Banking Modernization

Java JSP servlet application migrated to modern Spring Boot

Scenario:

The Challenge:
A legacy banking application built on JSP and Java servlets posed significant security risks and operational challenges. The code and complex manual deployment process threatened business continuity and regulatory compliance.

The Solution:
Rather than pursuing a high-risk “big bang” rewrite approach, I implemented an incremental modernization strategy, migrating features one-by-one from JSP and servlets to Spring Boot and Thymeleaf while maintaining operational continuity.

Legacy System:
– Deployed and analyzed complete legacy system.
– AWS EC2 with Tomcat and MySQL
– AWS Linux dependency resolution

AWS EC2 & Nginx:
– Incremental Migration Architechture on AWS Linux
– Nginx reverse proxy routing between legacy and modern pages
– Safe, gradual rollouts

CI/CD:
– Continuous Integration – Github Actions
– Continuous Delivery – Jenkins
– Custom deployment scripts

Modern System:
– Spring Boot 3 modernization
– MySQL to PostgreSQL migration
– Spring MVC with Thymeleaf

Enhanced Security:
– Legacy: Vulnerable Servlet based authentication
– Modern: Spring Security with JWT tokens
– Maintain existing user base

Dig into the details at the Github page.

Full Summary:

This project demonstrates enterprise-level legacy system modernization using the Strangler Fig pattern to incrementally migrate a banking application from JSP/Servlets to Spring Boot 3.x. Rather than a risky “big bang” rewrite, this approach maintains business continuity while delivering continuous value through feature-by-feature migration.

Why This Approach Matters

Zero business downtime during modernization process
Immediate security improvements addressing legacy servlet vulnerabilities and security gaps
Risk mitigation through feature flags and rollback capabilities
Continuous value delivery rather than waiting months/years for completion

Architecture & Migration Strategy

Current Infrastructure

┌─────────────────────────────────────────────────────────────┐
│                    AWS EC2 Instance                         │
├─────────────────────────────────────────────────────────────┤
│  ┌─────────────────┐    ┌─────────────────────────────────┐ │
│  │   Nginx Proxy   │    │     Migration Controller        │ │
│  │                 │    │   (Feature Flag Routing)        │ │
│  └─────────┬───────┘    └─────────────┬───────────────────┘ │
│            │                          │                     │
│  ┌─────────▼───────┐    ┌─────────────▼───────────────────┐ │
│  │ Legacy System   │    │    Modern System                │ │
│  │ JSP/Servlets    │    │    Spring Boot 3.x              │ │
│  │ Tomcat 10       │    │    Embedded Tomcat              │ │
│  └─────────────────┘    └─────────────────────────────────┘ │
├─────────────────────────────────────────────────────────────┤
│                     Database Layer                          │
│  ┌─────────────────┐              ┌─────────────────┐       │
│  │     MySQL       │     ────►    │   PostgreSQL    │       │
│  │  (Legacy Data)  │              │ (Modern Schema) │       │
│  └─────────────────┘              └─────────────────┘       │
└─────────────────────────────────────────────────────────────┘

Migration Phases

Phase 1: Legacy System Analysis

Deployed legacy JSP/Servlets application to production environment
Operational analysis through real-world usage and monitoring
Business logic documentation through black-box and code analysis
Infrastructure challenges identified and resolved

Phase 2: Database Migration

MySQL to PostgreSQL migration with zero-downtime strategy
Schema modernization while maintaining data compatibility
Dual-write validation to ensure data consistency
Performance optimization for banking transaction loads

Phase 3: CI/CD Integration

Jenkins pipeline integration for automated testing and deployment
Feature flag infrastructure for gradual traffic migration
Rollback automation for risk mitigation
Monitoring and alerting for migration health

Phase 4: Feature-by-Feature Migration

Authentication system (Spring Security + JWT)
Account management (Balance inquiry, transaction history)
Fund transfers (Internal and external bank transfers)
Complaint system (Customer service and resolution tracking)

Technical Implementation

Legacy System (Reference Baseline)

Framework: Traditional JSP/Servlets with manual MVC implementation
Application Server: Apache Tomcat 9
Database: MySQL 5.7 with banking transaction schema
Architecture: Servlet-based request handling with JSP views and manual routing

Modern System (Target Architecture)

Framework: Spring Boot 3.x with Spring Security
Database: PostgreSQL 15 with optimized schema design
API Design: RESTful services with comprehensive validation
Security: JWT authentication, SQL injection prevention, input sanitization
Testing: Comprehensive unit, integration, and migration verification tests

Key Technologies

Backend:
  - Java 17 (LTS)
  - Spring Boot 3.x
  - Spring Security 6
  - Spring Data JPA
  - PostgreSQL 15
  - MongoDB (complaints system)

Frontend:
  - Thymeleaf templates
  - Modern responsive CSS
  - Vanilla JavaScript
  - Bootstrap 5

Infrastructure:
  - AWS EC2
  - Nginx (reverse proxy)
  - Jenkins (CI/CD)
  - Docker (containerization)
  - Feature flags (gradual migration)