Spring MVC Legacy Migration: Extracting UI Logic for Micro-Frontend Architectures

Your Spring MVC monolith is a fossilized layer of JSPs, JSTL tags, and server-side state that prevents your team from moving at the speed of modern business. When every UI change requires a full redeploy of a 10-year-old WAR file, you aren't just dealing with technical debt—you’re facing an existential threat to your agility. The path forward is clear: you need to decouple the frontend into a Micro-Frontend (MFE) architecture. However, the manual process of spring legacy migration extracting logic from deeply nested JSP files is where most projects die.

Industry experts recommend a "Strangler Fig" approach to modernization, but the bottleneck remains the same: documentation. According to Replay’s analysis, 67% of legacy systems lack documentation, leaving architects to guess how complex server-side validation and session logic actually function. To move from a monolithic Spring MVC application to a modular React-based MFE, you need a repeatable framework for extraction.

TL;DR: Migrating Spring MVC to Micro-frontends requires decoupling server-side rendering from business logic. By using Replay for visual reverse engineering, teams can reduce the extraction time per screen from 40 hours to just 4 hours. This guide covers the extraction of UI logic, the transition to RESTful APIs, and the implementation of React-based components.

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The Strategic Necessity of Spring Legacy Migration Extracting#

The global technical debt bubble has reached $3.6 trillion, and a significant portion of that is locked within Java-based web applications. In a traditional Spring MVC setup, the UI is tightly coupled to the

object. Logic is scattered across files, custom tag libraries (), and Spring Controllers.

Video-to-code is the process of capturing user interactions within a legacy application and automatically generating production-ready frontend components and documentation.

When performing spring legacy migration extracting, you are essentially performing a "brain transplant." You are removing the rendering responsibility from the JVM and moving it to the browser (or a specialized Node.js edge). This allows for independent scaling, faster deployment cycles, and the ability to use modern design systems.

The Cost of Manual Extraction#

The traditional "rewrite from scratch" method is a recipe for disaster. Statistics show that 70% of legacy rewrites fail or exceed their timeline. The average enterprise rewrite timeline is 18 months—a period during which your competitors are shipping new features while you are simply trying to achieve parity with your old system.

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Step 1: Inventorying the UI with Visual Reverse Engineering#

Before you touch a single line of Java code, you must understand the current state. Most Spring MVC applications have "hidden" logic—JavaScript validation hidden in included headers, or conditional JSTL tags that only fire under specific user roles.

Instead of reading 100,000 lines of spaghetti code, use Replay to record real user workflows. By recording the application in action, you generate a visual map of every state, transition, and data requirement.

Identifying Component Boundaries#

In a Micro-Frontend architecture, you don't want a single monolithic React app. You want domain-driven components. During the spring legacy migration extracting phase, look for:

1. •Shared Widgets: Navigation bars, footers, and sidebars.
2. •Domain Modules: Claims processing, User profile management, or Product catalogs.
3. •Cross-cutting Concerns: Authentication headers and notification systems.

Replay's Flows feature allows you to see exactly how data moves through these components, making it easier to define the API contracts you'll need later.

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Step 2: Decoupling the Spring Controller#

The core of spring legacy migration extracting is transforming your

into a . In the legacy world, your controller likely looks like this:

java
@RequestMapping(value = "/userProfile", method = RequestMethod.GET)
public ModelAndView getUserProfile(@RequestParam("id") String userId) {
    User user = userService.findById(userId);
    ModelAndView mav = new ModelAndView("userProfileView");
    mav.addObject("user", user);
    mav.addObject("roles", roleService.getRoles(user));
    return mav;
}

This is problematic because the

(JSP) is what dictates the data structure. To migrate, you must extract the data into a DTO (Data Transfer Object) and return JSON.

The Modernized API Approach#

Your new endpoint should be agnostic of the UI. This allows multiple Micro-frontends to consume the same data. For more on this, see our article on Legacy Modernization Strategies.

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Step 3: Extracting UI Logic into React Components#

Once the data is exposed via an API, the next step in spring legacy migration extracting is recreating the UI. This is where most time is lost. Developers typically look at a JSP, try to find the CSS, and then manually rebuild the HTML in React.

With Replay, this process is automated. The platform takes the recording of the Spring MVC app and generates documented React components that mirror the legacy UI's behavior but use modern best practices (Tailwind CSS, TypeScript, and functional components).

Example: Extracted React Component#

Below is an example of what a component looks like after being extracted from a legacy Spring JSP form. Note the use of TypeScript for type safety—something missing in the original JSP.

typescript
import React, { useState, useEffect } from 'react';

interface UserProfileProps {
  userId: string;
  onUpdate: (data: any) => void;
}

const UserProfile: React.FC<UserProfileProps> = ({ userId, onUpdate }) => {
  const [user, setUser] = useState<any>(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    // Replaces the legacy Spring ModelAndView data fetch
    fetch(`/api/v1/users/${userId}`)
      .then(res => res.json())
      .then(data => {
        setUser(data);
        setLoading(false);
      });
  }, [userId]);

  if (loading) return <div>Loading legacy profile...</div>;

  return (
    <div className="p-6 bg-white shadow rounded-lg">
      <h2 className="text-2xl font-bold mb-4">User Profile</h2>
      <form onSubmit={(e) => { e.preventDefault(); onUpdate(user); }}>
        <div className="mb-4">
          <label className="block text-gray-700">Username</label>
          <input 
            type="text" 
            value={user.username} 
            onChange={(e) => setUser({...user, username: e.target.value})}
            className="mt-1 block w-full border-gray-300 rounded-md shadow-sm"
          />
        </div>
        <button type="submit" className="bg-blue-600 text-white px-4 py-2 rounded">
          Save Changes
        </button>
      </form>
    </div>
  );
};

export default UserProfile;

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Step 4: Implementing the Micro-Frontend Shell#

After spring legacy migration extracting individual components, you need a way to orchestrate them. A common pattern is using Module Federation or a "Shell" application that loads different MFEs based on the route.

