Legacy Java modernization: From Understand to Transform

On one of our client engagements, we ran a deep dead code analysis against a Java codebase of 654,273 lines. Roughly 275,000 of those lines sat in the business-logic layer that had been auto-translated from COBOL by a previous modernization vendor. After deep static and semantic analysis, we estimated that between 120,000 and 150,000 of those lines would not exist in a hand-written Java equivalent. Nearly half the code carried no semantic weight.
What matters more than the numbers is how we got to them, and why no off-the-shelf static analyzer would have produced the same answer. The ratios here are specific to this particular auto-translated project. Hand-written legacy systems behave very differently. Without structured understanding of the codebase before transformation, none of this would have surfaced, and the modernization plan would have been built around the wrong codebase.

Why understanding comes before transformation

Modernization teams routinely jump from "we have legacy code" to "let's prompt an AI to rewrite it." That approach fails at enterprise scale for a simple reason: the first question is not how do we migrate but what do we actually have.

This is also where the difference between prompt engineering and a modernization workflow becomes concrete. A prompt is a single instruction handed to a model. A workflow is a repeatable, governed sequence of operations with structured inputs, validated outputs, and traceable evidence. Prompts produce snippets. Workflows produce decisions that a CTO can defend in a steering committee.

Before any transformation, you need structured knowledge of the system you're working with: business documentation, dependency maps, architectural reconstruction, static and semantic findings. That knowledge becomes the substrate for every downstream change.  Business logic reconstruction and dependency mapping answer what is worth migrating. Dead code analysis answers a related but different question: how much of what you see is actually real?

A transformation pipeline applied to a codebase you don't understand is a parallel waste machine. It will faithfully migrate every dead branch, every ceremonial wrapper, every empty-string initializer into your modern stack. An AI agent asked to migrate tens of thousands of lines of structural boilerplate will produce tens of thousands of lines of structural boilerplate in the target language. The waste survives the transformation. This is also why "can AI agents migrate legacy code reliably?" is the wrong question. Reliability is a property of the workflow surrounding the agent, not of the agent itself.

Two categories of waste

Dead code analysis splits findings into two categories.

Strict dead code is lines whose execution has no observable effect. The IDE will usually flag these.

Translation overhead is lines that are syntactically alive but exist only because a mechanical translator emitted them. The IDE cannot see this because the surface code is well-formed; every statement looks like real work.

Static analysis tools handle the first category. The second is where the volume hides - and where modernization budgets quietly evaporate. Detecting it requires semantic reasoning, codebase-wide context, and pattern recognition that no IDE inspection provides.

The legacy codebase under analysis

The client owned a large back-office system originally written in COBOL. A prior modernization vendor had performed a mechanical COBOL-to-Java translation through a decompilation toolchain. The output Java code compiled and ran in production. There were no automated tests. The only validation performed at the time of translation was manual, and it had happened years before we arrived. By the time the system reached us, nobody on the team could fully describe what the code did - the institutional memory of the translation effort had moved on, and the surface code was opaque enough that no one was confident enough to touch it.

We began with the Understand phase, the first step of our modernization process, focused on reconstructing what the codebase actually does before any migration is scoped. The process runs on G.Tx, Grape Up's agentic platform for enterprise legacy modernization, which models Understand as a set of reusable workflows backed by AI agents, structured context, and engineering governance. The dead code analysis workflow produced the findings the rest of this article is built on.

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What static analysis caught

Some of the dead weight was syntactically obvious: indicator-variable boilerplate left over from COBOL host-variable conventions, redundant explicit casts preserved from the bytecode, discarded DAO results, duplicate branches in if-chains, redundant re-initializations of locals. The IDE could see all of it. In this codebase the relevant inspections had been silenced because the warning count was unusable. A finding technically visible to static analysis behaved, in practice, as if it were invisible.

Integer stationOutInd = 0;
// ... no writes anywhere ...
if (stationOutInd != 0) { stationOut = ""; } // always false

Even with the IDE's help, the visible findings explained only a small fraction of the auto-translated layer. The bigger story sat behind what the IDE could not see.

