Getting Started¶

Prerequisites¶

All runnable examples in this playbook require:

Java 25 (via SDKMAN: sdk install java 25-tem)
Maven 3.9+ (included as ./mvnw wrapper in each example)
Docker (for Testcontainers in integration tests)

Running an example¶

Each pattern has a self-contained Maven project under examples/:

git clone https://github.com/marvinrichter/jvm-modernisation-playbook
cd jvm-modernisation-playbook/examples/strangler-fig   # or branch-by-abstraction / acl

./mvnw spring-boot:run

Run the tests:

./mvnw verify

How the examples are structured¶

Each example contains:

Directory	Purpose
`src/main/java/de/marvinrichter/<pattern>/legacy/`	The legacy code you're migrating away from
`src/main/java/de/marvinrichter/<pattern>/newservice/`	The target hexagonal implementation
`src/main/resources/application.properties`	Feature flags and configuration
`src/test/`	Integration tests showing both old and new paths work

Pattern selection guide¶

Not sure which pattern fits your situation?

Is the legacy code accessed via HTTP?
  YES → Strangler Fig
  NO  → Is it a single class/service with a clear interface?
          YES → Branch-by-Abstraction
          NO  → Does the new system use a different domain model?
                  YES → Anti-Corruption Layer
                  NO  → Branch-by-Abstraction + Anti-Corruption Layer (combine them)

Relationship to `spring-hexagonal-archetype`¶

The "after" state in every example matches the package structure generated by spring-hexagonal-archetype. If you're starting a greenfield service alongside your legacy system, use the archetype to generate it — then use the patterns here to route traffic into it.