The SciJava app-launcher provides an entry point into SciJava applications, notably Fiji. Its design synergizes well with the Jaunch native launcher, although Jaunch is by no means the only solution for the native executable side; jpackage or jDeploy should also work for example.

The app-launcher's most major functions are:

• Ensure the version of Java being used is appropriate to app's requirements. In case it isn't, offer to upgrade Java by downloading a more appropriate version.

• Load classes dynamically, without relying on the system classpath.

• Display a splash window while the application is starting up.

Supported configuration

The app-launcher uses system properties to configure its behavior:

• scijava.app.name: Name of the application being launched. Used in message dialogs if/when interacting with the user.

• scijava.app.splash-image: Resource path to an image for the splash window, to be loaded using ClassLoader#getResource. It can be either its own file or stored within a JAR file, as long as the resource is on the classpath.

• scijava.app.look-and-feel: Fully qualified class name of a Swing LookAndFeel to be set before any UI components are created or shown. This can be useful to ensure a consistent appearance between the app-launcher splash and dialog UI with any later application UI, as well as to improve UI behavior on HiDPI displays using smarter Look & Feels such as FlatLaf.

• scijava.app.java-root: directory containing "bundled" installations of Java. The Java.root() method reports this value if it points to a valid directory. The Java.check() method will look here (via Java.root()) for which JVMs are already present locally, and also unpack any newly downloaded JVM into this directory.

• scijava.app.java-links: URL of a plain text file containing links to desirable Java bundles. The format is <platform>=<url> for each platform (OS+architecture) of Java that you want to support. For example:

  linux-arm64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-linux_aarch64.tar.gz
  linux-x64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-linux_x64.tar.gz
  macos-arm64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-macosx_aarch64.tar.gz
  macos-x64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-macosx_x64.tar.gz
  windows-arm64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-win_aarch64.zip
  windows-x64=https://cdn.azul.com/zulu/bin/zulu21.36.17-ca-jdk21.0.4-win_x64.zip
  

  The exact naming is up to you, but for a Java distribution to be downloaded, the scijava.app.platform property must be set and match one of the keys indicated within the fetched remote resource.

• scijava.app.java-version-minimum: The minimum version of Java required by the application. It can be a standalone number like 11, in which case it is treated as a major version, or a dot-separated sequence of digits, which case version comparisons are done digit by digit (see Versions.compare). This value is used by Java.check() (via Java.minimumVersion()) to warn the user accordingly if the running Java version is not good enough.

• scijava.app.java-version-recommended: The minimum version of Java the application would prefer to use. Same syntax as java-version-minimum. This value is used by Java.check() (via Java.recommendedVersion()) to warn the user accordingly if the running Java version is not ideal.

Provenance

The SciJava app-launcher evolved from the ImageJ Launcher, which was a prior solution for launching Fiji.

The imagej-launcher's ClassLauncher supported a couple of Fiji/ImageJ-specific flags that this SciJava app-launcher does not:

-ijjarpath <path>: This flag provided automatic loading of plugins in $HOME/.plugins as well as from the value(s) of the ij1.plugin.dirs system property, when <path> was set to plugins. To accomplish an equivalent thing with the app-launcher, use something like:

-jarpath plugins:"$HOME"/.plugins:/path1:/path2:/path3

rather than:

-Dij1.plugin.dirs=/path1:/path2:/path3 -jarpath plugins

This works because the org.scijava.launcher.ClassLauncher's -jarpath handling splits on colons/semicolons.

-ijcp <path1:path2:...>: This was an undocumented feature, not used by normal Fiji/ImageJ launches. You can accomplish something similar using -cp <path1:path2:...> instead.