Java 中关于自定义信号在Linux下的实现

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;