java 中关于自定义信号在linux下的实现

在java 中调用Signal的方法handle可以去注册一个信号的处理函数，方法的如下:

public static synchronized SignalHandler handle(Signal sig, SignalHandler handler) { .... }
比如常用的addShutdownHook钩子函数里，就是在Terminator.setup();的时候将Shutdown.exit 的函数注册到了信号 SHUTDOWN1_SIGNAL(SIGHUP)， SHUTDOWN2_SIGNAL(SIGINT)， SHUTDOWN3_SIGNAL(SIGTERM)中，当线程接受到信号时，通过调用函数Shutdown.exit的调用hook中的钩子函数。

在笔者的文章（java 中关于信号的处理在linux下的实现）也提到jdk如何处理信号的，那么调用handle里是不是直接就把这个方法注册进了信号处理呢？

请注意，handle是一个java的方法,而注册信号函数是c的代码，显然不能简单的将java的方法直接提供给c调用，其次信号处理函数是在内核态中处理，安全性和执行时间的长短将影响到内核的信号调度。

先看java源码,如下面所示

public static synchronized SignalHandler handle(Signal sig, SignalHandler handler) throws IllegalArgumentException { long newH = (handler instanceof NativeSignalHandler) ? ((NativeSignalHandler)handler).getHandler() : 2; long oldH = handle0(sig.number, newH); if (oldH == -1) { throw new IllegalArgumentException ("Signal already used by VM: " + sig); } signals.put(new Integer(sig.number), sig); synchronized (handlers) { SignalHandler oldHandler = (SignalHandler)handlers.get(sig); handlers.remove(sig); if (newH == 2) { handlers.put(sig, handler); } if (oldH == 0) { return SignalHandler.SIG_DFL; } else if (oldH == 1) { return SignalHandler.SIG_IGN; } else if (oldH == 2) { return oldHandler; } else { return new NativeSignalHandler(oldH); } } }在native code hand0里并没有将handle的方法传进去，只是传了一个整型值。

在c++代码中hand0里调用了函数JVM_RegisterSignal，具体来看一下这个函数的实现

JVM_ENTRY_NO_ENV(void*, JVM_RegisterSignal(jint sig, void* handler)) // Copied from classic vm // signals_md.c 1.4 98/08/23 void* newHandler = handler == (void *)2 ? os::user_handler() : handler; switch (sig) { /* The following are already used by the VM. */ case INTERRUPT_SIGNAL: case SIGFPE: case SIGILL: case SIGSEGV: /* The following signal is used by the VM to dump thread stacks unless ReduceSignalUsage is set, in which case the user is allowed to set his own _native_ handler for this signal; thus, in either case, we do not allow JVM_RegisterSignal to change the handler. */ case BREAK_SIGNAL: return (void *)-1; /* The following signals are used for Shutdown Hooks support. However, if ReduceSignalUsage (-Xrs) is set, Shutdown Hooks must be invoked via System.exit(), Java is not allowed to use these signals, and the the user is allowed to set his own _native_ handler for these signals and invoke System.exit() as needed. Terminator.setup() is avoiding registration of these signals when -Xrs is present. - If the HUP signal is ignored (from the nohup) command, then Java is not allowed to use this signal. */ case SHUTDOWN1_SIGNAL: case SHUTDOWN2_SIGNAL: case SHUTDOWN3_SIGNAL: if (ReduceSignalUsage) return (void*)-1; if (os::Linux::is_sig_ignored(sig)) return (void*)1; } void* oldHandler = os::signal(sig, newHandler); if (oldHandler == os::user_handler()) { return (void *)2; } else { return oldHandler; } JVM_END
void* newHandler = handler == (void *)2 ? os::user_handler() : handler;因为传进的值是2,那么真正在c++里的信号处理函数实际上os::user_handler(),同时jvm也保护了几个信号，不允许外部改变信号的处理函数。

一切豁然开朗，笔者的博客（java 中关于信号的处理在linux下的实现）已经提到过这个函数，通过os:signal_notify 去通知signal dispatcher 线程的os::signal_wait，也就是接受到信号的线程通过信号函数notify到处理信号的线程（signal dispatcher ），最后由该线程做后续的事情。

具体来看signal dispatcher 的thread entry

static void signal_thread_entry(JavaThread* thread, TRAPS) { .... default: { // Dispatch the signal to java HandleMark 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 ); } .... }
也就是在jvm的c++源码中，反调用了java的方法,也就是signal.java中的dispatch(int number)，方法dispatch中才是真正的调用了在文章开头提到的注册到Signal的方法handle。

dispatch方法中仍然起了一个新的线程去处理handle,这样就不会blocksignal dispatcher 线程。