perf: simplify CacheApiSettings singleton creation (#316)

core/src/main/java/io/github/xanthic/cache/core/CacheApiSettings.java

@@ -23,8 +23,6 @@
  */
 @Slf4j
 public final class CacheApiSettings {
-	private static volatile CacheApiSettings INSTANCE;
-
 	private final Map<Class<? extends CacheProvider>, CacheProvider> providers = Collections.synchronizedMap(new IdentityHashMap<>(16));
 	private final AtomicReference<Class<? extends CacheProvider>> defaultCacheProvider = new AtomicReference<>();
 
@@ -38,7 +36,7 @@ public final class CacheApiSettings {
 	private volatile MisconfigurationPolicy defaultMisconfigurationPolicy = MisconfigurationPolicy.IGNORE;
 
 	private CacheApiSettings() {
-		// restrict instantiation
+		this.populateProviders();
 	}
 
 	/**
@@ -102,31 +100,14 @@ public void registerCacheProvider(@NonNull Class<? extends CacheProvider> cacheP
 		}
 	}
 
-	/**
-	 * @return the cache api settings singleton
-	 */
-	@NotNull
-	public static CacheApiSettings getInstance() {
-		if (INSTANCE == null) {
-			synchronized (CacheApiSettings.class) {
-				if (INSTANCE == null) {
-					CacheApiSettings cacheApiSettings = new CacheApiSettings();
-					populateProviders(cacheApiSettings);
-					INSTANCE = cacheApiSettings;
-				}
-			}
-		}
-		return INSTANCE;
-	}
-
-	private static void populateProviders(CacheApiSettings cacheApiSettings) {
+	private void populateProviders() {
 		log.debug("Xanthic: Registering canonical cache providers from the classpath...");
 
 		AtomicInteger registered = new AtomicInteger();
 		Consumer<String> loadImpl = (providerClass) -> {
 			try {
 				Class<? extends CacheProvider> clazz = Class.forName(providerClass).asSubclass(CacheProvider.class);
-				cacheApiSettings.registerCacheProvider(clazz, null); // lazy, init if needed
+				registerCacheProvider(clazz, null); // lazy, init if needed
 				registered.incrementAndGet();
 			} catch (ClassNotFoundException cx) {
 				log.trace("Xanthic: Could not find optional cache provider " + providerClass);
@@ -148,4 +129,16 @@ private static void populateProviders(CacheApiSettings cacheApiSettings) {
 
 		log.debug("Xanthic: Loaded {} canonical cache provider(s) on settings construction!", registered.get());
 	}
+
+	/**
+	 * @return the cache api settings singleton
+	 */
+	@NotNull
+	public static CacheApiSettings getInstance() {
+		return SingletonHolder.INSTANCE;
+	}
+
+	private static class SingletonHolder {
+		private static final CacheApiSettings INSTANCE = new CacheApiSettings();
+	}
 }

core/src/test/java/io/github/xanthic/cache/core/CacheRegistrationTest.java

@@ -12,7 +12,10 @@
 import org.junit.jupiter.api.Test;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.Method;
 import java.time.Duration;
+import java.util.Map;
+import java.util.concurrent.atomic.AtomicReference;
 
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
@@ -23,10 +26,19 @@ public class CacheRegistrationTest {
 	@SneakyThrows
 	void beforeEachTest() {
 		// reset cache settings singleton
-		Field instanceField = CacheApiSettings.class.getDeclaredField("INSTANCE");
-		instanceField.setAccessible(true);
-		instanceField.set(null, null);
-		CacheApiSettings.getInstance();
+		CacheApiSettings instance = CacheApiSettings.getInstance();
+
+		Field providers = CacheApiSettings.class.getDeclaredField("providers");
+		providers.setAccessible(true);
+		((Map<?, ?>) providers.get(instance)).clear();
+
+		Field defaultProvider = CacheApiSettings.class.getDeclaredField("defaultCacheProvider");
+		defaultProvider.setAccessible(true);
+		((AtomicReference<?>) defaultProvider.get(instance)).set(null);
+
+		Method populate = CacheApiSettings.class.getDeclaredMethod("populateProviders");
+		populate.setAccessible(true);
+		populate.invoke(instance);
 	}
 
 	@Test