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