Search SL4J JDK provider in our own classpath

Using the previous behavior it would look for the service file and provider
in the server jar. First it requires relocating the service file to our JDK
provider and let the service only be queryable from our plugin jar.

Fixes #668

Search SL4J JDK provider in our own classpath

Using the previous behavior it would look for the service file and provider
in the server jar. First it requires relocating the service file to our JDK
provider and let the service only be queryable from our plugin jar.

Fixes #668

core/src/main/java/com/github/games647/fastlogin/core/CommonUtil.java

@@ -70,7 +70,24 @@ public class CommonUtil {
         return new String(chars);
     }
 
-    public static Logger createLoggerFromJDK(java.util.logging.Logger parent) {
+    /**
+     * This creates a SLF4J logger. In the process it initializes the SLF4J service provider. This method looks
+     * for the provider in the plugin jar instead of in the server jar when creating a Logger. The provider is only
+     * initialized once, so this method should be called early.
+     *
+     * The provider is bound to the service class `SLF4JServiceProvider`. Relocating this class makes it available
+     * for exclusive own usage. Other dependencies will use the relocated service too, and therefore will find the
+     * initialized provider.
+     *
+     * @param parent JDK logger
+     * @return slf4j logger
+     */
+    public static Logger initializeLoggerService(java.util.logging.Logger parent) {
+        // set the class loader to the plugin one to find our own SLF4J provider in the plugin jar and not in the global
+        ClassLoader oldLoader = Thread.currentThread().getContextClassLoader();
+
+        ClassLoader pluginLoader = CommonUtil.class.getClassLoader();
+        Thread.currentThread().setContextClassLoader(pluginLoader);
         try {
             parent.setLevel(Level.ALL);
 
@@ -82,6 +99,9 @@ public class CommonUtil {
             parent.log(Level.WARNING, "Cannot create slf4j logging adapter", reflectEx);
             parent.log(Level.WARNING, "Creating logger instance manually...");
             return LoggerFactory.getLogger(parent.getName());
+        } finally {
+            // restore previous class loader
+            Thread.currentThread().setContextClassLoader(oldLoader);
         }
     }