feat: Add JPMS compatibility to Jackson 2 JSON mapper

Configure ObjectMapper with JPMS-compatible settings:
- Disable CAN_OVERRIDE_ACCESS_MODIFIERS to prevent setAccessible() calls
- Add ParameterNamesModule to discover constructor parameter names from
  bytecode instead of reflection

This allows applications using the MCP SDK to operate without
`--add-opens` JVM flags, enabling full JPMS module encapsulation.

The SDK already compiles with `-parameters` flag, which is required
for ParameterNamesModule to function.

Signed-off-by: Nicholas Walter Knize <nknize@apache.org>

Author: nknize

mcp-json-jackson2/pom.xml

@@ -44,6 +44,11 @@
             <artifactId>jackson-databind</artifactId>
             <version>${jackson2.version}</version>
         </dependency>
+        <dependency>
+            <groupId>com.fasterxml.jackson.module</groupId>
+            <artifactId>jackson-module-parameter-names</artifactId>
+            <version>${jackson2.version}</version>
+        </dependency>
         <dependency>
             <groupId>com.networknt</groupId>
             <artifactId>json-schema-validator</artifactId>

mcp-json-jackson2/src/main/java/io/modelcontextprotocol/json/jackson2/JacksonMcpJsonMapperSupplier.java

@@ -4,6 +4,11 @@
 
 package io.modelcontextprotocol.json.jackson2;
 
+import com.fasterxml.jackson.databind.MapperFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.json.JsonMapper;
+import com.fasterxml.jackson.module.paramnames.ParameterNamesModule;
+
 import io.modelcontextprotocol.json.McpJsonMapper;
 import io.modelcontextprotocol.json.McpJsonMapperSupplier;
 
@@ -12,21 +17,41 @@
  * serialization and deserialization.
  * <p>
  * This implementation provides a {@link McpJsonMapper} backed by a Jackson
- * {@link com.fasterxml.jackson.databind.ObjectMapper}.
+ * {@link ObjectMapper} configured for JPMS (Java Platform Module System) compatibility.
  */
 public class JacksonMcpJsonMapperSupplier implements McpJsonMapperSupplier {
 
 	/**
 	 * Returns a new instance of {@link McpJsonMapper} that uses the Jackson library for
 	 * JSON serialization and deserialization.
 	 * <p>
-	 * The returned {@link McpJsonMapper} is backed by a new instance of
-	 * {@link com.fasterxml.jackson.databind.ObjectMapper}.
+	 * The returned {@link McpJsonMapper} is backed by a JPMS-compatible
+	 * {@link ObjectMapper} that does not require {@code --add-opens} JVM flags.
 	 * @return a new {@link McpJsonMapper} instance
 	 */
 	@Override
 	public McpJsonMapper get() {
-		return new JacksonMcpJsonMapper(new com.fasterxml.jackson.databind.ObjectMapper());
+		return new JacksonMcpJsonMapper(createJpmsCompatibleMapper());
+	}
+
+	/**
+	 * Creates an ObjectMapper configured for JPMS compatibility.
+	 * <p>
+	 * The mapper is configured to:
+	 * <ul>
+	 * <li>Not call {@code setAccessible()} on constructors/fields, avoiding the need for
+	 * {@code --add-opens} flags</li>
+	 * <li>Use the {@link ParameterNamesModule} to discover constructor parameter names
+	 * from bytecode (requires {@code -parameters} compiler flag, which is already
+	 * configured in the parent pom.xml)</li>
+	 * </ul>
+	 * @return a JPMS-compatible ObjectMapper
+	 */
+	private static ObjectMapper createJpmsCompatibleMapper() {
+		return JsonMapper.builder()
+			.disable(MapperFeature.CAN_OVERRIDE_ACCESS_MODIFIERS)
+			.addModule(new ParameterNamesModule())
+			.build();
 	}
 
 }

mcp-json-jackson2/src/test/java/io/modelcontextprotocol/json/jackson2/JpmsCompatibilityTests.java

