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feat: Introduce McpJsonMapper interface to decouple from Jackson (#543)
This pull request creates two modules, `mcp-json` and `mcp-json-jackson`. It removes the `com.fasterxml.jackson.core:jackson-databind` and `com.networknt:json-schema-validator` dependencies from the `mcp` module. The `mcp` module now only depends on `com.fasterxml.jackson.core:jackson-annotations`. To use Jackson, you have to add `mcp-jackson` to your dependencies in addition to `mcp`. I added the dependency `mcp-jackson` to both `mcp-spring-mvc` and `mcp-spring-webflux` to avoid a breaking change in those modules. It provides two [SPI](https://docs.oracle.com/javase/tutorial/sound/SPI-intro.html) `JsonSchemaValidatorSupplier` and `JacksonJsonSchemaValidatorSupplier` to allow easy replacement for consumers who don't want to use Jackson. This pull request also ensures no `McpJsonMapper` is instantiated if one is provided via a builder method. Only if the builders don't receive a `McpJsonMapper` mapper, one is instantiated in the `build` method of the builder. The logic behind this is to allow frameworks to provide a `McpJsonMapper` mapper singleton implementation and feed it to the builders without paying the price of instantiating `McpJsonMappers`, which will not be used. The goal is to be able to use the `ObjectMapper` singleton of an application also for the MCP code. Signed-off-by: Christian Tzolov <[email protected]>
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‎mcp-bom/pom.xml

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@@ -33,6 +33,20 @@
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<version>${project.version}</version>
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</dependency>
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<!-- MCP JSON -->
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<dependency>
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<groupId>io.modelcontextprotocol.sdk</groupId>
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<artifactId>mcp-json</artifactId>
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<version>${project.version}</version>
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</dependency>
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<!-- MCP JSON Jackson -->
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<dependency>
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<groupId>io.modelcontextprotocol.sdk</groupId>
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<artifactId>mcp-json-jackson2</artifactId>
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<version>${project.version}</version>
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</dependency>
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<!-- MCP Test -->
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<dependency>
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<groupId>io.modelcontextprotocol.sdk</groupId>

‎mcp-json-jackson2/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>io.modelcontextprotocol.sdk</groupId>
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<artifactId>mcp-parent</artifactId>
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<version>0.13.0-SNAPSHOT</version>
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</parent>
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<artifactId>mcp-json-jackson2</artifactId>
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<packaging>jar</packaging>
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<name>Java MCP SDK JSON Jackson</name>
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<description>Java MCP SDK JSON implementation based on Jackson</description>
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<url>https://github.com/modelcontextprotocol/java-sdk</url>
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<scm>
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<url>https://github.com/modelcontextprotocol/java-sdk</url>
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<connection>git://github.com/modelcontextprotocol/java-sdk.git</connection>
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<developerConnection>[email protected]/modelcontextprotocol/java-sdk.git</developerConnection>
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</scm>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-jar-plugin</artifactId>
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<configuration>
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<archive>
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<manifest>
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<addDefaultImplementationEntries>true</addDefaultImplementationEntries>
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</manifest>
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</archive>
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</configuration>
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</plugin>
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</plugins>
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</build>
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<dependencies>
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<dependency>
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<groupId>io.modelcontextprotocol.sdk</groupId>
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<artifactId>mcp-json</artifactId>
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<version>0.13.0-SNAPSHOT</version>
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</dependency>
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<dependency>
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<groupId>com.fasterxml.jackson.core</groupId>
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<artifactId>jackson-databind</artifactId>
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<version>${jackson.version}</version>
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</dependency>
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<dependency>
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<groupId>com.networknt</groupId>
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<artifactId>json-schema-validator</artifactId>
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<version>${json-schema-validator.version}</version>
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</dependency>
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</dependencies>
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</project>

‎mcp-json-jackson2/src/main/java/io/modelcontextprotocol/json/jackson/JacksonMcpJsonMapper.java

