Add caveat to warning msg about Java agents

spark-common/src/main/java/me/lucko/spark/common/SparkPlatform.java

@@ -53,6 +53,7 @@
 import me.lucko.spark.common.tick.TickReporter;
 import me.lucko.spark.common.util.BytebinClient;
 import me.lucko.spark.common.util.Configuration;
+import me.lucko.spark.common.util.SparkStaticLogger;
 import me.lucko.spark.common.util.TemporaryFiles;
 import me.lucko.spark.common.ws.TrustedKeyStore;
 import net.kyori.adventure.text.Component;
@@ -116,6 +117,7 @@ public class SparkPlatform {
 
     public SparkPlatform(SparkPlugin plugin) {
         this.plugin = plugin;
+        SparkStaticLogger.setLogger(plugin::log);
 
         this.temporaryFiles = new TemporaryFiles(this.plugin.getPluginDirectory().resolve("tmp"));
         this.configuration = new Configuration(this.plugin.getPluginDirectory().resolve("config.json"));

spark-common/src/main/java/me/lucko/spark/common/util/ClassFinder.java

@@ -26,6 +26,7 @@
 import java.lang.instrument.Instrumentation;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.logging.Level;
 
 /**
  * Uses {@link Instrumentation} to find a class reference for given class names.
@@ -34,15 +35,27 @@
  */
 public class ClassFinder {
 
-    private final Map<String, Class<?>> classes = new HashMap<>();
+    private static boolean warned = false;
 
-    public ClassFinder() {
-        Instrumentation instrumentation;
+    private static Instrumentation loadInstrumentation() {
+        Instrumentation instrumentation = null;
         try {
             instrumentation = ByteBuddyAgent.install();
+            if (!warned && JavaVersion.getJavaVersion() >= 21) {
+                warned = true;
+                SparkStaticLogger.log(Level.INFO, "If you see a warning above that says \"WARNING: A Java agent has been loaded dynamically\", it can be safely ignored.");
+                SparkStaticLogger.log(Level.INFO, "See here for more information: https://spark.lucko.me/docs/misc/Java-agent-warning");
+            }
         } catch (Exception e) {
-            return;
+            // ignored
         }
+        return instrumentation;
+    }
+
+    private final Map<String, Class<?>> classes = new HashMap<>();
+
+    public ClassFinder() {
+        Instrumentation instrumentation = loadInstrumentation();
         if (instrumentation == null) {
             return;
         }

spark-common/src/main/java/me/lucko/spark/common/util/JavaVersion.java

@@ -0,0 +1,44 @@
+/*
+ * This file is part of spark.
+ *
+ *  Copyright (c) lucko (Luck) <[email protected]>
+ *  Copyright (c) contributors
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package me.lucko.spark.common.util;
+
+public class JavaVersion {
+    ;
+
+    private static final int JAVA_VERSION;
+    static {
+        JAVA_VERSION = parseJavaVersion(System.getProperty("java.version"));
+    }
+
+    private static int parseJavaVersion(String version) {
+        if (version.startsWith("1.")) {
+            // Java 8 and below
+            return Integer.parseInt(version.substring(2, 3));
+        } else {
+            // Java 9 and above
+            return Integer.parseInt(version.split("\\.")[0]);
+        }
+    }
+
+    public static int getJavaVersion() {
+        return JAVA_VERSION;
+    }
+}

spark-common/src/main/java/me/lucko/spark/common/util/SparkStaticLogger.java

@@ -0,0 +1,61 @@
+/*
+ * This file is part of spark.
+ *
+ *  Copyright (c) lucko (Luck) <[email protected]>
+ *  Copyright (c) contributors
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package me.lucko.spark.common.util;
+
+import java.util.logging.Level;
+
+/**
+ * Special logger for use by classes that don't easily have access to a
+ * {@link me.lucko.spark.common.SparkPlatform} instance.
+ *
+ * <p>This avoids warnings on platforms like Paper that get upset if plugins use
+ * {@link System#out} or {@link System#err}.</p>
+ */
+public enum SparkStaticLogger {
+    ;
+
+    private static Logger logger = null;
+
+    public synchronized static void setLogger(Logger logger) {
+        if (SparkStaticLogger.logger == null) {
+            SparkStaticLogger.logger = logger;
+        }
+    }
+
+    public static void log(Level level, String msg) {
+        Logger logger = SparkStaticLogger.logger;
+        if (logger == null) {
+            if (level.intValue() >= 1000) {
+                System.err.println(msg);
+            } else {
+                System.out.println(msg);
+            }
+            return;
+        }
+
+        logger.log(level, msg);
+    }
+
+    public interface Logger {
+        void log(Level level, String msg);
+    }
+
+}

spark-common/src/main/java/me/lucko/spark/common/util/SparkThreadFactory.java

@@ -22,11 +22,12 @@
 
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.logging.Level;
 
 public class SparkThreadFactory implements ThreadFactory {
 
     public static final Thread.UncaughtExceptionHandler EXCEPTION_HANDLER = (t, e) -> {
-        System.err.println("Uncaught exception thrown by thread " + t.getName());
+        SparkStaticLogger.log(Level.SEVERE, "Uncaught exception thrown by thread " + t.getName());
         e.printStackTrace();
     };