@@ -0,0 +1,146 @@
+/*
+ * Copyright 2026 - 2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.json.jackson2;
+
+import com.fasterxml.jackson.databind.MapperFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.modelcontextprotocol.json.McpJsonMapper;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatNoException;
+
+/**
+ * Tests verifying JPMS (Java Platform Module System) compatibility.
+ * <p>
+ * These tests ensure that JSON deserialization of Java records works without requiring
+ * {@code --add-opens} JVM flags.
+ */
+public class JpmsCompatibilityTests {
+
+	private McpJsonMapper jsonMapper;
+
+	// Test records must be public for JPMS-compatible Jackson to access them
+	public record SimpleRecord(String name, String description) {
+	}
+
+	public record RecordWithMap(String type, Map<String, Object> properties) {
+	}
+
+	public record RecordWithList(List<String> items, boolean enabled) {
+	}
+
+	public record NestedRecord(String id, SimpleRecord nested) {
+	}
+
+	@BeforeEach
+	void setUp() {
+		jsonMapper = new JacksonMcpJsonMapperSupplier().get();
+	}
+
+	@Test
+	@DisplayName("Should deserialize simple record without reflection access")
+	void deserializeSimpleRecord() throws Exception {
+		String json = """
+				{
+					"name": "test-name",
+					"description": "A test description"
+				}
+				""";
+
+		assertThatNoException().isThrownBy(() -> {
+			SimpleRecord record = jsonMapper.readValue(json, SimpleRecord.class);
+			assertThat(record.name()).isEqualTo("test-name");
+			assertThat(record.description()).isEqualTo("A test description");
+		});
+	}
+
+	@Test
+	@DisplayName("Should deserialize record with map without reflection access")
+	void deserializeRecordWithMap() throws Exception {
+		String json = """
+				{
+					"type": "object",
+					"properties": {
+						"key1": "value1",
+						"key2": 42
+					}
+				}
+				""";
+
+		assertThatNoException().isThrownBy(() -> {
+			RecordWithMap record = jsonMapper.readValue(json, RecordWithMap.class);
+			assertThat(record.type()).isEqualTo("object");
+			assertThat(record.properties()).containsKey("key1");
+		});
+	}
+
+	@Test
+	@DisplayName("Should deserialize record with list without reflection access")
+	void deserializeRecordWithList() throws Exception {
+		String json = """
+				{
+					"items": ["a", "b", "c"],
+					"enabled": true
+				}
+				""";
+
+		assertThatNoException().isThrownBy(() -> {
+			RecordWithList record = jsonMapper.readValue(json, RecordWithList.class);
+			assertThat(record.enabled()).isTrue();
+			assertThat(record.items()).containsExactly("a", "b", "c");
+		});
+	}
+
+	@Test
+	@DisplayName("Should deserialize nested records without reflection access")
+	void deserializeNestedRecord() throws Exception {
+		String json = """
+				{
+					"id": "outer-id",
+					"nested": {
+						"name": "inner-name",
+						"description": "inner-description"
+					}
+				}
+				""";
+
+		assertThatNoException().isThrownBy(() -> {
+			NestedRecord record = jsonMapper.readValue(json, NestedRecord.class);
+			assertThat(record.id()).isEqualTo("outer-id");
+			assertThat(record.nested().name()).isEqualTo("inner-name");
+		});
+	}
+
+	@Test
+	@DisplayName("Should serialize and deserialize records round-trip")
+	void roundTripSerialization() throws Exception {
+		SimpleRecord original = new SimpleRecord("my-name", "my-description");
+
+		String json = jsonMapper.writeValueAsString(original);
+		SimpleRecord deserialized = jsonMapper.readValue(json, SimpleRecord.class);
+
+		assertThat(deserialized.name()).isEqualTo(original.name());
+		assertThat(deserialized.description()).isEqualTo(original.description());
+	}
+
+	@Test
+	@DisplayName("ObjectMapper should have JPMS-compatible configuration")
+	void verifyJpmsConfiguration() {
+		JacksonMcpJsonMapper jacksonMapper = (JacksonMcpJsonMapper) jsonMapper;
+		ObjectMapper objectMapper = jacksonMapper.getObjectMapper();
+
+		// Verify CAN_OVERRIDE_ACCESS_MODIFIERS is disabled
+		assertThat(objectMapper.isEnabled(MapperFeature.CAN_OVERRIDE_ACCESS_MODIFIERS))
+			.as("CAN_OVERRIDE_ACCESS_MODIFIERS should be disabled for JPMS compatibility")
+			.isFalse();
+	}
+
+}