Add appropriate message for people running on Java 9

src/metabase/plugins.clj

 (ns metabase.plugins
+  "The Metabase 'plugins' system automatically adds JARs in the `./plugins/` directory (parallel to `metabase.jar`) to
+  the classpath at runtime. This works great on Java 7 and 8, but never really worked properly on Java 9; as of 0.29.4
+  we're planning on telling people to just add extra external dependencies with `-cp` when using Java 9."
   (:require [clojure.java.io :as io]
             [clojure.tools.logging :as log]
             [dynapath.util :as dynapath]
             [metabase
              [config :as config]
-             [util :as u]])
-  (:import [java.net URL URLClassLoader]))
+             [util :as u]]
+            [puppetlabs.i18n.core :refer [trs]]))
+
+;;; +----------------------------------------------------------------------------------------------------------------+
+;;; |                                                   Java 7 + 8                                                   |
+;;; +----------------------------------------------------------------------------------------------------------------+
 
 (defn- plugins-dir
-  "The Metabase plugins directory. This defaults to `plugins/` in the same directory as `metabase.jar`, but can be configured via the env var `MB_PLUGINS_DIR`."
+  "The Metabase plugins directory. This defaults to `plugins/` in the same directory as `metabase.jar`, but can be
+  configured via the env var `MB_PLUGINS_DIR`."
   ^java.io.File []
   (let [dir (io/file (or (config/config-str :mb-plugins-dir)
                          (str (System/getProperty "user.dir") "/plugins")))]
@@ -24,15 +32,48 @@
   (let [sysloader (ClassLoader/getSystemClassLoader)]
     (dynapath/add-classpath-url sysloader (.toURL (.toURI jar-file)))))
 
-(defn load-plugins!
+(defn dynamically-add-jars!
   "Dynamically add any JARs in the `plugins-dir` to the classpath.
-   This is used for things like custom plugins or the Oracle JDBC driver, which cannot be shipped alongside Metabase for licensing reasons."
+   This is used for things like custom plugins or the Oracle JDBC driver, which cannot be shipped alongside Metabase
+  for licensing reasons."
   []
   (when-let [^java.io.File dir (plugins-dir)]
-    (log/info (format "Loading plugins in directory %s..." dir))
+    (log/info (trs "Loading plugins in directory {0}..." dir))
     (doseq [^java.io.File file (.listFiles dir)
             :when (and (.isFile file)
                        (.canRead file)
                        (re-find #"\.jar$" (.getPath file)))]
-      (log/info (u/format-color 'magenta "Loading plugin %s... %s" file (u/emoji "🔌")))
+      (log/info (u/format-color 'magenta (str (trs "Loading plugin {0}... " file) (u/emoji "🔌"))))
       (add-jar-to-classpath! file))))
+
+
+;;; +----------------------------------------------------------------------------------------------------------------+
+;;; |                                                     Java 9                                                     |
+;;; +----------------------------------------------------------------------------------------------------------------+
+
+(defn- show-java-9-message
+  "Show a message telling people how to add external dependencies on Java 9 if they have some in the `./plugins`
+  directory."
+  []
+  (when-let [plugins-dir (plugins-dir)]
+    (when (seq (.listFiles plugins-dir))
+      (log/info
+       (trs "It looks like you have some external dependencies in your Metabase plugins directory.")
+       (trs "With Java 9 or higher, Metabase cannot automatically add them to your classpath.")
+       (trs "Instead, you should include them at launch with the -cp option. For example:")
+       "\n\n    java -cp metabase.jar:plugins/* metabase.core\n\n"
+       (trs "See https://metabase.com/docs/latest/operations-guide/start.html#java-versions for more details.")
+       (trs "(If you're already running Metabase this way, you can ignore this message.)")))))
+
+
+;;; +----------------------------------------------------------------------------------------------------------------+
+;;; |                                                 load-plugins!                                                  |
+;;; +----------------------------------------------------------------------------------------------------------------+
+
+(defn load-plugins!
+  "Dynamically add JARs if we're running on Java 7 or 8. For Java 9, where this doesn't work, just display a nice
+  message instead."
+  []
+  (if (u/is-java-9-or-higher?)
+    (show-java-9-message)
+    (dynamically-add-jars!)))

src/metabase/util.clj

@@ -943,3 +943,13 @@
   (first (keep-indexed (fn [i x]
                          (when (pred x) i))
                        coll)))
+
+
+(defn is-java-9-or-higher?
+  "Are we running on Java 9 or above?"
+  []
+  (when-let [java-major-version (some-> (System/getProperty "java.version")
+                                        (s/split #"\.")
+                                        first
+                                        Integer/parseInt)]
+    (>= java-major-version 9)))