Add JPMS compatibility to Jackson JSON mapper

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#753 opened on Jan 16, 2026 0 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 2/5
Estimated time: 1-3 hours
Newbie friendliness: 35/100

Issue type: Bug
Clarity: Clearly specified
Activity status: Stale
Tech stack: java
Domain: api, backend

Research direction

Start at JacksonMcpJsonMapperSupplier and inspect how its ObjectMapper handles record constructors and parameter names. Verify the mapper configuration against the JPMS example, then confirm record deserialization works without --add-opens flags and preserves existing behavior.

Written by the indexing model from the issue text.

Description

bug P2

Expected Behavior

Applications using the MCP Java SDK should work with JPMS (Java Platform Module System) without requiring --add-opens JVM flags. Record deserialization should work through public constructors and parameter name discovery rather than reflective access.

  // Should work without --add-opens flags
  McpJsonMapper mapper = new JacksonMcpJsonMapperSupplier().get();
  MyRecord record = mapper.readValue(json, MyRecord.class);

Current Behavior

The JacksonMcpJsonMapperSupplier creates a plain ObjectMapper that uses reflection to access record constructors. This requires --add-opens JVM flags like:

  --add-opens java.base/java.lang.reflect=ALL-UNNAMED

Without these flags, deserialization of Java records fails with InaccessibleObjectException.

Context

We're integrating the MCP SDK into a JPMS-modularized application and cannot use --add-opens flags as they break module encapsulation.

The fix is straightforward:

Disable MapperFeature.CAN_OVERRIDE_ACCESS_MODIFIERS to prevent setAccessible() calls
Add ParameterNamesModule to discover constructor parameters from bytecode

The SDK already compiles with -parameters, so ParameterNamesModule will work without additional configuration.

Workaround: We currently maintain a custom MCP client with JPMS-compatible Jackson configuration, but would prefer to use the upstream SDK directly.

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Dominant language: Java
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