jons-mcp-java

MCP server that provides Java language intelligence through Eclipse JDT.LS.

This project is currently intended to be installed from a source checkout or from GitHub. It is not documented as a PyPI package.

The current public API is aligned with jons-mcp-typescript: public positions are one-based, successful tool responses use structured items and pagination shapes, and refactoring previews are read-only.

Requirements

• Python 3.10+

• Java 21+

• Eclipse JDT.LS

• A target Java workspace containing Gradle project markers: settings.gradle, settings.gradle.kts, build.gradle, or build.gradle.kts

On macOS with Homebrew:

brew install openjdk@21 jdtls

If Java is not on your PATH, set JAVA_HOME. If JDT.LS is installed somewhere custom, set JDTLS_HOME. The server also tries to resolve a jdtls executable on PATH.

If JDTLS_HOME points at a read-only install such as the Nix store, the server copies the small platform config_<os> directory into a writable cache under $XDG_CACHE_HOME/jons-mcp-java or ~/.cache/jons-mcp-java. Set JDTLS_CONFIG_DIR to choose a specific writable configuration directory.

Install and Run

From a source checkout:

git clone git@github.com:jonmmease/jons-mcp-java.git
cd jons-mcp-java
uv sync
uv run jons-mcp-java /path/to/java-workspace

From GitHub without a persistent checkout:

uvx --from git+https://github.com/jonmmease/jons-mcp-java.git \
  jons-mcp-java /path/to/java-workspace

Pass the Java workspace root as the optional positional argument, or set JONS_MCP_JAVA_WORKSPACE. The positional argument takes precedence. If neither is provided, the server uses the MCP process current working directory. All relative tool paths are resolved from this workspace root.

MCP Client Examples

Claude Code using a source checkout:

claude mcp add jons-mcp-java \
  -- uv run --project /path/to/jons-mcp-java \
  jons-mcp-java /path/to/java-workspace

Claude Code using GitHub:

claude mcp add jons-mcp-java \
  -- uvx --from git+https://github.com/jonmmease/jons-mcp-java.git \
  jons-mcp-java /path/to/java-workspace

Codex CLI using GitHub:

codex mcp add jons-mcp-java \
  -- uvx --from git+https://github.com/jonmmease/jons-mcp-java.git \
  jons-mcp-java /path/to/java-workspace

.mcp.json:

{
  "mcpServers": {
    "jons-mcp-java": {
      "command": "uvx",
      "args": [
        "--from",
        "git+https://github.com/jonmmease/jons-mcp-java.git",
        "jons-mcp-java",
        "/path/to/java-workspace"
      ]
    }
  }
}

Codex TOML:

[mcp_servers.jons-mcp-java]
command = "uvx"
args = [
  "--from",
  "git+https://github.com/jonmmease/jons-mcp-java.git",
  "jons-mcp-java",
  "/path/to/java-workspace",
]

Workspace and Path Behavior

The configured workspace root is the filesystem security boundary.

• Tool paths may be workspace-relative paths, absolute in-workspace paths, or file:// URIs.

• Relative paths are resolved from the configured workspace root, not from the MCP server process cwd.

• Paths containing .., paths outside the workspace, non-file URIs, malformed URIs, missing files, and symlink escapes are rejected before any filesystem or JDT.LS access.

• LSP locations outside the workspace may still be returned with "inWorkspace": false, but the server does not open or read external files.

Path and startup failures use a stable error shape:

{
  "status": "error",
  "error": {
    "type": "path_outside_workspace",
    "message": "Path resolves outside the configured workspace root.",
    "path": "../outside.java"
  }
}

Available Tools

The first file-backed call for a project starts JDT.LS lazily and usually returns:

{
  "status": "initializing",
  "message": "Starting project initialization; please retry shortly.",
  "project": "/path/to/java-workspace/app"
}

Retry the same tool after initialization finishes.

Tool Behavior

Tools that accept or return line and character use one-based positions, matching editor and agent Read output. If your editor shows line 28, pass line=28; returned ranges use the same convention. Use document_symbols to discover one-based symbol ranges when you do not already know a position.

Successful navigation tools return normalized items:

{
  "items": [
    {
      "uri": "file:///path/to/Main.java",
      "range": {
        "start": { "line": 12, "character": 8 },
        "end": { "line": 12, "character": 12 }
      },
      "inWorkspace": true
    }
  ],
  "totalItems": 1
}

references, document_symbols, workspace_symbols, and diagnostics return paginated results with items, totalItems, offset, limit, hasMore, and nextOffset.

Navigation tools preserve JDT.LS result order and return items plus totalItems. references, diagnostics, and symbol lists are sorted deterministically before pagination.

preview_rename is safe to inspect. It returns a flat list of file URI, one-based replacement range, newText, and inWorkspace values, plus totalEdits. It does not write to disk.

Freshness and Restart

The server tracks open LSP documents and compares disk metadata plus content hashes before read-style tool calls. If a file changed outside JDT.LS, the server sends a full-document didChange and didSave before requesting fresh language data.

diagnostics is file-scoped and always refreshes the target file before returning compiler diagnostics.

Use restart_server only as a fallback:

{ "file_path": "app/src/main/java/example/Main.java" }

With no file_path, restart_server stops all active JDT.LS clients. Servers restart lazily on the next file-backed tool call.

Troubleshooting

• JDT.LS not found: install JDT.LS, put jdtls on PATH, or set JDTLS_HOME.

• Java 21+ required: install Java 21+ or set JAVA_HOME.

• project_not_found: the file is inside the workspace, but not under a discovered Gradle root.

• path_outside_workspace: the path resolves outside the configured workspace root.

• project_startup_failed: the project import failed; check the JDT.LS stderr log under the generated workspace data directory in ~/.cache/jdtls-workspaces.

• Read-only JDTLS_HOME: set JDTLS_CONFIG_DIR to a writable directory, or let the server create its cached config copy automatically.