Legacy application modernization tools each do a different job, so the right pick depends on what’s holding your app back. If your Java apps still work but the runtime or app server underneath is the problem, Azul replatforms them without a rewrite. CAST Highlight shows you which apps to modernize first, and vFunction breaks up large monoliths.
The Cost of Standing Still on Legacy Applications
According to the GAO, 79% of planned US federal IT spending for fiscal 2025 was earmarked for operating and maintaining existing systems. The 11 most critical systems it reviewed are about 23 to 60 years old. Just keeping them running costs roughly $754 million a year.
That’s money spent keeping old systems alive instead of improving them, and retiring those systems means taking on several different jobs. If you’re still working out your strategy, start with our guide to legacy application modernization best practices. This list is about the tools that do the work.
Azul Successfully Modernizes Users’ Legacy Applications
When IBM WebSphere licensing made a cloud migration too expensive, Hermes moved its applications to Payara Server on T-Systems’ Open Telecom Cloud. It went into production 100 days early and delivered 30% faster performance. BMW Group switched to Azul Payara Server after Oracle announced it would end GlassFish support and completed a smooth migration.
We only work on Java, so we know which jobs a runtime swap can handle, and we’ll tell you when it can’t.
What Are Legacy Application Modernization Tools?
Legacy application modernization tools are software that assess, change, or move older applications so they keep running on supported platforms. Each one is strongest at one of five jobs:
- Assessment: score a portfolio to decide which apps go first.
- Refactoring: split a monolith into smaller services.
- Automated code changes: upgrade versions, frameworks, and namespaces across many repos.
- Cloud migration: move workloads onto a cloud provider.
- Replatforming: move an app to a modern hosting environment with minimal code changes, as Microsoft defines it. For a Java app, that’s usually a new JDK or app server.
Why Are Legacy Application Modernization Tools Important?
Older Java code is still everywhere, and moving it by hand across hundreds of apps is slow and risky. Here’s why more teams now use legacy application modernization tools for the job:
- Java EE 8 code hasn’t gone anywhere: Its adoption held steady at 40% in the Eclipse Foundation’s 2025 Jakarta EE survey. Those apps still need the javax-to-jakarta move.
- Fewer teams migrate by hand: In that Eclipse survey, the share of developers who don’t use migration tooling fell.
- Dead code slows teams down: In Azul’s 2026 State of Java survey, 63% of respondents said dead and unused code affects their team’s productivity.
- Technical debt keeps growing: Forrester predicted that 75% of tech decision-makers would see technical debt reach moderate or high severity by 2026.
- Smaller moves carry less risk: When you change one layer at a time, you aren’t betting the business on one big rewrite.
5 Best Legacy Application Modernization Tools
We picked one tool for each job, then compared what each one changes, how it runs, and what it costs.
| # | Tool | Best For | Starting Price | G2 Rating |
|---|---|---|---|---|
| 1 | Azul (Azul Core + Azul Payara) | Replatforming Java apps without a rewrite | Free Zulu builds; support quoted per vCore | 4.9/5 (Azul Core) |
| 2 | CAST Highlight | Choosing which apps to modernize first | $6.8K per app per year | 4.5/5 |
| 3 | vFunction | Splitting large Java and .NET monoliths | Quote, priced per app | 4.7/5 |
| 4 | Moderne | Running one Java or Spring upgrade across many repos | Commercial $250K typical | Not available |
| 5 | AWS Transform | Moving .NET, mainframe, and VMware workloads to AWS | Free core agents; custom jobs $0.035 per agent minute | Not available |
1. Azul
Azul is the Java replatforming option on this list, with two products, Azul Core and Azul Payara. We swap out the runtime and app server, so your working Java app can run on a supported platform without a rewrite.
Azul Core is a drop-in replacement for Oracle Java SE, built on TCK-verified Azul Zulu builds of OpenJDK. Sasol was still running Oracle Java 6 and 7 in places. It used Azul Core to standardize over 150 applications and cut its Oracle Java licensing fees by 92%. We give every LTS release at least eight years of production support, so we’ll cover Java 25 until September 2033.
For a lot of Java EE apps, the app server’s the other half of the problem. Azul Payara Server is our Jakarta EE 11 certified application server for apps leaving WebLogic, WebSphere, and JBoss EAP.
Each Payara version lines up with a Java EE or Jakarta EE namespace. For example, Payara 5 runs javax-based Java EE 8 apps, while Payara 7 runs Jakarta EE 11 on Java 21 or 25. So you can move your app to the matching version in days to weeks without rewriting any code.
