RFR 9: 8087286 Need a way to handle control-C and possibly some other signals

Peter Levart peter.levart at gmail.com
Tue Feb 2 11:44:04 UTC 2016

Hi Roger,

Is this public API supposed to replace sun.misc.Signal? I don't see why 
not. Comparing the APIs, thought they are different in style, I can't 
find a feature of sun.misc.Signal that wouldn't be supported by 
java.util.Signal except for NativeSignalHandler which in my 
understanding is currently only used to implement the 
SignalHandler.SIG_DFL and SignalHandler.SIG_IGN special "native" 
handlers. Is there any use of NativeSignalHandler to dispatch to 
arbitrary native procedure?

As to the implementation: Registering signal handlers with this API is 
protected by a runtime permission. Nevertheless, since this is public 
API and handlers are supplied by user code, should dispatching to the 
handlers be performed in a specially prepared Thread with no 
permissions? Current dispatch implementation constructs new Thread with 
constructor that uses the AccessController.getContext() inherited from 
the thread that calls the Signal.dispatch() method which is a special 
"Signal Dispatcher" thread spawned in the VM (os::signal_init). I 
haven't dug any deeper, but is it possible that "Signal Dispatcher" 
thread uses system protection domain?

As to the API. Is registerDefault() really needed to be exposed to the 
public? I can see it is currently used to install default non-native 
handler(s) for TERM, INT, HUP signals in the boot-up sequence in 
j.l.System. Do you want users to be able to override these default 
handlers? Also the new implementation only sets the defaultConsumer 
field which is used only when some normal handler is unregister-ed, so 
this does not work correctly...

Also if this is to become public API, There's a chance users would want 
to add a handler to the chain of existing handlers or override them. So 
what about an API that allows registering/unregistering a default 
(non-native) handler and other handlers above it in a uniform way, like:

public final class Signal {

     private static final ConcurrentMap<String, Signal>
         SIGNAL_BY_NAME = new ConcurrentHashMap<>(4);
     private static final ConcurrentMap<Integer, Signal>
         SIGNAL_BY_NUMBER = new ConcurrentHashMap<>(4);

     public static Signal of(String name) {
         Signal signal = SIGNAL_BY_NAME.get(name);
         if (signal != null) {
             return signal;

         int number;
         if (!name.startsWith("SIG") || name.length() <= 3 ||
             (number = findSignal0(name.substring(3))) < 0) {
             throw new UnsupportedOperationException("Unknown signal: " 
+ name);

         signal = SIGNAL_BY_NUMBER.computeIfAbsent(
             new Function<Integer, Signal>() {
                 public Signal apply(Integer number) {
                     return new Signal(name, number);

         SIGNAL_BY_NAME.putIfAbsent(name, signal);

         return signal;

     private final String name;
     private final int number;
     private volatile HandlerChain handlerChain;
     private long savedNativeHandler;

     private Signal(String name, int number) {
         this.name = name;
         this.number = number;

     public String name() { return name; }

     public int number() { return number; }

     public void raise() { raise0(number); }

     public void register(BiConsumer<Signal, Runnable> handler) {
         synchronized (this) {
             HandlerChain oldChain = handlerChain;
             handlerChain = new HandlerChain(handler, oldChain);
             if (oldChain == null) {
                 // set native to dispatch to Singnal.dispatch()
                 savedNativeHandler = handle1(2);

     public boolean unregister(BiConsumer<Signal, Runnable> handler) {
         synchronized (this) {
             HandlerChain oldChain = handlerChain;
             if (oldChain != null && oldChain.handler == handler) {
                 if (oldChain.next == null) {
                     // restore saved native handler
                     long oldNativeHandler = handle1(savedNativeHandler);
                     assert oldNativeHandler == 2L;
                 handlerChain = oldChain.next;
                 return true;
             } else {
                 return false;

     // following two should probably be hidden from public API

     public void nativeIgnore() {
         synchronized (this) {
             if (handlerChain == null) {
                 handle1(1); // ignore signal
             } else {
                 throw new IllegalStateException(
                     "Can't ignore signal after handlers have already 
been registered.");

     public void nativeDefault() {
         synchronized (this) {
             if (handlerChain == null) {
                 handle1(0); // default native handler
             } else {
                 throw new IllegalStateException(
                     "Can't restore signal after handlers have already 
been registered.");

     private long handle1(long nativeHandler) {
         long oldNativeHandler = handle0(number, nativeHandler);
         if (oldNativeHandler == -1L) {
             throw new UnsupportedOperationException(
                 "Signal already used by VM or OS: " + name);
         return oldNativeHandler;

      * Called by the VM to execute Java signal handlers.
     private static void dispatch(int number) {
         Signal signal = SIGNAL_BY_NUMBER.get(number);
         if (signal != null) {
             HandlerChain handlerChain = signal.handlerChain;
             if (handlerChain != null) {
                 new InnocuousThread(() -> handlerChain.accept(signal))

      * Find the signal number, given a name.
      * @param sigName the signal name
      * @return the signal number or -1 for unknown signals.
     private static native int findSignal0(String sigName);

     /* Registers a native signal handler, and returns the old handler.
      * Handler values:
      *   0     default handler
      *   1     ignore the signal
      *   2     call back to Signal.dispatch
      *   other arbitrary native signal handlers
      * @param nativeH the index or address of the new signal handler
      * @return the previous index or address
     private static native long handle0(int sig, long nativeH);

      * Raise a given signal number.
      * @param sig the signal number to raise
     private static native void raise0(int sig);

     private static class HandlerChain implements Consumer<Signal> {
         final BiConsumer<Signal, Runnable> handler;
         final HandlerChain next;

         HandlerChain(BiConsumer<Signal, Runnable> handler, HandlerChain 
next) {
             this.handler = handler;
             this.next = next;

         public void accept(Signal signal) {
             handler.accept(signal, () -> {
                 if (next != null) {

Regards, Peter

On 02/01/2016 05:02 PM, Roger Riggs wrote:
> Please review an API addition to handle signals such as SIGINT, 
> This JEP 260 motivated alternative to sun.misc.Signal supports the use 
> case for
> interactive applications that need to handle Control-C and other signals.
> The new java.util.Signal class provides a settable primary signal 
> handler and a default
> signal handler.  The primary signal handler can be unregistered and 
> handling is restored
> to the default signal handler.  System initialization registers 
> default signal handlers
> to terminate on SIGINT, SIGHUP, and SIGTERM.  Use of the Signal API 
> requires
> a permission if a SecurityManager is set.
> The sun.misc.Signal implementation is modified to be layered on a common
> thread and dispatch mechanism. The VM handling of native signals is 
> not affected.
> The command option to reduce signal use by the runtime with -Xrs is 
> unmodified.
> The changes to hotspot are minimal to rename the hardcoded callback to 
> the Java
> Signal dispatcher.
> Please review and comment on the API and implementation.
> javadoc:
>   http://cr.openjdk.java.net/~rriggs/signal-doc/
> Webrev:
> jdk:  http://cr.openjdk.java.net/~rriggs/webrev-signal-8087286/
> hotspot: http://cr.openjdk.java.net/~rriggs/webrev-hs-signal-8087286/
> Issue:
>    https://bugs.openjdk.java.net/browse/JDK-8087286
> JEP 260:
>   https://bugs.openjdk.java.net/browse/JDK-8132928
> Thanks, Roger

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