Handling signals in Java

Signals by definition are specific to the underlying implementation ( read, operating system) .

Java, being a platform independent language , it is often discouraged to write platform specific stuff except for the rarest cases and when there is a clear business justification for the same.

Java does have an undocumented API that talks about signals where we can override the default signal handler for a given signal.  Scenarios where this might be useful and necessary are when we try to release resources ( connections etc.).

Note: When we override default signal handlers, it is important to delegate the behavior to the default handler implementation (Hint: Save the old handler when overriding the same) after we finish the current implementation.

package experiment;

import java.util.HashMap;
import java.util.Map;
import java.util.logging.Logger;

import sun.misc.Signal;
import sun.misc.SignalHandler;

public class CustomSignalHandler implements SignalHandler
{

  private static final Logger               LOGGER   = Logger.getLogger(CustomSignalHandler.class.getName());

  private static Map<Signal, SignalHandler> handlers = new HashMap<Signal, SignalHandler>();


  @Override
  public void handle(Signal signal)
  {
    LOGGER.info("received " + signal);

    // Delegate to the existing handler after handling necessary clean-up.
    handlers.get(signal).handle(signal);
  }


  /**
   * Important: This API is not portable but heavily platform dependent as the signal name depends
   * on the underying operating system. 

   * 
   * @param signalName
   * @param signalHandler
   */
  public static void delegateHandler(final String signalName,
                                     final SignalHandler signalHandler)
  {
    try { 
      Signal signal = new Signal(signalName);
      SignalHandler oldhandler = Signal.handle(signal, signalHandler);
    } finally { 
      handlers.put(signal, oldhandler);
    } 
  }


  public static void main(String[] args)
  {
    final int LONG_TIME = 50000;

    SignalHandler example = new CustomSignalHandler();
    delegateHandler("TERM", example);
    delegateHandler("INT", example);
    delegateHandler("ABRT", example);

    try
    {
      Thread.sleep(LONG_TIME);
    }
    catch (InterruptedException ie)
    {
      ie.printStackTrace();
    }
  }
}