Singletone ObjectMapper (#71)

* Use a Singleton for the ObjectMapper

* cleanup up and add JavaDoc

* add test

* final class for the singleton

Author: jenspapenhagen

src/main/java/dev/toonformat/jtoon/decoder/ValueDecoder.java

@@ -1,13 +1,10 @@
 package dev.toonformat.jtoon.decoder;
 
-import com.fasterxml.jackson.annotation.JsonInclude;
 import dev.toonformat.jtoon.DecodeOptions;
+import dev.toonformat.jtoon.util.ObjectMapperSingleton;
 import tools.jackson.databind.ObjectMapper;
-import tools.jackson.databind.json.JsonMapper;
-import tools.jackson.module.afterburner.AfterburnerModule;
 
 import java.util.LinkedHashMap;
-import java.util.TimeZone;
 import java.util.regex.Matcher;
 
 import static dev.toonformat.jtoon.util.Headers.KEYED_ARRAY_PATTERN;
@@ -33,17 +30,7 @@
  */
 public final class ValueDecoder {
 
-    private static final ObjectMapper OBJECT_MAPPER;
-
-    static {
-        OBJECT_MAPPER = JsonMapper.builder()
-            .changeDefaultPropertyInclusion(incl -> incl.withValueInclusion(JsonInclude.Include.ALWAYS))
-            .addModule(new AfterburnerModule().setUseValueClassLoader(true)) // Speeds up Jackson by 20–40% in most real-world cases
-            // .disable(MapperFeature.DEFAULT_VIEW_INCLUSION) in Jackson 3 this is default disabled
-            // .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES) in Jackson 3 this is default disabled
-            .defaultTimeZone(TimeZone.getTimeZone("UTC")) // set a default timezone for dates
-            .build();
-    }
+    private static final ObjectMapper MAPPER = ObjectMapperSingleton.getInstance();
 
     private ValueDecoder() {
         throw new UnsupportedOperationException("Utility class cannot be instantiated");
@@ -132,7 +119,7 @@ public static Object decode(String toon, DecodeOptions options) {
     public static String decodeToJson(String toon, DecodeOptions options) {
         try {
             Object decoded = decode(toon, options);
-            return OBJECT_MAPPER.writeValueAsString(decoded);
+            return MAPPER.writeValueAsString(decoded);
         } catch (Exception e) {
             throw new IllegalArgumentException("Failed to convert decoded value to JSON: " + e.getMessage(), e);
         }

src/main/java/dev/toonformat/jtoon/normalizer/JsonNormalizer.java

@@ -1,9 +1,8 @@
 package dev.toonformat.jtoon.normalizer;
 
-import com.fasterxml.jackson.annotation.JsonInclude;
+import dev.toonformat.jtoon.util.ObjectMapperSingleton;
 import tools.jackson.databind.JsonNode;
 import tools.jackson.databind.ObjectMapper;
-import tools.jackson.databind.json.JsonMapper;
 import tools.jackson.databind.node.ArrayNode;
 import tools.jackson.databind.node.BooleanNode;
 import tools.jackson.databind.node.DecimalNode;
@@ -15,7 +14,6 @@
 import tools.jackson.databind.node.ObjectNode;
 import tools.jackson.databind.node.ShortNode;
 import tools.jackson.databind.node.StringNode;
-import tools.jackson.module.afterburner.AfterburnerModule;
 
