Legacy Application Modernization Best Practices

Modernizing legacy applications might sound like a dry, box-ticking IT initiative, but let’s be real: for most dev teams, it’s more like pulling out the engine of a moving car and swapping it mid-traffic. Still, it is necessary and is part of every application’s lifecycle as systems inevitably age. As a result, integrations may break, security holes can start to creep in, and eventually, even the most battle-hardened enterprise Java monolith can feel more like a liability than an asset. 

Despite this preamble, legacy application modernization doesn’t have a pure technical exercise dreaded by teams. With the right approach, it becomes a high-return investment that brings your software stack up to speed with the pace of business, tech as well as user expectations. In this blog post, we cover what legacy application modernization is, its benefits and challenges as well as best practices to help you plan a successful modernization strategy for your enterprise Java systems.  

What Is Legacy Application Modernization?  

Legacy application modernization is the process of reworking outdated software to align with current infrastructure, code and technical standards, compliance requirements as well as business goals. By strategically updating an application at its core, modernization can help optimize performance, enhance security and improve maintainability.  

This activity can take different forms, depending on the objectives, needs and allocated resources. The top seven strategies (or “Rs”) are: replacing, retaining, retiring, rehosting, replatforming (what some would describe as “lift and shift”), rewriting and refactoring. These differ in how much they alter/maintain the original application. More precisely, a modernization project can be as lightweight as encapsulating or rehosting an application or part of it or as ambitious as a full rebuild. While the specifics may vary, the goal is one: to make your legacy systems stay relevant and run better. 

In effect, the different modernization strategies typically share some common traits. More precisely, a modernization project involves updating architectures (think: moving from monoliths to microservices), improving performance through optimization, tightening security to handle modern threats, and future-proofing the system so you don’t end up in the same outdated spot within six months. 

While development teams should spend their entire time modernizing an application, they should approach modernization with the same mindset they bring to building a robust CI/CD pipeline: deliberate and iterative. In fact, it’s important to remember that modernization itself is never really "done." It's a moving target, as the definition of “modern” keeps evolving. Today’s best practices might be tomorrow’s technical debt. Frameworks get deprecated, security standards tighten, and business needs morph. So, while it’s tempting to treat modernization like a one-off milestone, it’s far more effective (and sustainable) to treat it as an ongoing process. 

Why Should You Modernize Legacy Applications?  

There are multiple reasons why you should modernize a legacy application, including: 

• Business growth opportunities
• Security and compliance requirements
   

Business Growth Opportunities 

Let’s face it: legacy applications are often the quiet blockers behind stalled initiatives. Say your team wants to adopt Jakarta EE 10 features, such as enhanced dependency injection or improved security APIs, but your core application is still running on Java EE 8. You can't just drop in the new libraries and call it a day.  

The namespace shift from javax.* to jakarta.* alone breaks half your dependencies, and migrating isn't as simple as a search-and-replace. You're suddenly dealing with incompatible frameworks, unsupported libraries and custom code that hasn’t been touched since the early 2010s.  

Modernizing can help: 

• Align applications with evolving business processes and demands 

• Move faster with automation, microservices and containerized workflows 

• Integrate with new technologies (ML, real-time data pipelines, APIs, etc.) 

Case Study: Hermes transitioned its business-critical applications to the cloud using Payara Server Enterprise, resulting in faster performance (30% better) and cost reductions. 

Security and Compliance 

It’s not only about adopting the latest tools. Old apps are like abandoned buildings: plenty of ways for the bad guys to get in. Many legacy systems rely on deprecated libraries or outdated encryption methods, putting the entire organization at risk. Regulations like CRA, HIPAA or ISO standards aren’t getting looser, either. A modernization can therefore: 

• Close vulnerabilities present in outdated systems 

• Meet industry regulations and standards  

Consider BMW Group’s move: When Oracle discontinued commercial support for GlassFish, the company pivoted to Payara Server to maintain stability and compliance.  

App Modernization Common Challenges 

We’re not sugar coating it: application modernization can be tough. In fact, 79% of app modernization projects fail, according to a 2022 report. Common issues include: 

• Cost and compatibility: Updating software costs money and migrating from legacy applications may require updating multiple elements to ensure compatibility with every tool, integration and dependency you need. 

• Downtime and migration risks: Every developer dreads the moment a mission-critical app goes dark mid-migration. Data loss, misconfigurations, broken dependencies are all issues that can affect migration projects and the applications involved.  

Still, these aren’t reasons to delay a much needed legacy application modernization project. They’re reasons to prepare. 

Best Practices for Legacy Application Modernization  

With only about 21% of application modernization efforts considered successful, legacy application modernization can feel pretty intimidating. But if you dig into why most of these projects fall short, it usually comes down to one element: poor planning, which sets a shaky foundation from the start. In effect, the 2022 research report ‘Why App Modernization Projects Fail’ lists these as the top reasons cited by organizations: 

• Failure to accurately set expectations 

• Too expensive  

• Required organizational structure changes 

• Lack of intelligent tools 

• Inadequate skills or training  

• Too complex  

By adhering to the following suggested best practices, you can help make sure your modernization project is a successful one.  

1. Conduct a Comprehensive Assessment

You wouldn't renovate a house without checking the foundation first. The same goes for code. Execute a thorough analysis that includes: 

• Mapping out all system components: what stays, what goes and what needs replacing 

• Calculating ROI: Not every system needs a full rebuild. Sometimes rehosting does the trick. Know what you’ll gain and what it’ll cost. 

