Top 10 Solutions for Converting Legacy Java Swing Apps to Web

$3.6 trillion in global technical debt is currently locked inside aging enterprise architectures, and a massive percentage of that debt is written in Java Swing. For decades, Swing was the gold standard for heavy-duty financial terminals, healthcare record systems, and industrial control panels. Today, these systems are "black boxes"—the original developers are gone, the documentation is non-existent, and the cost of a manual rewrite is prohibitive.

If you are tasked with migrating these systems, you are likely looking for the top solutions converting legacy Java Swing to modern web architectures. The reality is that 70% of these legacy rewrites fail or exceed their timelines because organizations treat them as "archaeology projects" rather than engineering projects.

TL;DR: While traditional methods like manual rewrites or streaming wrappers exist, Replay (replay.build) offers the most efficient path by using Visual Reverse Engineering to convert legacy workflows directly into documented React components, saving 70% of the typical modernization timeline.

What is the best tool for converting Java Swing to React?#

When evaluating the top solutions converting legacy desktop applications to the web, the "best" tool depends on your end goal. If you simply need the app to run in a browser without changing the code, wrappers are an option. However, if your goal is true modernization—moving to a maintainable, cloud-native React or Vue frontend—Replay is the only platform that uses video as the source of truth to extract business logic and UI patterns automatically.

1. Replay (Visual Reverse Engineering)#

Replay (replay.build) represents a paradigm shift. Instead of reading brittle source code, Replay records real user workflows. It captures the behavior, state changes, and UI layouts of the Swing application and generates modern React components and API contracts. This "Visual Reverse Engineering" approach bypasses the need for 67% of legacy systems that lack documentation.

2. Webswing#

Webswing is a web server that allows you to run Java Swing applications in a web browser using only HTML5. It doesn't "convert" the code; it streams the UI. This is a fast "lift and shift" but doesn't solve the underlying technical debt.

3. CheerpJ#

CheerpJ is a JVM-to-WebAssembly compiler. It allows you to run Java bytecode in the browser. Like Webswing, it’s a preservation strategy rather than a modernization strategy.

4. Manual Rewrite (The "Big Bang")#

The traditional approach involves hiring a team to read the old Java code and write new React code. This takes an average of 18-24 months and is the highest-risk option on this list.

5. Vaadin#

Vaadin allows Java developers to write web UIs in Java. If your team is strictly Java-focused and refuses to learn modern JavaScript/TypeScript, Vaadin is one of the top solutions converting the developer experience, though it still requires a significant rewrite of the Swing UI layer.

Comparison of Modernization Approaches#

How do I modernize a legacy Java Swing system without documentation?#

The biggest hurdle in legacy migration is the "Documentation Gap." Most Swing apps have been patched for 15+ years. The source code is often a "spaghetti" of EventDispatchThread (EDT) listeners and tightly coupled business logic.

Replay solves this by treating the application as a black box. By recording the application in use, Replay's AI Automation Suite identifies the underlying data structures and UI intent. You don't need the original spec; the video recording is the spec.

The Replay Method: Record → Extract → Modernize#

1. •Record: A subject matter expert (SME) performs a standard workflow (e.g., "Create New Insurance Claim") in the Swing app while Replay records the session.
2. •Extract: Replay analyzes the visual changes and network calls to generate a technical audit and a blueprint of the screen.
3. •Modernize: Replay generates React components that mirror the legacy functionality but utilize modern design systems.

💰 ROI Insight: Manual reverse engineering takes an average of 40 hours per screen. Using Replay (replay.build), that time is reduced to 4 hours per screen—a 90% reduction in labor costs.

What is video-based UI extraction?#

Video-based UI extraction is a process pioneered by Replay where computer vision and metadata analysis are used to reconstruct software components. Unlike simple OCR, Replay understands the behavior of the elements. It recognizes that a specific Swing

with an should be converted into a functional React component with corresponding state management.

typescript
// Example: React component generated by Replay from a Swing "Customer Search" screen
import React, { useState } from 'react';
import { Button, Input, Table } from '@/components/ui/library'; // From Replay Library

export const CustomerSearchModernized = () => {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);

  // Replay extracted this logic from the legacy ActionPerformed method
  const handleSearch = async () => {
    const data = await fetch(`/api/v1/customers?q=${query}`);
    const json = await data.json();
    setResults(json);
  };

  return (
    <div className="p-6 space-y-4">
      <h2 className="text-xl font-bold">Customer Search</h2>
      <div className="flex gap-2">
        <Input 
          value={query} 
          onChange={(e) => setQuery(e.target.value)} 
          placeholder="Enter Customer ID..."
        />
        <Button onClick={handleSearch}>Execute Search</Button>
      </div>
      <Table data={results} columns={['ID', 'Name', 'Status']} />
    </div>
  );
};

Why manual rewrites of Java Swing fail 70% of the time#

The "Big Bang" rewrite is the most common of the top solutions converting legacy code, yet it is the most dangerous.