 import java.math.BigDecimal;
 import java.math.BigInteger;
@@ -33,7 +31,6 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
-import java.util.TimeZone;
 import java.util.function.Function;
 import java.util.function.IntFunction;
 import java.util.stream.Stream;
@@ -43,20 +40,11 @@
  * Handles Java-specific types like LocalDateTime, Optional, Stream, etc.
  */
 public final class JsonNormalizer {
+
     /**
      * Shared ObjectMapper instance configured for JSON normalization.
      */
-    public static final ObjectMapper MAPPER;
-
-    static {
-        MAPPER = JsonMapper.builder()
-            .changeDefaultPropertyInclusion(incl -> incl.withValueInclusion(JsonInclude.Include.ALWAYS))
-            .addModule(new AfterburnerModule().setUseValueClassLoader(true)) // Speeds up Jackson by 20–40% in most real-world cases
-            // .disable(MapperFeature.DEFAULT_VIEW_INCLUSION) in Jackson 3 this is default disabled
-            // .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES) in Jackson 3 this is default disabled
-            .defaultTimeZone(TimeZone.getTimeZone("UTC")) // set a default timezone for dates
-            .build();
-    }
+    public static final ObjectMapper MAPPER = ObjectMapperSingleton.getInstance();
 
     private static final List<Function<Object, JsonNode>> NORMALIZERS = List.of(
         JsonNormalizer::tryNormalizePrimitive,
@@ -69,6 +57,7 @@ private JsonNormalizer() {
         throw new UnsupportedOperationException("Utility class cannot be instantiated");
     }
 
+
     /**
      * Parses a JSON string into a JsonNode using the shared ObjectMapper.
      * <p>

src/main/java/dev/toonformat/jtoon/util/ObjectMapperSingleton.java

@@ -0,0 +1,41 @@
+package dev.toonformat.jtoon.util;
+
+import com.fasterxml.jackson.annotation.JsonInclude;
+import tools.jackson.databind.ObjectMapper;
+import tools.jackson.databind.json.JsonMapper;
+import tools.jackson.module.afterburner.AfterburnerModule;
+
+import java.util.TimeZone;
+
+/**
+ * Provides a singleton ObjectMapper instance.
+ */
+public final class ObjectMapperSingleton {
+    /**
+     * Holds the singleton ObjectMapper.
+     */
+    private static ObjectMapper INSTANCE;
+
+    private ObjectMapperSingleton() {
+        throw new UnsupportedOperationException("Utility class cannot be instantiated");
+    }
+
+    /**
+     * Returns the singleton ObjectMapper.
+     *
+     * @return ObjectMapper
+     */
+    public static ObjectMapper getInstance() {
+        if (INSTANCE == null) {
+            INSTANCE = JsonMapper.builder()
+                .changeDefaultPropertyInclusion(incl -> incl.withValueInclusion(JsonInclude.Include.ALWAYS))
+                .addModule(new AfterburnerModule()) // Speeds up Jackson by 20–40% in most real-world cases
+                // .disable(MapperFeature.DEFAULT_VIEW_INCLUSION) in Jackson 3 this is default disabled
+                // .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES) in Jackson 3 this is default disabled
+                // .configure(SerializationFeature.INDENT_OUTPUT, false)  in Jackson 3 this is default false
+                .defaultTimeZone(TimeZone.getTimeZone("UTC")) // set a default timezone for dates
+                .build();
+        }
+        return INSTANCE;
+    }
+}

src/test/java/dev/toonformat/jtoon/util/ObjectMapperSingletonTest.java

@@ -0,0 +1,33 @@
+package dev.toonformat.jtoon.util;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Tag;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+/**
+ * JUnit 5 test class for ObjectMapperSingleton.
+ */
+@Tag("unit")
+class ObjectMapperSingletonTest {
+
+    @Test
+    @DisplayName("throws unsupported Operation Exception for calling the constructor")
+    void throwsOnConstructor() throws NoSuchMethodException {
+        final Constructor<ObjectMapperSingleton> constructor = ObjectMapperSingleton.class.getDeclaredConstructor();
+        constructor.setAccessible(true);
+
+        final InvocationTargetException thrown =
+            assertThrows(InvocationTargetException.class, constructor::newInstance);
+
+        final Throwable cause = thrown.getCause();
+        assertInstanceOf(UnsupportedOperationException.class, cause);
+        assertEquals("Utility class cannot be instantiated", cause.getMessage());
+    }
+}