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What semantic analysis revealed

The architectural patterns were harder. Each one looked like ordinary Java to an analyzer. Each line allocated, called, or assigned something. The waste was architectural, not syntactic, and only became visible once we looked at the codebase as a whole.

The ValueHolder marshalling dance. Wrapper-class boilerplate emulating COBOL's BY REFERENCE.  Every multi-output call became three lines of wrap-call-unwrap, often on the same variable repeatedly:

copyCountHolder = new ValueHolder(Integer.class, (Object) copyCount);
returnCode = printFilter.searchStationCopyCount(
	stationPrint, "DOCUMENT_TYPE_A", (ValueHolder<Integer>) copyCountHolder
);
copyCount = (Integer) copyCountHolder.getValue();

In idiomatic Java the same sites collapse to a return value, a record, or a small result class.

Section-global state emulation. COBOL paragraphs share state through working storage, a flat namespace visible to every paragraph. The translator preserved that model by giving each service module its own Context class and turning every former local variable into a context field accessed through a wrapping getter on every term of every expression.

this.getServiceContext().setBrand(this.getServiceContext().getBrandCode());
this.getServiceContext().getInvoice().setBrandCode(this.getServiceContext().getBrand());

The deeper finding came from cross-referencing reads and writes: many context fields were written by exactly one paragraph and read by exactly that same paragraph. They had no business being state at all. They were locals masquerading as state because the translator did not know the difference.

DTO bloat. COBOL PIC X(n) working-storage fields default to spaces, not null. The translator preserved the equivalent by initializing every Java string field to `""`. Every COBOL 01-level record became a Java DTO with one field, one getter, one setter, and one empty-string initializer per string field.

The IDE's redundant-initializer inspection only fires when the explicit value matches the JVM default. "" is not the default for String (which is null), so the inspection treated every empty-string initializer as intentional.

A few smaller patterns followed the same logic: identity assignments via UxRuntime.assign for COBOL MOVE statements that needed no coercion, and UxRuntime.memset calls on Java objects that did nothing. Each was invisible to static analysis because each looked like a real method call.

The same translator habits also produced latent correctness bugs, not just overhead. Methods that take a String parameter and reassign it across dozens of lines (a literal translation of COBOL BY REFERENCE) silently lose every write at return, because Java is pass-by-value for object references:

public void formatLetterMessage(Long period, Long invoiceId, String message) {
	// 50+ lines of work, repeatedly reassigning `message`
	message = StringUtils.replaceCharAt(message, charPos, ' ');
	// method ends — every write is lost
}

Elsewhere in the same codebase, the translator used ValueHolder precisely to emulate pass-by-reference correctly. The pattern of forgetting to wrap is the bug. Try/catch blocks that perform conditional database lookups and write a result through a setter, only to be overwritten by an unconditional setter immediately after the block, fall in the same category: dead code at the line level, latent defect at the behaviour level. In a system without automated tests, neither shape had any chance of being noticed.

Aggregate picture of the dead code

In this particular auto-translated codebase, strict dead code accounted for roughly 5–10% of the 275,000-line business-logic layer. Translation overhead accounted for another 35–45%. Together, roughly 45–55% of the auto-translated layer would not exist in a hand-written Java equivalent - between 120,000 and 150,000 lines of code carrying no semantic weight.

The bulk of that volume came from a small number of patterns:

These ratios reflect this specific auto-translated project. Other codebases, especially hand-written legacy systems, distribute their waste very differently. The methodology generalizes; the percentages do not.

In the worst-affected individual methods, 30–50% of the body was dead or boilerplate at the line level. A developer reading those methods was spending up to one line out of every two on mechanical noise before reaching anything that described the actual business behaviour.

