Spring Boot 3.5 EOL: Planning a Spring Boot 3 → 4 Migration Before End of Life

As Spring Boot 3.5 end-of-life (EOL) approaches, many organizations are realizing that applications they rely on every day will soon stop receiving official security updates.

Once Spring Boot reaches end of life, new vulnerabilities are no longer patched, increasing security exposure and compliance risk. For many teams, the assumed solution is a Spring Boot migration — but migrating from Spring Boot 3 to Spring Boot 4 is rarely a simple dependency upgrade.

To help teams plan realistically before Spring Boot 3.5 EOL becomes a hard deadline, I built the Spring Boot 3 → 4 Migration Estimator. It’s a practical tool that gives application owners early insight into how long a migration is likely to take, enabling more informed decision-making.

This post explains what the estimator is, how to use it, and why it matters as Spring Boot end-of-life approaches. To use the estimator, click here.

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What the Spring Boot 3 → 4 Migration Estimator Is

The Spring Boot 3 → 4 Migration Estimator is a planning tool for teams preparing for Spring Boot 3 end-of-life.

Rather than treating the upgrade as a version bump, the estimator models the real engineering effort behind a Spring Boot migration, including:

• Application size and codebase complexity
• Number of Spring Boot applications affected by Spring Boot 3.5 EOL
• Team velocity and available engineering capacity
• Use of automated migration tools such as OpenRewrite
• Direct usage of Spring projects (Data, Security, Cloud, Web, Batch, Integration, and more)
• Mandatory baseline upgrades introduced by Spring Boot 4
  
  • JDK upgrades
  • Servlet container upgrades
  • Hibernate, Kotlin, JUnit, and Jackson version changes

The result is a range of weeks, reflecting uncertainty, risk, velocity, and the realities of production migrations.

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What the estimator is not

This tool is intentionally transparent. It is not:

• ❌ A guaranteed delivery timeline
• ❌ A fixed migration plan
• ❌ A replacement for hands-on technical analysis

What it does provide is clarity early in the planning process:

Is this Spring Boot migration likely to take weeks — or many months?

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How to Use the Spring Boot Migration Estimator

The estimator is designed to be fast, opinionated, and useful even when details are incomplete.

Step 1: Define your Spring Boot migration scope

Start by describing your environment:

• How many Spring Boot applications are impacted by Spring Boot 3.5 EOL?
• How large are those applications on average?
• How many developers can realistically work on the migration?
• Will you use automated migration tools to reduce effort?

Default values are preselected based on common Spring Boot upgrade patterns, allowing teams to generate a realistic estimate quickly.

Step 2: Identify Spring project dependencies

Next, select the Spring projects your applications depend on directly:

• Spring Data
• Spring Security
• Spring Cloud
• Spring Web
• Spring Batch, Integration, Retry, and others

Each additional Spring dependency increases migration complexity, testing scope, and the likelihood of breaking changes.

If your applications use additional Spring modules, the Other option allows you to account for that complexity.

Step 3: Account for baseline upgrades and breaking changes

Spring Boot 4 introduces mandatory platform changes that frequently represent the bulk of migration risk and effort.

The estimator explicitly models work related to:

• JDK upgrades (17 → 21 → 25)
• Servlet container upgrades (Tomcat 11, Jetty 12.3, Servlet 6.1)
• Replacing Undertow, which is not compatible with Spring Boot 4
• Hibernate 7.1+ upgrades
• Kotlin 2.2+ upgrades
• JUnit 6.0+ upgrades
• Mandatory Jackson 3 upgrades

These are common sources of test failures, production issues, and security vulnerabilities during rushed Spring Boot migrations.

Step 4: Interpret results against Spring Boot end-of-life

The estimator produces a migration timeline range in weeks, adjusted for:

• Application count
• Team velocity
• Automation usage

It also compares your projected completion window against the June 30, 2026 Spring Boot 3 end-of-life deadline, clearly indicating whether your current plan finishes before or after official support ends.

Built-in tooltips explain assumptions and help teams understand why certain inputs increase migration time.

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Why Spring Boot 3.5 EOL Planning Matters

After Spring Boot reaches end of life:

• Newly discovered vulnerabilities remain unpatched
• Security findings accumulate
• Compliance requirements become harder to meet
• Emergency, high-risk migrations become more likely

Too often, teams underestimate the effort required for a Spring Boot migration until time is already running out.

The goal of this estimator is to surface that reality early — while safer options still exist.

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When Migrating Before EOL Isn’t the Right Option

For many organizations, the estimator reveals a difficult truth: completing a Spring Boot 3 → 4 migration before EOL is unrealistic without unacceptable risk.

That doesn’t mean continuing to run vulnerable software is the only alternative.

HeroDevs provides commercial end-of-life support for Spring Boot 3, helping organizations:

• Receive ongoing security patches after Spring Boot 3.5 EOL
• Reduce exposure to known and emerging vulnerabilities
• Maintain compliance requirements
• Avoid rushed, high-risk Spring Boot migrations
• Plan upgrades on a business-aligned timeline

This approach allows teams to stay secure while maintaining control over when — and how — they migrate.

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Stay Secure After Spring Boot 3.5 EOL

Not ready to migrate to Spring Boot 4 yet?
Spring Boot 3.5 EOL doesn’t have to mean running vulnerable applications. HeroDevs provides commercial Spring Boot 3 end-of-life support, including security patches and vulnerability remediation, so you can upgrade on your own timeline.

→ Learn more about HeroDevs Spring Boot 3 EOL Support

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Make Spring Boot End-of-Life a Planned Event

Spring Boot 3.5 end-of-life doesn’t need to trigger a crisis.

Whether you migrate immediately to Spring Boot 4 or extend Spring Boot 3 safely with HeroDevs support, the most important step is making the decision early — with realistic expectations and a clear understanding of risk.

The Spring Boot 3 → 4 Migration Estimator exists to help teams do exactly that.

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