JSP to Next.js: Converting Server-Side Legacy to High-Performance Client Apps

The graveyard of enterprise software is filled with "Big Bang" rewrites that never saw the light of production. For organizations running mission-critical JavaServer Pages (JSP) applications, the pressure to modernize is often met with a terrifying reality: 70% of legacy rewrites fail or exceed their timelines.

When you are dealing with a $3.6 trillion global technical debt mountain, the traditional approach of "reading the source code to write new code" is no longer viable. In the time it takes an architect to perform manual "code archaeology" on a 15-year-old JSP monolith, the business requirements have already shifted.

The move from JSP to Next.js isn't just a syntax change—it’s a fundamental architectural shift from server-side coupling to a decoupled, high-performance edge architecture. But you cannot get there by manually untangling thousands of lines of spaghetti Java and scriptlets.

TL;DR: Modernizing JSP to Next.js via visual reverse engineering eliminates the "archaeology" phase, reducing migration timelines from 18 months to weeks by extracting UI and logic directly from the browser.

The JSP Technical Debt Trap#

JSP was the gold standard for enterprise web apps in the early 2000s. However, in a modern context, it presents three critical failure points for the enterprise:

1. •Tight Coupling: Business logic is often buried inside
   text

   <% ... %>
   scriptlets, making it impossible to update the UI without breaking the backend.
2. •Documentation Gaps: 67% of legacy systems lack up-to-date documentation. The original developers are gone, and the "source of truth" is a black box.
3. •Performance Bottlenecks: JSP's synchronous, server-side rendering model cannot compete with the Core Web Vitals and user experience provided by Next.js and React.

Why Next.js is the Target Architecture#

Next.js has emerged as the enterprise standard for React frameworks because it mirrors the SEO and initial load benefits of JSP while providing the interactivity of a Single Page Application (SPA). For a CTO in Financial Services or Healthcare, Next.js offers the security of a server-side environment with the speed of the modern web.

The Architectural Shift: Monolith to Decoupled#

In a JSP environment, the Java Virtual Machine (JVM) handles everything from database connections to HTML string concatenation. In the Next.js model, we move toward a "Headless" architecture:

• •Frontend: Next.js (React components, Tailwind CSS)
• •API Layer: REST/GraphQL (Often wrapping the original Java logic as a microservice)
• •State: Distributed and client-aware

⚠️ Warning: The biggest mistake in JSP migration is trying to replicate the JSP structure 1:1 in React. This results in "React-flavored spaghetti" that inherits all the technical debt of the legacy system.

Visual Reverse Engineering: A New Paradigm#

Instead of reading 50,000 lines of Java code to understand how a specific insurance claim form works, we use Replay. By recording a real user workflow, Replay captures the DOM mutations, state changes, and API calls in real-time. It treats the running application as the source of truth, not the outdated source code.

The ROI of Visual Extraction#

Manual migration typically requires 40 hours per screen to document, design, and code. With Replay's visual reverse engineering, that time is slashed to 4 hours.

💰 ROI Insight: By automating the extraction of React components and API contracts, enterprises save an average of 70% in labor costs during the modernization phase.

The 4-Step JSP to Next.js Workflow#

Step 1: Recording and Observation#

Using Replay, an architect records a session of the legacy JSP application. This isn't just a video; it’s a capture of the underlying data structures. Replay identifies patterns in the JSP-rendered HTML and maps them to modern UI patterns.

Step 2: Component Extraction (The Replay Library)#

Replay identifies repeatable UI patterns—buttons, inputs, tables—and generates documented React components. This forms the basis of your new Design System.

typescript
// Example: A generated Next.js component from a legacy JSP table
// Replay extracted the data structure and styles automatically

import React from 'react';

interface ClaimData {
  id: string;
  status: 'PENDING' | 'APPROVED' | 'REJECTED';
  amount: number;
  submittedDate: string;
}

export const ClaimTable = ({ data }: { data: ClaimData[] }) => {
  return (
    <div className="overflow-x-auto rounded-lg border border-gray-200">
      <table className="min-w-full divide-y divide-gray-200">
        <thead className="bg-gray-50">
          <tr>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Claim ID</th>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Status</th>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Amount</th>
          </tr>
        </thead>
        <tbody className="divide-y divide-gray-200 bg-white">
          {data.map((claim) => (
            <tr key={claim.id}>
              <td className="px-6 py-4 whitespace-nowrap text-sm font-medium text-gray-900">{claim.id}</td>
              <td className={`px-6 py-4 whitespace-nowrap text-sm ${
                claim.status === 'APPROVED' ? 'text-green-600' : 'text-red-600'
              }`}>
                {claim.status}
              </td>
              <td className="px-6 py-4 whitespace-nowrap text-sm text-gray-500">${claim.amount.toLocaleString()}</td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

