SNT Java Version Bridge

A lightweight Java 8-compiled JAR that provides a graceful upgrade path for SNT users on legacy Fiji installations.

Problem

SNT now requires Java 21, but many Fiji installations still run Java 8. The Fiji Updater has no mechanism (at least for now)to serve different JARs based on JVM version. It always delivers the latest artifact. When Java 8 users update, they pull the Java 21-compiled SNT JAR and encounter a cryptic UnsupportedClassVersionError the moment they try to use any SNT command.

Solution

This bridge JAR is compiled for Java 8 and registered as a SciJava Service. During Fiji's context initialization, all Service initialize() methods are called, and since this JAR targets Java 8, it loads successfully on any Fiji.

The bridge checks the running JVM version:

• Java 21+: Does nothing. SNT loads and works normally.
• Java 8 (or any version < 21): Displays a dialog explaining that SNT requires Fiji-Latest (Java 21), with clickable links to the download page and the Image.sc forum.

SNT menu entries may (will!?) still appear, but clicking them on Java 8 will fail. The startup dialog ensures users understand why before they encounter any error.

Implementation

  Fiji startup (Java 8)          Fiji startup (Java 21)
           │                              │
      Context init                   Context init
           │                              │
     Load Services                  Load Services
           │                              │
  ┌──────────-───────┐          ┌──────────────-───┐
  │ JavaVersionBridge│          │ JavaVersionBridge│
  │ (Java 8 bytecode)│          │ (Java 8 bytecode)│
  │ ✓ Loads fine     │          │ ✓ Loads fine     │
  │                  │          │                  │
  │ Detects Java 8   │          │ Detects Java 21  │
  │ → Shows dialog   │          │ → Returns (noop) │
  └───────────────-──┘          └───────────────-──┘
           │                              │
    User clicks SNT                 User clicks SNT
           │                              │
    UnsupportedClass-               SNT loads and
    VersionError (but               works normally
    user already knows
    why from the dialog)

Deployment

1. Build this project: mvn clean package
2. Upload snt-bridge-<version>.jar to the Neuroanatomy update site alongside the Java 21-compiled SNT JARs
3. Both JARs live on the same update site — the bridge handles version detection at runtime

NB: Do not change the compiler source/target from 1.8. The whole point of this JAR is that it must load on Java 8!

Updating the Required Version

If a future SNT release raises the minimum to, say, Java 25, edit the REQUIRED_JAVA_VERSION constant in JavaVersionBridge.java