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/*
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* Copyright 2025 - 2025 the original author or authors.
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*/
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package io.modelcontextprotocol.json.jackson;
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import com.fasterxml.jackson.databind.JavaType;
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import com.fasterxml.jackson.databind.ObjectMapper;
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import io.modelcontextprotocol.json.McpJsonMapper;
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import io.modelcontextprotocol.json.TypeRef;
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import java.io.IOException;
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/**
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* Jackson-based implementation of JsonMapper. Wraps a Jackson ObjectMapper but keeps the
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* SDK decoupled from Jackson at the API level.
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*/
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public final class JacksonMcpJsonMapper implements McpJsonMapper {
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private final ObjectMapper objectMapper;
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/**
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* Constructs a new JacksonMcpJsonMapper instance with the given ObjectMapper.
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* @param objectMapper the ObjectMapper to be used for JSON serialization and
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* deserialization. Must not be null.
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* @throws IllegalArgumentException if the provided ObjectMapper is null.
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*/
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public JacksonMcpJsonMapper(ObjectMapper objectMapper) {
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if (objectMapper == null) {
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throw new IllegalArgumentException("ObjectMapper must not be null");
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}
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this.objectMapper = objectMapper;
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}
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/**
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* Returns the underlying Jackson {@link ObjectMapper} used for JSON serialization and
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* deserialization.
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* @return the ObjectMapper instance
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*/
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public ObjectMapper getObjectMapper() {
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return objectMapper;
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}
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@Override
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public <T> T readValue(String content, Class<T> type) throws IOException {
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return objectMapper.readValue(content, type);
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}
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@Override
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public <T> T readValue(byte[] content, Class<T> type) throws IOException {
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return objectMapper.readValue(content, type);
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}
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@Override
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public <T> T readValue(String content, TypeRef<T> type) throws IOException {
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JavaType javaType = objectMapper.getTypeFactory().constructType(type.getType());
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return objectMapper.readValue(content, javaType);
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}
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@Override
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public <T> T readValue(byte[] content, TypeRef<T> type) throws IOException {
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JavaType javaType = objectMapper.getTypeFactory().constructType(type.getType());
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return objectMapper.readValue(content, javaType);
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}
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@Override
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public <T> T convertValue(Object fromValue, Class<T> type) {
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return objectMapper.convertValue(fromValue, type);
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}
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@Override
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public <T> T convertValue(Object fromValue, TypeRef<T> type) {
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JavaType javaType = objectMapper.getTypeFactory().constructType(type.getType());
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return objectMapper.convertValue(fromValue, javaType);
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}
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@Override
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public String writeValueAsString(Object value) throws IOException {
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return objectMapper.writeValueAsString(value);
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}
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@Override
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public byte[] writeValueAsBytes(Object value) throws IOException {
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return objectMapper.writeValueAsBytes(value);
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}
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}

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

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/*
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* Copyright 2025 - 2025 the original author or authors.
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*/
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package io.modelcontextprotocol.json.jackson;
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import io.modelcontextprotocol.json.McpJsonMapper;
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import io.modelcontextprotocol.json.McpJsonMapperSupplier;
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/**
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* A supplier of {@link McpJsonMapper} instances that uses the Jackson library for JSON
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* serialization and deserialization.
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* <p>
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* This implementation provides a {@link McpJsonMapper} backed by a Jackson
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* {@link com.fasterxml.jackson.databind.ObjectMapper}.
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*/
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public class JacksonMcpJsonMapperSupplier implements McpJsonMapperSupplier {
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/**
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* Returns a new instance of {@link McpJsonMapper} that uses the Jackson library for
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* JSON serialization and deserialization.
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* <p>
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* The returned {@link McpJsonMapper} is backed by a new instance of
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* {@link com.fasterxml.jackson.databind.ObjectMapper}.
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* @return a new {@link McpJsonMapper} instance
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*/
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@Override
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public McpJsonMapper get() {
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return new JacksonMcpJsonMapper(new com.fasterxml.jackson.databind.ObjectMapper());
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}
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}

‎mcp/src/main/java/io/modelcontextprotocol/spec/DefaultJsonSchemaValidator.java renamed to ‎mcp-json-jackson2/src/main/java/io/modelcontextprotocol/json/schema/jackson/DefaultJsonSchemaValidator.java

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/*
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* Copyright 2024-2024 the original author or authors.
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*/
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package io.modelcontextprotocol.spec;
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package io.modelcontextprotocol.json.schema.jackson;
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import java.util.Map;
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import java.util.Set;
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import java.util.concurrent.ConcurrentHashMap;
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import io.modelcontextprotocol.json.schema.JsonSchemaValidator;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import com.networknt.schema.SpecVersion;
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import com.networknt.schema.ValidationMessage;
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import io.modelcontextprotocol.util.Assert;
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/**
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* Default implementation of the {@link JsonSchemaValidator} interface. This class
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* provides methods to validate structured content against a JSON schema. It uses the
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@Override
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public ValidationResponse validate(Map<String, Object> schema, Object structuredContent) {
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Assert.notNull(schema, "Schema must not be null");
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Assert.notNull(structuredContent, "Structured content must not be null");
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if (schema == null) {
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throw new IllegalArgumentException("Schema must not be null");
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}
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if (structuredContent == null) {
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throw new IllegalArgumentException("Structured content must not be null");
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}
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try {
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‎mcp-json-jackson2/src/main/java/io/modelcontextprotocol/json/schema/jackson/JacksonJsonSchemaValidatorSupplier.java

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/*
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* Copyright 2025 - 2025 the original author or authors.
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*/
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package io.modelcontextprotocol.json.schema.jackson;
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import io.modelcontextprotocol.json.schema.JsonSchemaValidator;
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import io.modelcontextprotocol.json.schema.JsonSchemaValidatorSupplier;
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/**
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* A concrete implementation of {@link JsonSchemaValidatorSupplier} that provides a
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* {@link JsonSchemaValidator} instance based on the Jackson library.
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*
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* @see JsonSchemaValidatorSupplier
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* @see JsonSchemaValidator
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*/
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public class JacksonJsonSchemaValidatorSupplier implements JsonSchemaValidatorSupplier {
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/**
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* Returns a new instance of {@link JsonSchemaValidator} that uses the Jackson library
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* for JSON schema validation.
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* @return A {@link JsonSchemaValidator} instance.
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*/
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@Override
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public JsonSchemaValidator get() {
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return new DefaultJsonSchemaValidator();
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}
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}

