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package sun.misc;
import java.util.Hashtable;

This class provides ANSI/ISO C signal support. A Java program can register signal handlers for the current process. There are two restrictions:
  • Java code cannot register a handler for signals that are already used by the Java VM implementation. The Signal.handle function raises an IllegalArgumentException if such an attempt is made.
  • When Signal.handle is called, the VM internally registers a special C signal handler. There is no way to force the Java signal handler to run synchronously before the C signal handler returns. Instead, when the VM receives a signal, the special C signal handler creates a new thread (at priority Thread.MAX_PRIORITY) to run the registered Java signal handler. The C signal handler immediately returns. Note that because the Java signal handler runs in a newly created thread, it may not actually be executed until some time after the C signal handler returns.

Signal objects are created based on their names. For example:

new Signal("INT");
constructs a signal object corresponding to SIGINT, which is typically produced when the user presses Ctrl-C at the command line. The Signal constructor throws IllegalArgumentException when it is passed an unknown signal.

This is an example of how Java code handles SIGINT:

SignalHandler handler = new SignalHandler () {
    public void handle(Signal sig) {
      ... // handle SIGINT
    }
};
Signal.handle(new Signal("INT"), handler);
Author: Sheng Liang, Bill Shannon
See Also:
Since: 1.2
/** * This class provides ANSI/ISO C signal support. A Java program can register * signal handlers for the current process. There are two restrictions: * <ul> * <li> * Java code cannot register a handler for signals that are already used * by the Java VM implementation. The <code>Signal.handle</code> * function raises an <code>IllegalArgumentException</code> if such an attempt * is made. * <li> * When <code>Signal.handle</code> is called, the VM internally registers a * special C signal handler. There is no way to force the Java signal handler * to run synchronously before the C signal handler returns. Instead, when the * VM receives a signal, the special C signal handler creates a new thread * (at priority <code>Thread.MAX_PRIORITY</code>) to * run the registered Java signal handler. The C signal handler immediately * returns. Note that because the Java signal handler runs in a newly created * thread, it may not actually be executed until some time after the C signal * handler returns. * </ul> * <p> * Signal objects are created based on their names. For example: * <blockquote><pre> * new Signal("INT"); * </blockquote></pre> * constructs a signal object corresponding to <code>SIGINT</code>, which is * typically produced when the user presses <code>Ctrl-C</code> at the command line. * The <code>Signal</code> constructor throws <code>IllegalArgumentException</code> * when it is passed an unknown signal. * <p> * This is an example of how Java code handles <code>SIGINT</code>: * <blockquote><pre> * SignalHandler handler = new SignalHandler () { * public void handle(Signal sig) { * ... // handle SIGINT * } * }; * Signal.handle(new Signal("INT"), handler); * </blockquote></pre> * * @author Sheng Liang * @author Bill Shannon * @see sun.misc.SignalHandler * @since 1.2 */
public final class Signal { private static Hashtable<Signal,SignalHandler> handlers = new Hashtable<>(4); private static Hashtable<Integer,Signal> signals = new Hashtable<>(4); private int number; private String name; /* Returns the signal number */ public int getNumber() { return number; }
Returns the signal name.
See Also:
Returns:the name of the signal.
/** * Returns the signal name. * * @return the name of the signal. * @see sun.misc.Signal#Signal(String name) */
public String getName() { return name; }
Compares the equality of two Signal objects.
Params:
  • other – the object to compare with.
Returns:whether two Signal objects are equal.
/** * Compares the equality of two <code>Signal</code> objects. * * @param other the object to compare with. * @return whether two <code>Signal</code> objects are equal. */
public boolean equals(Object other) { if (this == other) { return true; } if (other == null || !(other instanceof Signal)) { return false; } Signal other1 = (Signal)other; return name.equals(other1.name) && (number == other1.number); }
Returns a hashcode for this Signal.
Returns: a hash code value for this object.
/** * Returns a hashcode for this Signal. * * @return a hash code value for this object. */
public int hashCode() { return number; }
Returns a string representation of this signal. For example, "SIGINT" for an object constructed using new Signal ("INT").
Returns:a string representation of the signal
/** * Returns a string representation of this signal. For example, "SIGINT" * for an object constructed using <code>new Signal ("INT")</code>. * * @return a string representation of the signal */
public String toString() { return "SIG" + name; }
Constructs a signal from its name.
Params:
  • name – the name of the signal.
Throws:
See Also:
/** * Constructs a signal from its name. * * @param name the name of the signal. * @exception IllegalArgumentException unknown signal * @see sun.misc.Signal#getName() */
public Signal(String name) { number = findSignal(name); this.name = name; if (number < 0) { throw new IllegalArgumentException("Unknown signal: " + name); } }
Registers a signal handler.
Params:
  • sig – a signal
  • handler – the handler to be registered with the given signal.
Throws:
See Also:
@resultthe old handler
/** * Registers a signal handler. * * @param sig a signal * @param handler the handler to be registered with the given signal. * @result the old handler * @exception IllegalArgumentException the signal is in use by the VM * @see sun.misc.Signal#raise(Signal sig) * @see sun.misc.SignalHandler * @see sun.misc.SignalHandler#SIG_DFL * @see sun.misc.SignalHandler#SIG_IGN */
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 or OS: " + sig); } signals.put(sig.number, sig); synchronized (handlers) { SignalHandler oldHandler = 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); } } }
Raises a signal in the current process.
Params:
  • sig – a signal
See Also:
/** * Raises a signal in the current process. * * @param sig a signal * @see sun.misc.Signal#handle(Signal sig, SignalHandler handler) */
public static void raise(Signal sig) throws IllegalArgumentException { if (handlers.get(sig) == null) { throw new IllegalArgumentException("Unhandled signal: " + sig); } raise0(sig.number); } /* Called by the VM to execute Java signal handlers. */ private static void dispatch(final int number) { final Signal sig = signals.get(number); final SignalHandler handler = handlers.get(sig); Runnable runnable = new Runnable () { public void run() { // Don't bother to reset the priority. Signal handler will // run at maximum priority inherited from the VM signal // dispatch thread. // Thread.currentThread().setPriority(Thread.NORM_PRIORITY); handler.handle(sig); } }; if (handler != null) { new Thread(runnable, sig + " handler").start(); } } /* Find the signal number, given a name. Returns -1 for unknown signals. */ private static native int findSignal(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 */ private static native long handle0(int sig, long nativeH); /* Raise a given signal number */ private static native void raise0(int sig); }