Add shutdown hook

Add shutdown hook to Runtime which will call EngineWrapper.close.  This should help ensure Engine.close is called (and therefore license is released) in some situations where finally block may be subverted, for example if System.exit is called within user's Consumer<Utilities> accept method.

Author: JuicyDragon

Java/src/main/java/com/nuix/javaenginesimple/EngineWrapper.java

@@ -26,7 +26,7 @@
  * @author Jason Wells
  *
  */
-public class EngineWrapper {
+public class EngineWrapper implements AutoCloseable {
 	// Obtain a logger instance for this class
 	private final static Logger logger = Logger.getLogger("EngineWrapper");
 
@@ -35,6 +35,8 @@ public class EngineWrapper {
 	private File nuixBaseDirectory = null;
 	private Engine engine = null;
 
+	private Thread shutdownHook = null;
+	
 	// We will use this while iterating licenses to determine which one to acquire.
 	// By default this filter accepts any license.
 	private LicenseFilter licenseFilter = new LicenseFilter();
@@ -55,6 +57,15 @@ public EngineWrapper(File nuixBaseDirectory){
 		File engineLibDirectory = new File(nuixBaseDirectory,"lib");
 		logger.info(String.format("Setting 'nuix.libdir' to: %s", engineLibDirectory.getAbsolutePath()));
 		System.setProperty("nuix.libdir", engineLibDirectory.getAbsolutePath());
+		
+		// Whenever we create an instance of the engine to hand over to the user, we will register
+		// our shutdown hook to close this instance.  This helps to ensure that the license is released
+		// in some scenarios where the finally block may not execute, such as code calling System.exit before
+		// returning from user provided Consumer<Utilities>.
+		shutdownHook = new Thread(() -> {
+			try { close(); } catch (Exception e)
+			{ logger.error("Error in shutdown hook",e); }
+		});
 	}
 
 	/***
@@ -101,6 +112,11 @@ public void withDongleLicense(Consumer<Utilities> consumer) throws Exception{
 					if(licenseObtained){
 						Utilities utilities = engine.getUtilities();
 						ThirdPartyDependencyChecker.logAllDependencyInfo(utilities);
+						
+						// Setup our shutdown hook in case user terminates before consumer returns
+						logger.info("Adding shutdown hook for EngineWrapper::close");
+						Runtime.getRuntime().addShutdownHook(shutdownHook);
+						
 						logger.info("License was obtained, providing Utilities object to consumer...");
 						consumer.accept(utilities);
 					}
@@ -112,10 +128,7 @@ public void withDongleLicense(Consumer<Utilities> consumer) throws Exception{
 				logger.error("Error while creating Engine instance",engineException);
 				throw engineException;
 			} finally {
-				if(engine != null){
-					logger.info("Closing Engine instance...");
-					engine.close();
-				}
+				close();
 			}
 		} catch (Exception globalContainerException) {
 			logger.error("Error while creating GlobalContainer",globalContainerException);
@@ -209,6 +222,11 @@ public void execute(CredentialsCallbackInfo info) {
 					if(licenseObtained){
 						Utilities utilities = engine.getUtilities();
 						ThirdPartyDependencyChecker.logAllDependencyInfo(utilities);
+						
+						// Setup our shutdown hook in case user terminates before consumer returns
+						logger.info("Adding shutdown hook for EngineWrapper::close");
+						Runtime.getRuntime().addShutdownHook(shutdownHook);
+						
 						logger.info("License was obtained, providing Utilities object to consumer...");
 						consumer.accept(utilities);
 					} else {
@@ -222,10 +240,7 @@ public void execute(CredentialsCallbackInfo info) {
 				logger.error("Error while creating Engine instance",engineException);
 				throw engineException;
 			} finally {
-				if(engine != null){
-					logger.info("Closing Engine instance...");
-					engine.close();
-				}
+				close();
 			}
 		} catch (Exception globalContainerException) {
 			logger.error("Error while creating GlobalContainer",globalContainerException);
@@ -302,6 +317,11 @@ public void execute(CredentialsCallbackInfo info) {
 					if(licenseObtained){
 						Utilities utilities = engine.getUtilities();
 						ThirdPartyDependencyChecker.logAllDependencyInfo(utilities);
+
+						// Setup our shutdown hook in case user terminates before consumer returns
+						logger.info("Adding shutdown hook for EngineWrapper::close");
+						Runtime.getRuntime().addShutdownHook(shutdownHook);
+						
 						logger.info("License was obtained, providing Utilities object to consumer...");
 						consumer.accept(utilities);
 					} else {
@@ -315,10 +335,7 @@ public void execute(CredentialsCallbackInfo info) {
 				logger.error("Error while creating Engine instance",engineException);
 				throw engineException;
 			} finally {
-				if(engine != null){
-					logger.info("Closing Engine instance...");
-					engine.close();
-				}
+				close();
 			}
 		} catch (Exception globalContainerException) {
 			logger.error("Error while creating GlobalContainer",globalContainerException);
@@ -477,4 +494,16 @@ public LicenseFilter getLicenseFilter() {
 	public void setLicenseFilter(LicenseFilter licenseFilter) {
 		this.licenseFilter = licenseFilter;
 	}
+
+	@Override
+	public void close() throws Exception {
+		if(engine != null) {
+			logger.info("Closing engine instance");
+			engine.close();
+		}
+		
+		// Remove our shutdown hook since engine has now been closed
+		logger.info("Removing shutdown hook to EngineWrapper::close");
+		Runtime.getRuntime().removeShutdownHook(shutdownHook);
+	}
 }