• Documenting dependencies: You’ll be surprised how many legacy applications rely on that one ancient EJB 2.1 bean or that obscure JAXB library. Don’t get blindsided. 

Think of this as your modernization playbook. It’ll save you time, stress, unexpected cost rises and unplanned fire drills later. 

2. Select the Right Modernization Strategy

There are seven “Rs” you can pick from to modernize your legacy application. Since these have different requirements and output, selecting one that matches your own needs and goals is key. Choose the right approach based on what you uncovered in your assessment. In particular, note that not every legacy system needs a total rewrite. At the same time, AI is changing how modernization can be done. Let’s have a look at some of the most common “Rs” companies pick and what they involve: 

Rehosting 

Rehosting means you are moving the entire application to a new environment. No major code changes, just a different home. This option is ideal for stable apps that just need a modern infrastructure, as it can: 

• Simply lift and shift to a better suited environment 

• Preserve existing functionality 

• Minimize initial investment in time, cost and human resources 

There is a risk of carrying over old inefficiencies. However, following a successful rehosting, organization can then plan future upgrades.  

Refactoring 

When refactoring, you tweak parts of the code to support a different runtime and framework. Through refactoring, you can optimize critical components: 

• Improve specific parts of the codebase 

• Enhance performance bottlenecks 

• Maintain core functionality  

This method is ideal for applications with significant technical debt but solid core functionality. However, it requires surgical precision to ensure a smooth transition and effective post-modernization operations.  

Rebuilding 

Typically rebuilding is the big one. You’re tearing down the old structure and building fresh. It’s a complete rewrite that preserves core functionality and business rules. Since you create new solutions, some experts don’t consider rebuilding a modernization strategy. As a result of it, you can: 

• Develop with modern architecture 

• Implement current best practices 

• Build for future scalability 

Rebuilding is resource-heavy, but often worth it when legacy applications are deeply flawed. 

How AI Is Changing Java Application Modernization

GenAI tools have the potential to shift the balance between rehosting, refactoring and rebuilding. What used to be lengthy and risky could be automated, partially rewritten, and iteratively modernized with AI-assisted tooling. For example:

• IDEs like IntelliJ and frameworks like OpenRewrite can assist with automated javax.* to jakarta.* migration

• LLMs are being piloted
  in enterprise codebases to rewrite services and identify dead code, accelerating modernization efforts

The result: rebuilding and rewriting may not necessarily be as complex as they used to be. AI-assisted rebuilds or microservice extraction are becoming more viable first-class modernization strategies.

3. Implement Cloud-Native Architecture

Cloud isn’t just a buzzword, and if you are planning a modernization, chances are you already have eyes on becoming cloud-native some time in the future. This is totally understandable, as it can support agile, scalable software. Here’s how to go about it: 

• Adopt modern, modular, cloud-native design patterns: 

• • Microservices may not be right for every application or every modernization project, but where they work, they shine  

• • More modern containerized solutions can help you scale resources automatically 

• • Containers and microservices can help you ensure resilient operations 

• Use tools for container management and orchestration: 

• • Deploy on Kubernetes (K8s) for robust management – and if you are not a K8s master, leverage a solution like Payara Cloud or Payara Qube 

• • Package applications in Docker containers 

• • Implement service discovery and load balancing 

• Choose cloud-optimized runtimes like Payara Platform: 

• • Leverage built-in cloud-native capabilities 

• • Supports MicroProfile, which is a set of enterprise Java APIs and technologies, designed to help with the challenge of building microservices architectures in Java  

• • Enable automated scaling and monitoring for effective resource utilization and application performance 

4. Prioritize Testing and Continuous Delivery

Testing shouldn’t be just a final checklist item during modernization. When you’re transforming a legacy application, every change introduces the risk of breaking long-standing (and often undocumented) behavior. The older the system, the more likely it is to contain brittle, business-critical logic that’s been patched over for years without test coverage. 

Here’s what effective testing looks like during modernization to help ensure reliable deployments: 

• Establish a baseline 

• Adopt and incremental approach and create CI/CD pipelines 

• Monitor deployment success 

• Enable quick rollbacks to address issues promptly if anything fails 

5. Monitor and Optimize Post-Migration

A legacy application modernization project is not done just because the application is live on a shiny new platform (or other). Monitoring and continuous improvement are your best allies to make sure everything is running smoothly and system efficiency is maximized: 

• Track and analyze metrics, logs and traces with key tools that give you a live pulse of your systems

• Analyze performance data: Maybe you overprovisioned Kubernetes resources. Maybe that new endpoint has a subtle memory leak. You won’t know unless you’re watching 

• Optimize resource usage 

• Iterate: modernization offers an opportunity to make sure you regularly improve your application

Conclusions 

Want to learn more about legacy application modernization? Check our guide now!

While typically considered a pure technical upgrade or a burden to an already strained development team, legacy application modernization represents a strategic shift that can improve developer productivity and profitability alike. Done right, it clears the path for innovation, reduces operational risk and makes applications easier to manage and interact with. 

Yes, the process is challenging, takes time and it never really ends. But when you are no longer stuck babysitting ancient services or duct-taping dependencies just to add a new feature and you can really focus on pure innovation, it all starts to feel worth it. Plus, you don’t have to do it alone. Services like Payara Accelerator exist specifically to guide teams through these complex modernization efforts. With the right support, that mountain becomes a roadmap and companies can successfully complete their modernizations in a timely manner.