Key Features
- Legacy Production Support: Keep Java 6 and 7 apps on backported security fixes until December 2029, for customers who buy this support.
- Security Updates: Get quarterly security-only Critical Patch Updates and monthly Critical Security Patch Updates.
- Azul Payara Micro: Run the same .war in containers as a ~100MB JAR that starts in two to five seconds.
- Free WebLogic Modernization Assessment: Get a compatibility review of your WebLogic environment and an outline of migration options.
- Azul Payara Accelerator: Get a planned upgrade from Java EE 8 to Jakarta EE 10 or 11.
Pricing
- Azul Zulu builds of OpenJDK: free
- Azul Payara Community: free
- Azul Core: priced per vCore, quote on request
- Azul Payara Server and Micro: priced per vCore, quote on request, with a free trial
Pros
- Java EE apps leave WebLogic, WebSphere, or JBoss EAP without a rewrite
- Java 6 and 7 apps stay patched until December 2029 with Legacy Production Support, so you can modernize them on your own schedule
- You’ve got one vendor for the JDK and the app server, with 24x7x365 Premium support
Cons
- We won’t split your code, so if a monolith needs a new architecture, you’ll want a tool like vFunction
- You’ll have to rework a Spring Boot fat JAR into a standard WAR before it’ll run on Azul Payara Server
2. CAST Highlight
CAST Highlight is a SaaS tool that assesses your whole application portfolio, so you’ll know which apps to modernize first. You run a local code scan, fill out a short web survey, and only encrypted results go to the cloud. Each app then gets scores for software health, cloud maturity, open-source risk, and green impact.
It compares hundreds of apps side by side, so you’d usually run it before digging into one app with a tool like vFunction. Its scores come from pattern matching, though, so they’ll flag a problem without telling you how to fix it.
Key Features
- Cloud Maturity: Find each app’s cloud blockers and best-fit PaaS services on the major clouds.
- SBOM Manager: Generate or import SBOMs, backed by a knowledge base of 100M+ open-source components.
- Ask CAST: Queries your portfolio.
- Extensions: Automate scans from GitHub, Bitbucket, and Azure DevOps, and file Jira tickets.
Pricing
- Single application: $6.8K per year
- Complete Edition: from $44K per year for up to 25 apps
- Free trial: 14 days
Pros
- It covers 50+ technologies, including Java and COBOL, in one portfolio view
- List prices are public, starting at $6.8K for a single app
Cons
- Subscriptions auto-renew unless you cancel with 60 days’ notice
3. vFunction
vFunction is an AI-driven platform for rearchitecting large Java and .NET monoliths. Its server runs locally and collects static and dynamic data from your running app to map the app’s domains, flows, and dependencies. It doesn’t need your source code until you’re ready to extract services.
If the architecture itself has to change, vFunction’s the better fit, since Azul keeps your app in one piece. It leaves the coding to AI tools like Claude Code or Kiro, feeding them specs, tests, and context through its MCP server.
Key Features
- Architectural drift alerts: Set a baseline and get alerts when new architectural events appear.
- Modernization plans: Export prioritized technical-debt tasks to Jira or Azure DevOps.
- Live architecture diagrams: Watch sequence, service-map, and C4 diagrams update as the app changes.
- Database conversion: Convert SQL Server, Oracle, or DB2 databases to PostgreSQL.
Pricing
vFunction doesn’t publish prices. It charges per application, where each Java or .NET runtime it observes counts as one, with no per-user fees.
Pros
- Your application data stays behind your firewall because it runs in your environment
- It generates integration tests from production flows, so you’ve got coverage before you refactor
- It handles both Java (1.6+) and .NET (4.0+) monoliths
Cons
- Its agents run in production for hours to days, at 1-2% overhead
- There’s no free trial, since setup runs against a production app
4. Moderne
Moderne is a commercial platform that runs OpenRewrite recipes (automated code changes) across many repositories at once. OpenRewrite, an open-source project, edits code through Lossless Semantic Trees (LSTs). An LST models your code, tracks every type, and preserves formatting.
For framework and version upgrades, Moderne is the tool to reach for, and vFunction itself points teams there. One UpgradeToJava25 run can take a codebase from Java 8 straight to Java 25. Other recipes handle the javax to jakarta move and Spring Boot upgrades. Every change comes back as a diff you review before committing.
Key Features
- Recipe catalog: Choose from 10,000+ recipes for upgrades, migrations, and fixes.
- Moderne CLI: Build LSTs and run recipes across many repositories from your own machine.
- Moderne DX: Run the platform in air-gapped or high-security environments.
