Add shutdown hook functionality (broken, #6)

Doesn't really work right now, will likely be removed

base/src/main/java/de/staropensource/sosengine/base/Engine.java

@@ -32,6 +32,7 @@ import de.staropensource.sosengine.base.exceptions.dependency.UnmetDependenciesE
 import de.staropensource.sosengine.base.internal.events.InternalEngineShutdownEvent;
 import de.staropensource.sosengine.base.internal.types.DependencySubsystemVector;
 import de.staropensource.sosengine.base.logging.CrashHandler;
+import de.staropensource.sosengine.base.logging.InitLogger;
 import de.staropensource.sosengine.base.logging.Logger;
 import de.staropensource.sosengine.base.logging.LoggerInstance;
 import de.staropensource.sosengine.base.types.DependencyVector;
@@ -40,6 +41,7 @@ import de.staropensource.sosengine.base.types.immutable.ImmutableLinkedList;
 import de.staropensource.sosengine.base.utility.DependencyResolver;
 import de.staropensource.sosengine.base.utility.Miscellaneous;
 import de.staropensource.sosengine.base.utility.PlaceholderEngine;
+import lombok.AccessLevel;
 import lombok.Getter;
 import lombok.Setter;
 import org.jetbrains.annotations.NotNull;
@@ -107,6 +109,21 @@ public final class Engine extends SubsystemClass {
     @Getter
     private @NotNull EngineState state = EngineState.UNKNOWN;
 
+    /**
+     * Contains a list of all registered subsystems.
+     * The list is sorted after initialization order.
+     *
+     * @since v1-alpha1
+     * -- GETTER --
+     * Returns a list of all registered subsystems.
+     * The list is sorted after initialization order.
+     *
+     * @return subsystem list
+     * @since v1-alpha1
+     */
+    @Getter
+    private @NotNull ImmutableLinkedList<@NotNull DependencySubsystemVector> subsystems = new ImmutableLinkedList<>();
+
     /**
      * Contains the engine's shutdown handler.
      * The shutdown handler is responsible for
@@ -136,19 +153,39 @@ public final class Engine extends SubsystemClass {
     private @NotNull ShutdownHandler shutdownHandler = new Engine.JvmShutdownHandler();
 
     /**
-     * Contains a list of all registered subsystems.
-     * The list is sorted after initialization order.
+     * Contains the JVM shutdown hook thread,
+     * which ensures that the engine is fully shut
+     * down before the JVM exits.
      *
-     * @since v1-alpha1
-     * -- GETTER --
-     * Returns a list of all registered subsystems.
-     * The list is sorted after initialization order.
-     *
-     * @return subsystem list
-     * @since v1-alpha1
+     * @see EngineInternals#installSafetyShutdownHook(boolean)
+     * @since v1-alpha4
      */
-    @Getter
-    private @NotNull ImmutableLinkedList<@NotNull DependencySubsystemVector> subsystems = new ImmutableLinkedList<>();
+    @Getter(AccessLevel.MODULE)
+    private final @NotNull Thread safetyShutdownHook = Thread.ofPlatform()
+            .name("Engine shutdown thread")
+            .group(getThreadGroup())
+            .unstarted(() -> {
+                // To avoid race condition or something
+                try {
+                    Thread.sleep(1000);
+                } catch (InterruptedException ignored) {}
+
+                // Check if already shutting down
+                switch (state) {
+                    case UNKNOWN, SHUTDOWN, CRASHED -> {
+                        return;
+                    }
+                }
+
+                // Print warning about shutdown
+                logger.warn("Trying to shut down engine using shutdown hook.\nThis approach to shutting down the engine and JVM is NOT RECOMMENDED, please use Engine#shutdown() instead.");
+
+                // Shutdown
+                Engine.getInstance().shutdown();
+
+                // Print last message
+                InitLogger.warn(getClass(), "ENGINE", EngineInformation.getVersioningCodename(), "Engine successfully shut down using shutdown hook. PLEASE USE Engine#shutdown() INSTEAD OF System#exit() or Runtime#exit()!");
+            });
 
