feature: Feature flag utility

.gitignore

@@ -1,6 +1,7 @@
 # Gradle
 .gradle/
 /build/
+buildSrc/build/
 
 # IDE
 .idea/

build.gradle.kts

@@ -10,7 +10,7 @@ plugins {
 
 
 group = "io.flamingock"
-val declaredVersion = "1.5.3-SNAPSHOT"
+val declaredVersion = "1.6.0-SNAPSHOT"
 version = VersionManager.resolveVersion(declaredVersion, project.hasProperty("release"))
 
 repositories {

src/main/java/io/flamingock/internal/util/FeatureFlag.java

@@ -0,0 +1,219 @@
+/*
+ * Copyright 2025 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.flamingock.internal.util;
+
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Supplier;
+
+/**
+ * Simple, internal, in-memory feature flag registry.
+ *
+ * <p>Flags are identified by name and are disabled unless explicitly enabled. There is no external
+ * configuration source: flags are only turned on and off from code, so they cannot be overridden by
+ * the user of the library.
+ *
+ * <p>Flags can be queried, to be used in a regular condition:
+ *
+ * <pre>{@code
+ * FeatureFlag.enable("my-feature");
+ *
+ * if (FeatureFlag.isEnabled("my-feature")) {
+ *     ...
+ * }
+ * }</pre>
+ *
+ * <p>or used to guard an action directly:
+ *
+ * <pre>{@code
+ * FeatureFlag.ifEnabled("my-feature", () -> service.doNewThing());
+ * FeatureFlag.ifEnabledOrElse("my-feature", () -> newWay(), () -> oldWay());
+ *
+ * String value = FeatureFlag.getIfEnabled("my-feature", () -> newValue(), "fallback");
+ * }</pre>
+ *
+ * <p>The state is global and shared by the whole JVM. This class is thread-safe.
+ */
+public final class FeatureFlag {
+
+    private static final Map<String, Boolean> FLAGS = new ConcurrentHashMap<>();
+
+    private FeatureFlag() {
+    }
+
+    /**
+     * Enables the given feature.
+     *
+     * @param feature feature name
+     * @return whether the feature was enabled before this call
+     */
+    public static boolean enable(String feature) {
+        return set(feature, true);
+    }
+
+    /**
+     * Disables the given feature.
+     *
+     * @param feature feature name
+     * @return whether the feature was enabled before this call
+     */
+    public static boolean disable(String feature) {
+        return set(feature, false);
+    }
+
+    /**
+     * Sets the state of the given feature.
+     *
+     * @param feature feature name
+     * @param enabled new state
+     * @return whether the feature was enabled before this call
+     */
+    public static boolean set(String feature, boolean enabled) {
+        return isEnabled(FLAGS.put(validateFeature(feature), enabled));
+    }
+
+    /**
+     * Removes the given feature from the registry, so it goes back to its default state(disabled).
+     *
+     * @param feature feature name
+     * @return whether the feature was enabled before this call
+     */
+    public static boolean remove(String feature) {
+        return isEnabled(FLAGS.remove(validateFeature(feature)));
+    }
+
+    /**
+     * Removes every feature from the registry. Mainly intended for tests.
+     */
+    public static void clear() {
+        FLAGS.clear();
+    }
+
+    /**
+     * Returns whether the given feature is enabled. Unknown features are considered disabled.
+     *
+     * @param feature feature name
+     * @return true if the feature is enabled
+     */
+    public static boolean isEnabled(String feature) {
+        return isEnabled(feature, false);
+    }
+
+    /**
+     * Returns whether the given feature is enabled, providing the value to be used when the feature
+     * hasn't been explicitly set.
+     *
+     * @param feature      feature name
+     * @param defaultValue value returned when the feature is not registered
+     * @return true if the feature is enabled
+     */
+    public static boolean isEnabled(String feature, boolean defaultValue) {
+        Boolean state = FLAGS.get(validateFeature(feature));
+        return state != null ? state : defaultValue;
+    }
+
+    /**
+     * Returns whether the given feature is disabled. Unknown features are considered disabled.
+     *
+     * @param feature feature name
+     * @return true if the feature is disabled
+     */
+    public static boolean isDisabled(String feature) {
+        return !isEnabled(feature);
+    }
+
+    /**
+     * Returns whether the given feature is disabled, providing the value to be used when the feature
+     * hasn't been explicitly set.
+     *
+     * @param feature      feature name
+     * @param defaultValue state assumed when the feature is not registered
+     * @return true if the feature is disabled
+     */
+    public static boolean isDisabled(String feature, boolean defaultValue) {
+        return !isEnabled(feature, defaultValue);
+    }
+
+    /**
+     * Runs the given action only if the feature is enabled.
+     *
+     * @param feature feature name
+     * @param action  action to run when the feature is enabled
+     */
+    public static void ifEnabled(String feature, Runnable action) {
+        Objects.requireNonNull(action, "action must not be null");
+        if (isEnabled(feature)) {
+            action.run();
+        }
+    }
+
+    /**
+     * Runs the given action only if the feature is disabled.
+     *
+     * @param feature feature name
+     * @param action  action to run when the feature is disabled
+     */
+    public static void ifDisabled(String feature, Runnable action) {
+        Objects.requireNonNull(action, "action must not be null");
+        if (isDisabled(feature)) {
+            action.run();
+        }
+    }
+
+    /**
+     * Runs one action or the other, depending on the state of the feature.
+     *
+     * @param feature       feature name
+     * @param action        action to run when the feature is enabled
+     * @param fallbackAction action to run when the feature is disabled
+     */
+    public static void ifEnabledOrElse(String feature, Runnable action, Runnable fallbackAction) {
+        Objects.requireNonNull(action, "action must not be null");
+        Objects.requireNonNull(fallbackAction, "fallbackAction must not be null");
+        if (isEnabled(feature)) {
+            action.run();
+        } else {
+            fallbackAction.run();
+        }
+    }
+
+    /**
+     * Returns the value provided by the supplier if the feature is enabled, or the fallback value
+     * otherwise. The supplier is only invoked when the feature is enabled.
+     *
+     * @param feature  feature name
+     * @param supplier supplier invoked when the feature is enabled
+     * @param fallback value returned when the feature is disabled
+     * @param <T>      returned type
+     * @return the supplied value or the fallback
+     */
+    public static <T> T getIfEnabled(String feature, Supplier<T> supplier, T fallback) {
+        Objects.requireNonNull(supplier, "supplier must not be null");
+        return isEnabled(feature) ? supplier.get() : fallback;
+    }
+
+    private static boolean isEnabled(Boolean state) {
+        return state != null && state;
+    }
+
+    private static String validateFeature(String feature) {
+        if (StringUtil.isEmpty(feature)) {
+            throw new IllegalArgumentException("feature must not be null or empty");
+        }
+        return feature;
+    }
+}