Migration guide
Spring Boot teams fail on COBOL when they port paragraphs instead of rules. This guide shows how to turn copybooks, COMP-3 fields and PERFORM chains into contracts a Java team can build against.
By Pranav Kulkarni, Head of Language Engineering · Updated 15 Oct 2026
Technical syntax challenges
PIC S9(9)V99 COMP-3 occupies 6 bytes ((11 + 1) / 2) and carries an implied decimal point that never appears in the data. In Java it must become BigDecimal with scale 2 and an explicit RoundingMode; mapping it to double silently loses cents in accrual loops.
COBOL truncates intermediate results to the receiving field unless the statement says ROUNDED, and the default ROUNDED mode is half-up (away from zero). A Java port that calls setScale(2, RoundingMode.HALF_EVEN) everywhere disagrees with production on a measurable share of postings.
88 ACCT-DORMANT VALUE 'D' 'X'. means one business state maps to two stored codes. The rule extractor must carry both values into the enum or decision table, otherwise the Java service rejects historical records written with the older code.
PERFORM 2000-CALC THRU 2999-EXIT executes every paragraph between the two labels in source order, including any added later by maintenance. The real call graph only exists after the control-flow graph is built across the THRU range.
A record that redefines the same 40 bytes as a personal or a business layout is a discriminated union. Java needs a sealed interface or two record types selected by the discriminator field, not one class with every field nullable.
Extraction workflow
The sequence a PYD ENTERPRISES Discovery Scan runs for this stack. Every output links back to source file and line range.
Export the COBOL members, every COPYLIB in the concatenation order your compile JCL uses, and the DB2 DDL. Concatenation order matters when two libraries hold a copybook with the same name.
PYD ENTERPRISES parses each program with the IBM Enterprise COBOL dialect, expands copybooks with REPLACING, and computes the byte offset, length and usage of every data item, including REDEFINES variants.
Static CALLs, CICS LINK and XCTL targets and PERFORM THRU ranges are resolved into one graph, so dead paragraphs and fall-through paths are visible before any rule is named.
Program slicing on each output field isolates the statements that compute business values. File status checks, DISPLAY tracing and abend handling are classified as plumbing and listed separately.
Decisions become DMN 1.5 tables with FEEL expressions, records become JSON Schema 2020-12 with decimal constraints, and program entry points become OpenAPI 3.1 operations, each linked to file and line range.
Boundary fixtures are generated from real branch conditions (for example balance 1499.99 and 1500.00). The Java team runs them against the new service in CI until every fixture matches the legacy result.
Legacy in, specification out
Left: the COBOL construct as it sits in production. Right: the reviewed artifact your Java Spring Boot engineers build against. PYD ENTERPRISES produces the spec and the tests; your team writes the service.
LEGACY SOURCE
1 01 LN-ACCRUAL-AREA.2 05 LN-PRIN-BAL PIC S9(9)V99 COMP-3.3 05 LN-ANN-RATE PIC S9(3)V9(6) COMP-3.4 05 LN-DAILY-INT PIC S9(7)V99 COMP-3.5 05 LN-BASIS-CODE PIC X(01).6 88 BASIS-360 VALUE '0'.7 88 BASIS-365 VALUE '5' '6'.8 *9 2100-CALC-DAILY-INT.10 IF BASIS-36011 COMPUTE LN-DAILY-INT ROUNDED =12 LN-PRIN-BAL * LN-ANN-RATE / 36013 ELSE14 COMPUTE LN-DAILY-INT ROUNDED =15 LN-PRIN-BAL * LN-ANN-RATE / 36516 END-IF.17 2100-EXIT.18 EXIT.
EXTRACTED ARTIFACT
1{2 "id": "LNACCR-0412",3 "name": "Daily interest accrual by day-count basis",4 "hitPolicy": "UNIQUE",5 "inputs": [6 { "name": "basisCode", "type": "string", "source": "LN-BASIS-CODE" }7 ],8 "outputs": [9 { "name": "dailyInterest", "type": "decimal", "scale": 2, "rounding": "HALF_UP" }10 ],11 "rules": [12 { "when": ["\"0\""], "then": ["principal * annualRate / 360"] },13 { "when": ["\"5\", \"6\""], "then": ["principal * annualRate / 365"] },14 { "when": ["-"], "then": ["principal * annualRate / 365"], "note": "ELSE branch: any other code" }15 ],16 "types": {17 "principal": "decimal(11,2) from S9(9)V99 COMP-3, 6 bytes",18 "annualRate": "decimal(9,6) from S9(3)V9(6) COMP-3, 5 bytes"19 },20 "source": "LNACCR02.cbl:1187-1204"21}
Architectural best practices
In depth
A transpiled Spring service keeps the COBOL shape: one class per program, fields named WS-TEMP-AMT-2, and control flow driven by flags set three paragraphs earlier. It compiles, it passes a smoke test, and nobody on the Java team can change it safely. The faster route is to agree what the program does first.
PYD ENTERPRISES does not write the Spring application. It produces the decision tables, JSON Schemas, OpenAPI operations and fixtures that describe the legacy behavior, each traceable to source lines, so your engineers write idiomatic services and prove they match. Analysts review every extracted rule beside the COBOL that produced it before it reaches a backlog.
Precision bugs are the most common post-cutover defect in COBOL to Java programs. The storage layout tells you the declared precision, but the arithmetic tells you the effective precision: COBOL computes intermediates with enough digits for the operands and then truncates or rounds into the receiver.
S9(7)V99 COMP-3 becomes decimal(9,2), carried in JSON Schema as a string with a pattern or as a number with multipleOf: 0.01.COMPUTE A = B * C / D multiplies first, so the Java expression must not divide first.RoundingMode.DOWN in Java.FAQ
No. PYD ENTERPRISES extracts specifications: business rules as DMN 1.5, data contracts as JSON Schema, interfaces as OpenAPI 3.1 and behavioral fixtures. Your engineers write the Spring Boot services and use the fixtures to prove equivalence.
Each packed field keeps its total digits and scale from the PIC clause. JSON Schema exports carry them as decimal strings or numbers with multipleOf constraints, and the rule metadata records the original usage and byte length so Java teams pick BigDecimal with the right scale.
Every executable path is accounted for: extracted as a rule, classified as plumbing, or flagged for review. Flagged paths, such as dynamic CALL targets built at runtime, appear in the review queue with their source lines.
Request a codebase scan and review extracted rules, contracts and fixtures for your own COBOL estate, inside your VPC.