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Java Compatibility

Requirement: Java 21

This project requires Java 21 (LTS). The CI workflow, devcontainer, and all scripts are configured for Java 21.

Why Not Java 8

The original project ran on Java 8. The upgrade to Java 21 was driven by:

  1. Datomic Pro — the actively maintained Datomic distribution targets Java 11+
  2. Pedestal 0.7 — uses Jetty 11, which requires Java 11+
  3. Security — Java 8 is past end of public updates
  4. Ecosystem — modern Clojure libraries increasingly require Java 11+

Datomic Free + Java 21: Incompatible

Datomic Free 0.9.5703 fails on Java 21 with an SSL handshake timeout. The peer cannot connect to the transactor. This is caused by changes in Java's TLS implementation that break ActiveMQ Artemis (Datomic's internal messaging). Datomic Free is unmaintained — no fix is available.

Solution: Migrate to Datomic Pro (com.datomic/peer), which supports Java 21.

Pedestal + Jetty Constraint

Pedestal 0.7.0 uses Jetty 11. Pedestal 0.7.1+ uses Jetty 12.

Jetty 12 removes ScopedHandler, causing NoClassDefFoundError when figwheel-main is running (figwheel-main's Ring adapter depends on Jetty 11 classes).

Current pin: Pedestal 0.7.0 with Jetty 11. This constraint is about Pedestal/Jetty compatibility, not Java version.

javax.servlet-api

Java 9+ removed javax.servlet from the default classpath. Added explicitly:

[javax.servlet/javax.servlet-api "4.0.1"]

CI Configuration

.github/workflows/continuous-integration.yml uses setup-java@v4 with temurin distribution, Java 21.