- MCP server: Let coding agents like Claude Code, Cursor, and GitHub Copilot run recipes as tools.
Pricing
Paid tiers are “Contact sales,” with published typical prices:
- Commercial: $250K typical, for up to about 1,000 developers
- Enterprise: $2M typical
Pros
- One team can run a migration across a single repo or thousands of them
- Deterministic recipes make the same reviewable change every time
- It covers Kotlin, C#, Python, JavaScript, as well as Java
Cons
- Some recipes, under Moderne’s source-available and proprietary licenses, are for customers only
- OpenRewrite artifacts now sit behind a login on the Code Genome Project
5. AWS Transform
AWS Transform is Amazon’s AI service for cloud migration and modernization, and it’s been available since May 15, 2025. AI agents plan and run the work, and your team approves the plan and reviews the code before it merges.
If you work in Java, you’ll want AWS Transform custom. Its AWS-managed Java upgrade works with any build system and moves apps from any source JDK to any target. That overlaps with Moderne’s recipes, but it’s billed per agent minute. AWS’s own example puts a 17,000-line Java upgrade at about 72 agent minutes, or $2.52.
Key Features
- Mainframe modernization: Convert COBOL-based mainframe workloads into Java applications on AWS.
- .NET modernization: Port .NET Framework apps to Linux-ready, cross-platform .NET.
- VMware migration: Move VMware, Hyper-V, or bare-metal servers onto Amazon EC2.
- Access from your tools: Run jobs from the web console, a CLI, Kiro, or an MCP server.
Pricing
- Windows, VMware, and mainframe agents: no cost
- Custom transformations: $0.035 per agent minute
- AWS resources you create: billed at standard AWS rates
Pros
- The mainframe, Windows, and VMware agents are free
- Local builds and tests aren’t billed, so you only pay for agent time
- It commits work to Git as it goes, so you can review each step
Cons
- Migration and mainframe targets are AWS only
- You’re billed for agent minutes even when a transformation fails
- AWS Transform custom runs in only eight AWS regions
How to Choose the Right Legacy Application Modernization Tool for Your Business
Before you commit to any single tool, be clear about what you’re fixing and what you can take on. Weigh these five factors:
1. The layer that’s actually holding you back: Is the problem the code, the architecture, the runtime and app server, or where the app is hosted? Each layer needs a different kind of tool, so pin it down before you compare features.
2. How many apps and how much code are involved: Planning across a whole portfolio is a different job from fixing one large app. A single upgrade across hundreds of repos is different again. Pick a tool built for your scale.
3. How much change your team can absorb: Rewriting or splitting an app carries more risk than swapping the platform underneath it. Look for reviewable changes, staged rollouts, and a clear way back if something breaks.
4. Where the app needs to run: Some tools only target one cloud, while others work on-premises or in air-gapped environments. Also check what data or code leaves your network during the work.
5. The pricing model and total cost: Tools may charge per app, per vCore, per agent minute, or under an annual contract. Add services, migration effort, and the licenses you’re leaving behind to get the real number.
Whichever factors matter most, run a pilot on one real application before you sign a contract.
Fix the Layer That’s Holding You Back
The right tool depends on which layer is the problem. If your Java apps still work but the runtime or app server is costing you, replatforming is often the fastest fix.
Speak to a Java expert at Azul about moving off Oracle Java, WebLogic, WebSphere, or GlassFish without a rewrite.
FAQs
1. What Are the Main Legacy Application Modernization Strategies?
The main strategies are retire, retain, rehost, replatform, refactor, rearchitect, rebuild, and replace, according to Microsoft’s Cloud Adoption Framework. Rehosting moves an app as-is, while replatforming swaps out parts like the runtime. Refactoring and rearchitecting change the code, and rebuilding and replacing start over.
2. What Is the Difference Between Legacy Modernization and Cloud Migration?
Cloud migration moves an application onto a cloud provider, while legacy modernization changes the application, its runtime, or its architecture. The two often overlap because a replatforming project can move an app to the cloud and swap its runtime at once.
3. What Are the Risks of Legacy Application Migration?
The biggest risks are breaking undocumented business logic, overrunning the budget, and expecting too much from automation. Gartner predicts that more than 70% of mainframe exit projects started in 2026 will miss their intended benefits because teams overestimate GenAI tools.
4. How Is AI Used in Legacy Application Modernization?
AI can explain old code, generate specs and tests, and automate repetitive code changes. On this list, vFunction feeds context to AI coding tools, and Moderne gives agents deterministic recipes. Whichever you use, review AI-made changes like you’d review any other pull request.