     /**
      * Initializes the StarOpenSource Engine.
@@ -170,8 +207,9 @@ public final class Engine extends SubsystemClass {
 
             logger.info("Initializing engine");
             initializeClasses();         // Initialize classes
-            if (checkEnvironment())
+            if (checkEnvironment())      // Check environment
                 throw new IllegalStateException("Running in an incompatible environment");
+            ensureEnvironment();         // Prepare the environment and ensure safety
             populateCrashContent();      // Populate crash content
             cacheEvents();               // Cache event listeners
             startThreads();              // Start threads
@@ -197,6 +235,19 @@ public final class Engine extends SubsystemClass {
         logger.info("Initialized sos!engine %engine_version% (commit %engine_git_commit_id_long%-%engine_git_branch%, dirty %engine_git_dirty%) in " + initTime + "ms");
     }
 
+    /**
+     * Initializes all classes.
+     *
+     * @since v1-alpha0
+     */
+    private void initializeClasses() {
+        logger.verb("Initializing engine classes");
+        new EngineInternals();
+        new PlaceholderEngine();
+
+        EngineInformation.update();
+    }
+
     /**
      * Checks if the environment is compatible with the engine build.
      *
@@ -212,15 +263,12 @@ public final class Engine extends SubsystemClass {
     }
 
     /**
-     * Initializes all classes.
+     * Ensures the execution safety of the environment.
      *
-     * @since v1-alpha0
+     * @since v1-alpha4
      */
-    private void initializeClasses() {
-        logger.verb("Initializing engine classes");
-        new PlaceholderEngine();
-
-        EngineInformation.update();
+    private void ensureEnvironment() {
+        EngineInternals.getInstance().installSafetyShutdownHook(true);
     }
 
     /**
@@ -412,6 +460,12 @@ public final class Engine extends SubsystemClass {
      * @since v1-alpha0
      */
     public synchronized void shutdown(@Range(from = 0, to = 255) int exitCode) {
+        switch (state) {
+            case UNKNOWN, SHUTDOWN, CRASHED -> {
+                return;
+            }
+        }
+
         logger.info("Shutting engine down");
         if (state != EngineState.CRASHED)
             state = EngineState.SHUTDOWN;
@@ -425,12 +479,19 @@ public final class Engine extends SubsystemClass {
         // Flush log messages
         Logger.flushLogMessages();
 
+        // Disable safety shutdown hook
+        try {
+            Runtime.getRuntime().removeShutdownHook(safetyShutdownHook);
+        } catch (Exception ignored) {}
+
+        // Send events
         logger.verb("Notifiying classes about shutdown");
         new EngineShutdownEvent().callEvent();
 
         logger.verb("Notifying subsystems about shutdown");
         new InternalEngineShutdownEvent().callEvent();
 
+        // Invoke shutdown handler
         logger.verb("Invoking shutdown handler (code " + exitCode + ")");
         shutdownHandler.shutdown((short) exitCode);
     }
@@ -497,8 +558,18 @@ public final class Engine extends SubsystemClass {
         /** {@inheritDoc} */
         @Override
         public void shutdown(short exitCode) {
-            logger.warn("Terminating JVM");
+            // Check if already shutting down
+            try {
+                Thread thread = Thread.ofVirtual().unstarted(() -> {});
 
+                Runtime.getRuntime().addShutdownHook(thread);
+                Runtime.getRuntime().removeShutdownHook(thread);
+            } catch (IllegalStateException exception) {
+                logger.warn("Terminating JVM: Already shutting down, skipping");
+                return;
+            }
+
+            logger.warn("Terminating JVM");
             System.exit(exitCode);
         }
     }

