CardDemo Java modernization: Spring Boot foundation (copybooks → JPA model)

[devanshi-gpta]

Summary

First layer of the COBOL → Java/Spring Boot migration: a standalone Maven module in java/ that carries the data model every migrated program depends on. No app/ artifact is touched; the COBOL/CICS/VSAM/JCL sources remain the reference implementation.

What it establishes:

• Copybook → entity mapping. Each app/cpy/*.cpy record layout becomes a JPA entity whose Javadoc names the copybook and VSAM file, and whose fields carry the original PIC clause: PIC X(n) → String with length = n, PIC 9(n) → Integer/Long by digit count, PIC S9(m)V99 → BigDecimal(m+2, 2). Group keys (TRAN-CAT-KEY, DIS-GROUP-KEY) become @Embeddable composite ids.
• VSAM KSDS → Spring Data. Repositories mirror the access paths the COBOL uses, including the alternate-index reads (findByAccountIdOrderByCardNumber for the CARDDATA/CARDXREF AIX paths).
• COMMAREA → session DTO. CardDemoCommarea reproduces COCOM01Y field-for-field; pseudo-conversational state will live in the HTTP session under CardDemoCommarea.SESSION_KEY instead of being passed on EXEC CICS RETURN.
• Zoned decimal handling. CobolUtils.decimal decodes the sign overpunch in the sample extracts ({=+0, A–I=+1..+9, }=-0, J–R=-1..-9), so "0000009190}" at scale 2 → -919.00. rounded() gives COBOL ROUNDED (HALF_UP) semantics for the interest math landing in a later PR.
• Load JCL → SeedDataLoader. Replaces ACCTFILE/CARDFILE/CUSTFILE/XREFFILE/DISCGRP/TCATBALF/TRANTYPE/TRANCATG/DUSRSECJ by parsing app/data/ASCII at startup (toggle: carddemo.data.load-on-startup).

Worth flagging: acctdata.txt puts the disclosure group value (A000000000) in the ACCT-ADDR-ZIP byte range and leaves ACCT-GROUP-ID blank. The loader follows the CVACT01Y offsets rather than "fixing" the data, so interest calculation will fall back to the DEFAULT disclosure group exactly as CBACT04C does on the mainframe.

Tests: CobolUtilsTest (overpunch/rounding/padding), SeedDataLoaderTest (each layout asserted against the real sample records), and a context test asserting 50 accounts/cards/customers load.

Follow-up PRs (batch programs CB*, online programs CO*) branch off this one.

Link to Devin session: https://app.devin.ai/sessions/5409e398c7304f249331d08190d3abd3
Requested by: @devanshi-gpta

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