[jOOQ/jOOQ#8781] Improve error msg when org.jooq.util class not found

Users keep running into issues when they don't see why their
org.jooq.util classes from the code generator cannot be found. This
change helps them better find the new package qualification of types in
the jOOQ-meta and jOOQ-codegen modules.

jOOQ-codegen/src/main/java/org/jooq/codegen/GenerationTool.java

@@ -875,12 +875,7 @@ public class GenerationTool {
             // [#2283] If no explicit class loader was provided try loading the class
             // with "default" techniques
             if (loader == null) {
-                try {
-                    return Class.forName(className);
-                }
-                catch (ClassNotFoundException e) {
-                    return Thread.currentThread().getContextClassLoader().loadClass(className);
-                }
+                return loadClass0(className);
             }
 
             // Prefer the explicit class loader if available
@@ -892,10 +887,22 @@ public class GenerationTool {
         catch (ClassNotFoundException e) {
             String message = null;
 
-            // [#7556]
+            // [#7556] [#8781]
             if (className.startsWith("org.jooq.util.")) {
+                String alternative = null;
+
+                alternativeLoop:
+                for (String pkg : new String[] { "org.jooq.meta", "org.jooq.meta.extensions", "org.jooq.codegen", "org.jooq.codegen.maven" }) {
+                    try {
+                        alternative = loadClass0(className.replace("org.jooq.util", pkg)).getName();
+                        break alternativeLoop;
+                    }
+                    catch (ClassNotFoundException ignore) {}
+                }
+
                 log.warn("Type not found", message =
-                    "Your configured org.jooq.util type was not found.\n"
+                    "Your configured " + className + " type was not found.\n"
+                  + (alternative != null ? ("Did you mean " + alternative + "?\n") : "")
                   + "Do note that in jOOQ 3.11, jOOQ-meta and jOOQ-codegen packages have been renamed. New package names are:\n"
                   + "- org.jooq.meta\n"
                   + "- org.jooq.meta.extensions\n"
@@ -919,6 +926,15 @@ public class GenerationTool {
         }
     }
 
+    private Class<?> loadClass0(String className) throws ClassNotFoundException {
+        try {
+            return Class.forName(className);
+        }
+        catch (ClassNotFoundException e) {
+            return Thread.currentThread().getContextClassLoader().loadClass(className);
+        }
+    }
+
     private static String trim(String string) {
         return (string == null ? null : string.trim());
     }