According to Replay's analysis, teams that use a component library approach during migration see a 70% increase in developer velocity in subsequent quarters. By building a Design System early in the extraction process, you ensure consistency across all migrated modules.

Routing and Integration#

The legacy Spring MVC app can continue to serve the main layout while you "strangle" specific routes. For example,

might still be handled by Spring MVC, but is proxied to your new React MFE.

typescript
// mfe-config.ts
export const mfeConfig = {
  profile: {
    endpoint: 'https://profile-mfe.enterprise.com/remoteEntry.js',
    scope: 'profile',
    module: './ProfileModule'
  },
  billing: {
    endpoint: 'https://billing-mfe.enterprise.com/remoteEntry.js',
    scope: 'billing',
    module: './BillingModule'
  }
};

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Step 5: Handling State and Session Migration#

The hardest part of spring legacy migration extracting is dealing with

. Legacy Spring apps often store user data, shopping carts, or multi-step form progress in the server's memory.

1. •Extract to Redis: Move session state from the JVM memory to a distributed cache like Redis.
2. •JWT Authentication: Transition from session cookies to JSON Web Tokens (JWT) to allow your MFEs to authenticate against multiple backend microservices.
3. •State Management: Use tools like React Query or Redux to manage the state that was previously handled by Spring's
   text

   RedirectAttributes
   .

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A Step-by-Step Framework for Spring Legacy Migration Extracting#

To ensure success, follow this architectural checklist:

1. Visual Audit#

Record every "happy path" and "edge case" workflow using Replay. This creates your source of truth. If the legacy app has 50 screens, you should have 50 recordings. This bypasses the need for non-existent documentation.

2. Component Library Generation#

Use the Replay Library to convert those recordings into a standardized component library. This ensures that the "New" app looks and feels like the "Old" app, reducing the need for extensive user retraining.

3. API Facade Layer#

Don't rewrite your entire backend at once. Create a REST facade over your existing Spring services. This allows the React MFE to consume data without knowing that the backend is still a monolith.

4. Incremental Deployment#

Deploy one MFE at a time. Use a load balancer (like Nginx or AWS ALB) to route specific paths to the new frontend. This minimizes risk and provides immediate value to stakeholders.

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Overcoming the "Documentation Gap"#

The biggest hurdle in spring legacy migration extracting is the 67% of systems that lack documentation. When you don't know why a specific JSP tag exists, you are afraid to remove it.

Visual Reverse Engineering solves this by documenting the behavior rather than the code. By observing the output of the Spring MVC rendering engine, Replay can infer the logic required to reproduce that UI in React. This shifts the focus from "What does the Java code say?" to "What does the user actually see and do?"

Industry experts recommend that for any application over 5 years old, you should assume the code comments are lies and the documentation is obsolete. The only truth is the running application.

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Frequently Asked Questions#

How do I handle custom JSTL tags during extraction?#

Custom JSTL tags often contain complex business logic or formatting rules. During spring legacy migration extracting, these should be converted into React "Helper" functions or specialized UI components. Replay identifies the rendered output of these tags, allowing you to recreate the visual result in React without having to manually parse the underlying Java tag library code.

Can I migrate from Spring MVC to React without a full rewrite?#

Yes, using the Strangler Fig pattern. You keep the Spring MVC monolith running and replace individual pages or sections with React components. By using Replay, you can automate the extraction of the UI layer, which is usually the most time-consuming part of the process, saving up to 70% of the migration time.

What happens to my server-side validation logic?#

Validation logic is often duplicated in the JSP (via JavaScript) and the Spring Controller (via JSR-303/Hibernate Validator). When extracting, you should move this logic to a shared validation schema (like Zod or Yup) in the frontend for immediate feedback, while maintaining the Java validation in your new REST API for security.

Is Micro-Frontend architecture right for every Spring migration?#

Not necessarily. If your application is small and managed by a single team, a monolithic React SPA might be simpler. However, for large enterprises with multiple teams working on different domains (e.g., Billing, Support, User Management), an MFE architecture allows for independent deployment cycles and prevents the "frontend monolith" problem.

How does Replay handle SOC2 and HIPAA requirements during migration?#

Replay is built for regulated environments. It offers SOC2 compliance, is HIPAA-ready, and provides on-premise deployment options. This ensures that even when recording sensitive financial or healthcare workflows for the purpose of spring legacy migration extracting, your data remains secure and compliant within your own infrastructure.

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Conclusion: The Path to Modernization#

The days of 24-month migration projects are over. The $3.6 trillion technical debt problem requires a new approach—one that leverages AI and visual reverse engineering to bypass the manual labor of the past. By focusing on spring legacy migration extracting through a visual lens, you can transform your aging Spring MVC monolith into a high-performance, Micro-Frontend architecture in weeks rather than years.

Stop digging through JSPs and start building the future. According to Replay's analysis, the move to automated extraction isn't just a technical preference; it's a financial necessity for any enterprise looking to survive the next decade of digital transformation.

Ready to modernize without rewriting? Book a pilot with Replay