Why this changes the modernization conversation

1. Migration cost estimation collapses by half when overhead is removed. Quoting "275,000 lines to migrate" anchors the budget. Quoting "approximately 130,000 lines of real logic, plus 145,000 lines of removable overhead" reframes the engagement entirely — both in scope and in sequencing. This is, in concrete terms, the ROI of a feasibility analysis for legacy modernization: half the scope you were about to budget for is not real.
2. AI-assisted transformation amplifies whatever you feed it. An agent asked to migrate the ValueHolder dance will faithfully migrate the ValueHolder dance. The Understand phase puts the cleanup before the migration, not after.
3. Test generation is more reliable on real code than on boilerplate. Auto-generated tests for dead branches and clobbered setters pass but verify nothing. Understand-phase outputs allow downstream test-generation workflows to skip dead surface area entirely — which matters even more in systems like this one, where no test suite existed and behaviour had to be reconstructed from code rather than from assertions.
4. Static analysis alone is insufficient. The IDE saw a small fraction of the problem here. The remaining required semantic analysis aware of the translator's idioms.
5. Modernizing in place is a real option. Not every legacy system needs to be rewritten or replatformed. Once dead code and hidden dependencies are mapped, in-place hardening like removing overhead, recovering documentation, restoring testability, is often the higher-ROI path. The decision to migrate, modernize in place, or split the system between the two should follow the analysis, not precede it.
6. Human-in-the-loop validation remains mandatory. Several findings were latent correctness bugs hiding behind dead code. Auto-deleting them without engineering review would be reckless. The output of a dead code analysis workflow is a curated, traceable finding set, not a green-light-to-delete list.

How the G.Tx Understand operationalizes this

The dead code analysis workflow produces, for each finding, a classification of what is dead, the location in the codebase, and the rationale explaining why it qualifies as dead. Aggregate counts per classification are available as well, so engineering teams can see both the individual evidence and the overall distribution of waste across the codebase. Every classification is traceable back to source locations or runtime evidence.

And dead code is not only a code-level phenomenon. The same analytical lens applies one layer up: endpoints that no client has called in years, scheduled jobs that nobody remembers writing, service modules whose only consumer was decommissioned long ago, infrastructure quietly burning budget for traffic that no longer exists. Code-level dead code is a maintainability and correctness problem. Functionality-level dead code is a cost and risk problem. Both belonging the Understand phase, because both shape the same decision: what is worth migrating, what is worth hardening in place, and what should simply be turned off.

That last point matters for hallucination control. Models hallucinate when they infer from incomplete context. The artifacts produced during Understand, classified findings, traceable evidence, mapped dependencies, are exactly the grounding downstream agents need during transformation. Hallucination is reduced before any code is touched, because the model has real evidence to work with instead of having to guess at the codebase.

Lessons for legacy modernization

• Quantify before you migrate. The single most valuable artifact in a modernization engagement is a defensible number for real code volume. Without it, every estimate is fiction.
• Auto-translation defers cost; it does not eliminate it. A clean compile is not evidence of a maintainable codebase. In this engagement, half the code was overhead, and the only thing keeping that overhead in production was that nobody was confident enough to touch it. The code was unreadable, undocumented, and unverified. Removing anything felt riskier than leaving everything.
• The IDE is not enough. Roughly 90% of the waste in this codebase was invisible to off-the-shelf static analysis. Semantic, codebase-aware analysis closed the gap.
• Dead code is a correctness signal, not just a hygiene problem. Several of the patterns we found were latent bugs the team did not know they had.
• Modernization is an orchestration problem. No single prompt, agent, or tool produces findings of this kind. It takes reusable workflows, curated context, structured outputs, and disciplined human review.

Modernization decisions made without an Understand phase are decisions made about the wrong codebase. In this engagement, the "wrong codebase" was roughly twice the size of the real one, and the real one was the only one worth migrating.

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If you suspect your own auto-translated or long-lived legacy system is carrying overhead nobody has measured, the G.Tx Understand phase exists precisely for that conversation. Reach out - we'll start with a focused feasibility analysis for legacy modernization and produce a defensible picture of what you actually have.

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