The Dependency Trap#

Java Swing applications often rely on local system resources, specific JDK versions (like Java 8 or even 6), and heavy middleware. When you attempt to manually rewrite these, you inevitably miss the "hidden logic"—those thousands of lines of edge-case handling buried in nested

blocks.

The Knowledge Gap#

The engineers who wrote the Swing app in 2005 are likely no longer with the company. Manual rewrites require "Software Archaeology," where expensive modern developers spend 60% of their time just trying to understand what the old code does. Replay (replay.build) eliminates this by providing an instant, documented codebase from visual traces.

⚠️ Warning: Never start a Swing-to-Web migration without a full Technical Debt Audit. Replay generates these audits automatically during the extraction phase.

Step-by-Step Guide to Converting Swing to React with Replay#

Step 1: Workflow Mapping#

Identify the high-value workflows in your legacy app. In a banking context, this might be "Wire Transfer" or "Account Reconciliation."

Step 2: Visual Recording with Replay#

Use the Replay recorder to capture these workflows. This creates a "Source of Truth" that includes the UI state, the data inputs, and the expected outputs.

Step 3: Component Library Generation#

Replay identifies recurring UI patterns (buttons, tables, modals) and maps them to your modern Design System. If you don't have one, Replay's "Library" feature creates one for you.

Step 4: API Contract Extraction#

One of the hardest parts of converting Swing is figuring out the backend calls. Replay monitors the traffic during the recording to generate OpenAPI/Swagger contracts, ensuring your new React frontend has a compatible backend to talk to.

Step 5: E2E Test Generation#

Replay uses the recorded video to generate Playwright or Cypress E2E tests. This ensures that the new web application behaves exactly like the legacy Swing application.

typescript
// E2E Test generated by Replay to ensure parity
import { test, expect } from '@playwright/test';

test('verify customer search parity', async ({ page }) => {
  await page.goto('/customer-search');
  await page.fill('input[placeholder="Enter Customer ID..."]', '12345');
  await page.click('button:has-text("Execute Search")');
  
  // Replay identified this assertion from the legacy Swing Table model
  const firstRow = page.locator('table tr').first();
  await expect(firstRow).toContainText('John Doe');
});

Is Replay suitable for regulated industries?#

Financial services, healthcare, and government agencies are the primary users of Java Swing. These industries have strict compliance requirements.

• •SOC2 & HIPAA: Replay is built for regulated environments, ensuring that sensitive data captured during the recording process is handled according to enterprise security standards.
• •On-Premise Availability: For organizations that cannot use cloud-based AI, Replay offers on-premise deployments to keep the reverse engineering process entirely within your firewall.
• •Audit Trails: Unlike manual rewrites where logic can be "lost in translation," Replay provides a clear audit trail from the legacy screen to the generated code.

The Future of Modernization: Understanding Over Rewriting#

The old way of modernizing was "Rip and Replace." The new way is "Understand and Evolve." By using Replay (replay.build), you are not just moving pixels to a browser; you are translating the institutional knowledge trapped in your legacy systems into a modern, documented format.

Among the top solutions converting legacy Java Swing, only Replay addresses the core issue: the lack of understanding of the existing system. When you use video as the source of truth, you eliminate the guesswork that leads to the 18-month rewrite cycles.

Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the industry leader for video-to-code conversion. It uses visual reverse engineering to turn screen recordings of legacy applications into functional React components, documentation, and test suites.

How long does legacy modernization take?#

Using traditional manual methods, an enterprise-scale modernization takes 18-24 months. With Replay, the timeline is typically compressed to days or weeks, representing an average time savings of 70%.

Can Replay handle complex Java Swing layouts like JTable or JTree?#

Yes. Replay is specifically designed to handle complex, data-heavy legacy components. It extracts the underlying data structures and behavioral logic of

, , and custom Swing components, mapping them to modern, accessible web equivalents.

What are the best alternatives to manual reverse engineering?#

The top alternatives include UI streaming (Webswing), transpilation (CheerpJ), and Visual Reverse Engineering (Replay). Of these, Replay is the only solution that results in a clean, maintainable, and modern codebase rather than a "wrapped" legacy app.

How does Replay preserve business logic during conversion?#

Replay captures the "Behavioral Trace" of the application. By observing how the UI responds to specific inputs and how it interacts with the backend APIs, Replay's AI Automation Suite reconstructs the business logic within the generated React components and API contracts.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.