base/src/main/java/de/staropensource/sosengine/base/EngineInternals.java

@@ -0,0 +1,117 @@
+/*
+ * STAROPENSOURCE ENGINE SOURCE FILE
+ * Copyright (c) 2024 The StarOpenSource Engine Contributors
+ * Licensed under the GNU Affero General Public License v3
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.staropensource.sosengine.base;
+
+import de.staropensource.sosengine.base.exceptions.NoAccessException;
+import de.staropensource.sosengine.base.logging.LoggerInstance;
+import de.staropensource.sosengine.base.types.InternalAccessArea;
+import lombok.Getter;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Class which allows access to the internals
+ * and changing the behaviour of the engine.
+ *
+ * @since v1-alpha4
+ */
+@SuppressWarnings({ "JavadocDeclaration" })
+public final class EngineInternals {
+    /**
+     * Contains the class instance.
+     *
+     * @since v1-alpha4
+     * -- GETTER --
+     * Returns the class instance.
+     *
+     * @return class instance unless the engine is uninitialized
+     * @since v1-alpha4
+     */
+    @Getter
+    private static EngineInternals instance;
+
+    /**
+     * Contains the {@link LoggerInstance} for this instance.
+     *
+     * @see LoggerInstance
+     * @since v1-alpha4
+     */
+    private static final LoggerInstance logger = new LoggerInstance.Builder().setClazz(EngineInternals.class).setOrigin("ENGINE").build();
+
+    /**
+     * Contains all disabled internal access areas.
+     *
+     * @since v1-alpha4
+     */
+    @Getter
+    private final @NotNull List<@NotNull InternalAccessArea> restrictedAreas = new ArrayList<>();
+
+    /**
+     * Constructs this class.
+     *
+     * @since v1-alpha4
+     */
+     public EngineInternals() {
+         // Only allow one instance
+         if (instance == null && Engine.getInstance() != null)
+             instance = this;
+         else
+             logger.crash("Only one instance of this class is allowed, use getInstance() instead of creating a new instance");
+    }
+
+    /**
+     * Allows disabling internal engine access.
+     *
+     * @since v1-alpha4
+     */
+    public void restrictAccess(@NotNull InternalAccessArea area) {
+        if (area == InternalAccessArea.ALL) {
+            List<@NotNull InternalAccessArea> areas = new ArrayList<>(List.of(InternalAccessArea.values()));
+            areas.remove(InternalAccessArea.ALL);
+            restrictedAreas.addAll(areas);
+        }
+
+        restrictedAreas.add(area);
+    }
+
+    /**
+     * Installs or uninstalls the JVM shutdown
+     * hook, which prevents the JVM from exiting
+     * before the engine has fully shut down.
+     * Highly recommended to keep enabled.
+     *
+     * @param status {@code true} to install, {@code false} otherwise
+     * @throws NoAccessException when restricted
+     * @since v1-alpha4
+     */
+    public void installSafetyShutdownHook(boolean status) throws NoAccessException {
+        if (restrictedAreas.contains(InternalAccessArea.SAFETY_SHUTDOWN_HOOK))
+            throw new NoAccessException("The internal access area SAFETY_SHUTDOWN_HOOK has been restricted");
+
+        try {
+            if (status)
+                Runtime.getRuntime().addShutdownHook(Engine.getInstance().getSafetyShutdownHook());
+            else
+                Runtime.getRuntime().removeShutdownHook(Engine.getInstance().getSafetyShutdownHook());
+        } catch (IllegalArgumentException | IllegalStateException ignored) {}
+    }
+}

base/src/main/java/de/staropensource/sosengine/base/logging/Logger.java

@@ -200,6 +200,9 @@ public final class Logger {
         if (format == null)
             format = EngineConfiguration.getInstance().getLoggerTemplate();
 
+        // Replace placeholders using PlaceholderEngine
+        format = PlaceholderEngine.getInstance().process(format);
+
         // Replace logger placeholders (no colors)
         format = new LogClass(issuerClass).replace(format);
         format = new de.staropensource.sosengine.base.internal.placeholders.logger.LogLevel(level).replace(format);
@@ -256,7 +259,7 @@ public final class Logger {
         format = new LogColorPrimary(level).replace(format);
         format = new LogColorSecondary(level).replace(format);
 
-        // Replace placeholders using PlaceholderEngine
+        // Replace placeholders using PlaceholderEngine again
         format = PlaceholderEngine.getInstance().process(format);
 
         // Invoke LoggerImpl#postPlaceholder

base/src/main/java/de/staropensource/sosengine/base/types/InternalAccessArea.java

@@ -0,0 +1,46 @@
+/*
+ * STAROPENSOURCE ENGINE SOURCE FILE
+ * Copyright (c) 2024 The StarOpenSource Engine Contributors
+ * Licensed under the GNU Affero General Public License v3
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package de.staropensource.sosengine.base.types;
+
+import de.staropensource.sosengine.base.EngineInternals;
+
+/**
+ * Specifies multiple areas of internal engine access.
+ * Used for restricting access of internals.
+ *
+ * @see EngineInternals#restrictAccess(InternalAccessArea)
+ * @since v1-alpha4
+ */
+public enum InternalAccessArea {
+    /**
+     * Refers to all areas.
+     *
+     * @since v1-alpha4
+     */
+    ALL,
+
+    /**
+     * Refers to the toggling of the JVM shutdown hook, which
+     * prevents JVM shutdowns without the engine first shutting down.
+     *
+     * @since v1-alpha4
+     */
+    SAFETY_SHUTDOWN_HOOK,
+}