‎mcp-json-jackson2/src/main/resources/META-INF/services/io.modelcontextprotocol.json.McpJsonMapperSupplier

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io.modelcontextprotocol.json.jackson.JacksonMcpJsonMapperSupplier

‎mcp-json-jackson2/src/main/resources/META-INF/services/io.modelcontextprotocol.json.schema.JsonSchemaValidatorSupplier

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io.modelcontextprotocol.json.schema.jackson.JacksonJsonSchemaValidatorSupplier

‎mcp-json/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>io.modelcontextprotocol.sdk</groupId>
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<artifactId>mcp-parent</artifactId>
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<version>0.13.0-SNAPSHOT</version>
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</parent>
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<artifactId>mcp-json</artifactId>
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<packaging>jar</packaging>
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<name>Java MCP SDK JSON Support</name>
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<description>Java MCP SDK JSON Support API</description>
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<url>https://github.com/modelcontextprotocol/java-sdk</url>
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<scm>
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<url>https://github.com/modelcontextprotocol/java-sdk</url>
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<connection>git://github.com/modelcontextprotocol/java-sdk.git</connection>
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<developerConnection>[email protected]/modelcontextprotocol/java-sdk.git</developerConnection>
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</scm>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-jar-plugin</artifactId>
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<configuration>
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<archive>
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<manifest>
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<addDefaultImplementationEntries>true</addDefaultImplementationEntries>
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</manifest>
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</archive>
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</configuration>
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</plugin>
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</plugins>
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</build>
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<dependencies>
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</dependencies>
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</project>

‎mcp-json/src/main/java/io/modelcontextprotocol/json/McpJsonInternal.java

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/*
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* Copyright 2025 - 2025 the original author or authors.
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*/
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package io.modelcontextprotocol.json;
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import java.util.ServiceLoader;
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import java.util.concurrent.atomic.AtomicReference;
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import java.util.stream.Stream;
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/**
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* Utility class for creating a default {@link McpJsonMapper} instance. This class
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* provides a single method to create a default mapper using the {@link ServiceLoader}
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* mechanism.
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*/
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final class McpJsonInternal {
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private static McpJsonMapper defaultJsonMapper = null;
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/**
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* Returns the cached default {@link McpJsonMapper} instance. If the default mapper
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* has not been created yet, it will be initialized using the
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* {@link #createDefaultMapper()} method.
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* @return the default {@link McpJsonMapper} instance
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* @throws IllegalStateException if no default {@link McpJsonMapper} implementation is
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* found
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*/
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static McpJsonMapper getDefaultMapper() {
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if (defaultJsonMapper == null) {
30+
defaultJsonMapper = McpJsonInternal.createDefaultMapper();
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}
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return defaultJsonMapper;
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}
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/**
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* Creates a default {@link McpJsonMapper} instance using the {@link ServiceLoader}
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* mechanism. The default mapper is resolved by loading the first available
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* {@link McpJsonMapperSupplier} implementation on the classpath.
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* @return the default {@link McpJsonMapper} instance
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* @throws IllegalStateException if no default {@link McpJsonMapper} implementation is
41+
* found
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*/
43+
static McpJsonMapper createDefaultMapper() {
44+
AtomicReference<IllegalStateException> ex = new AtomicReference<>();
45+
return ServiceLoader.load(McpJsonMapperSupplier.class).stream().flatMap(p -> {
46+
try {
47+
McpJsonMapperSupplier supplier = p.get();
48+
return Stream.ofNullable(supplier);
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}
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catch (Exception e) {
51+
addException(ex, e);
52+
return Stream.empty();
53+
}
54+
}).flatMap(jsonMapperSupplier -> {
55+
try {
56+
return Stream.ofNullable(jsonMapperSupplier.get());
57+
}
58+
catch (Exception e) {
59+
addException(ex, e);
60+
return Stream.empty();
61+
}
62+
}).findFirst().orElseThrow(() -> {
63+
if (ex.get() != null) {
64+
return ex.get();
65+
}
66+
else {
67+
return new IllegalStateException("No default McpJsonMapper implementation found");
68+
}
69+
});
70+
}
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72+
private static void addException(AtomicReference<IllegalStateException> ref, Exception toAdd) {
73+
ref.updateAndGet(existing -> {
74+
if (existing == null) {
75+
return new IllegalStateException("Failed to initialize default McpJsonMapper", toAdd);
76+
}
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else {
78+
existing.addSuppressed(toAdd);
79+
return existing;
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}
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});
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}
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}

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