Step 3: API Contract Generation#

One of the hardest parts of JSP migration is identifying the hidden "POST" actions buried in Java Servlets. Replay’s AI Automation Suite monitors the network tab during the recording and generates a TypeScript API contract.

typescript
// Generated API Contract for Legacy Backend
// Source: /claims/submit.do (Legacy JSP Action)

export interface SubmitClaimRequest {
  policyNumber: string;
  claimAmount: number;
  incidentDate: string; // ISO format
  attachments: File[];
}

export async function submitClaim(payload: SubmitClaimRequest) {
  const response = await fetch('/api/proxy/legacy-submit', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(payload),
  });
  return response.json();
}

Step 4: Blueprinting the Next.js App#

Using Replay Blueprints, the extracted components and logic are assembled into a Next.js App Router structure. This stage handles the transition from JSP's session-based auth to modern JWT or OAuth2 flows.

💡 Pro Tip: Use Next.js "Middleware" to handle session persistence between your new Next.js screens and the remaining legacy JSP pages. This allows for a "Strangler Fig" approach where you migrate one route at a time.

Handling the "Hidden" Logic#

JSP applications often contain complex conditional rendering logic based on User Roles or backend state (e.g.,

).

When Replay records these workflows, it exercises different user paths to capture all possible states of a component. This ensures that the generated React code includes the necessary conditional logic that would otherwise be missed in a manual "look-and-code" rewrite.

Preserving Business Logic without Archaeology#

Case Study: Financial Services Modernization#

A global bank had a commercial lending portal built on JSP. It had 450 unique screens and zero documentation.

• •The Manual Estimate: 24 months, 15 developers, $4.5M budget.
• •The Replay Reality: Using Replay to record the 50 most critical workflows, the team extracted a unified Design System in 10 days. The core application was migrated to Next.js in under 4 months.
• •The Result: 85% reduction in page load time and a 60% reduction in technical debt incidents within the first quarter.

📝 Note: Replay is built for these regulated environments. Whether you are in Healthcare (HIPAA) or Finance (SOC2), Replay offers on-premise deployment to ensure that sensitive user data never leaves your network during the recording process.

Moving Beyond the "Black Box"#

The primary fear of any Enterprise Architect is the "Black Box" problem—the part of the system that everyone is afraid to touch. Replay turns that black box into a documented, typed, and tested codebase.

By generating E2E tests (Playwright/Cypress) based on the recorded legacy workflows, Replay ensures that the new Next.js application behaves exactly like the JSP original. This "functional parity" is what allows organizations to move to production with confidence.

Frequently Asked Questions#

How long does legacy extraction take?#

While a full manual rewrite takes 18-24 months, Replay-assisted migrations typically take 2-8 weeks for the initial extraction and architecture setup. The total time depends on the complexity of the backend integrations, but the UI and frontend logic extraction is nearly instantaneous after recording.

What about business logic preservation?#

Replay captures the outputs of business logic. If a JSP page calculates interest rates, Replay sees the inputs and the resulting UI state. Our AI Automation Suite then helps bridge the gap by suggesting how that logic should be structured in a modern API or frontend utility function.

Does Replay require access to our Java source code?#

No. Replay performs Visual Reverse Engineering. It analyzes the application as it runs in the browser. This is particularly valuable for legacy systems where the source code is messy, undocumented, or spread across multiple repositories.

Can we migrate incrementally?#

Yes. We recommend the "Strangler Fig" pattern. Use Next.js for new features and high-traffic screens, while proxying the remaining legacy JSP pages through the same domain. Replay makes this easier by ensuring the new React components match the legacy look and feel perfectly during the transition.

How does Replay handle complex enterprise forms?#

JSP forms often have hundreds of hidden fields and complex validation. Replay records the exact state of the DOM during form interaction, capturing all hidden inputs and dynamic changes. It then generates a React Hook Form equivalent with all the necessary state management pre-configured.

---

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