. Once we decide about// that we should fix this.sig = os::signal_wait();}if (sig == os::sigexitnum_pd()) {// Terminate the signal threadreturn;}switch (sig) {case SIGBREAK: {// Check if the signal is a trigger to start the Attach Listener - in that// case don't print stack traces.if (!DisableAttachMechanism && AttachListener::is_init_trigger()) {continue;}// Print stack traces// Any SIGBREAK operations added here should make sure to flush// the output stream (e.g. tty->flush()) after output. See 4803766.// Each module also prints an extra carriage return after its output.VM_PrintThreads op;VMThread::execute(&op);VM_PrintJNI jni_op;VMThread::execute(&jni_op);VM_FindDeadlocks op1(tty);VMThread::execute(&op1);Universe::print_heap_at_SIGBREAK();if (PrintClassHistogram) {VM_GC_HeapInspection op1(gclog_or_tty, true /* force full GC before heap inspection */,true /* need_prologue */);VMThread::execute(&op1);}if (JvmtiExport::should_post_data_dump()) {JvmtiExport::post_data_dump();}break;}default: {// Dispatch the signal to javaHandleMark hm(THREAD);klassOop k = SystemDictionary::resolve_or_null(vmSymbolHandles::sun_misc_Signal(), THREAD);KlassHandle klass (THREAD, k);if (klass.not_null()) {JavaValue result(T_VOID);JavaCallArguments args;args.push_int(sig);JavaCalls::call_static(&result,klass,vmSymbolHandles::dispatch_name(),vmSymbolHandles::int_void_signature(),&args,THREAD);}if (HAS_PENDING_EXCEPTION) {// tty is initialized early so we don't expect it to be null, but// if it is we can't risk doing an initialization that might// trigger additional out-of-memory conditionsif (tty != NULL) {char klass_name[256];char tmp_sig_name[16];const char* sig_name = "UNKNOWN";instanceKlass::cast(PENDING_EXCEPTION->klass())->name()->as_klass_external_name(klass_name, 256);if (os::exception_name(sig, tmp_sig_name, 16) != NULL)sig_name = tmp_sig_name;warning("Exception %s occurred dispatching signal %s to handler""- the VM may need to be forcibly terminated",klass_name, sig_name );}CLEAR_PENDING_EXCEPTION;}}}}
}
可以看到通过os::signal_wait();等待信号，而在linux里是通过sem_wait()来实现，接受到SIGBREAK(linux 中的QUIT)信号的时候（关于信号处理请参考笔者的另一篇博客:java 中关于信号的处理在linux下的实现），第一次通过调用 AttachListener::is_init_trigger（）初始化attach listener线程，详细见2.Attach Listener 线程。
- 第一次收到信号，会开始初始化，当初始化成功，将会直接返回，而且不返回任何线程stack的信息（通过socket file的操作返回），并且第二次将不在需要初始化。如果初始化不成功，将直接在控制台的outputstream中打印线程栈信息。
- 第二次收到信号，如果已经初始化过，将直接在控制台中打印线程的栈信息。如果没有初始化，继续初始化，走和第一次相同的流程。
2. Attach Listener 线程
Attach Listener 线程是负责接收到外部的命令，而对该命令进行执行的并且吧结果返回给发送者。在jvm启动的时候，如果没有指定+StartAttachListener，该线程是不会启动的，刚才我们讨论到了在接受到quit信号之后，会调用 AttachListener::is_init_trigger（）通过调用用AttachListener::init（）启动了Attach Listener 线程，同时在不同的操作系统下初始化，在linux中 是在attachListener_Linux.cpp文件中实现的。
在linux中如果发现文件.attach_pid#pid存在，才会启动attach listener线程，同时初始化了socket 文件，也就是通常jmap,jstack tool干的事情，先创立attach_pid#pid文件，然后发quit信号,通过这种方式暗式的启动了Attach Listener线程（见博客：http://blog.csdn.net/raintungli/article/details/7023092）。
线程的实现在 attach_listener_thread_entry 方法体中实现
static void attach_listener_thread_entry(JavaThread* thread, TRAPS) {os::set_priority(thread, NearMaxPriority);if (AttachListener::pd_init() != 0) {return;}AttachListener::set_initialized();for (;;) {AttachOperation* op = AttachListener::dequeue(); if (op == NULL) {return; // dequeue failed or shutdown}ResourceMark rm;bufferedStream st;jint res = JNI_OK;// handle special detachall operationif (strcmp(op->name(), AttachOperation::detachall_operation_name()) == 0) {AttachListener::detachall();} else {// find the function to dispatch tooAttachOperationFunctionInfo* info = NULL;for (int i=0; funcs[i].name != NULL; i++) {const char* name = funcs[i].name;assert(strlen(name) <= AttachOperation::name_length_max, "operation <= name_length_max");if (strcmp(op->name(), name) == 0) {info = &(funcs[i]);break;}}// check for platform dependent attach operationif (info == NULL) {info = AttachListener::pd_find_operation(op->name());}if (info != NULL) {// dispatch to the function that implements this operationres = (info->func)(op, &st);} else {st.print("Operation %s not recognized!", op->name());res = JNI_ERR;}}// operation complete - send result and output to clientop->complete(res, &st);}
}在AttachListener::dequeue(); 在liunx里的实现就是监听刚才创建的socket的文件，如果有请求进来，找到请求对应的操作，调用操作得到结果并把结果写到这个socket的文件，如果你把socket的文件删除，jstack/jmap会出现错误信息 unable to open socket file:........
我们经常使用 kill -3 pid的操作打印出线程栈信息，我们可以看到具体的实现是在Signal Dispatcher 线程中完成的，因为kill -3 pid 并不会创建.attach_pid#pid文件，所以一直初始化不成功，从而线程的栈信息被打印到控制台中。