#1467 Implement messages file migration

- Create messages updater called when a messages YML file is loaded
  - Work in progress
- Does not yet include changes to any message keys

src/main/java/fr/xephi/authme/message/updater/JarMessageSource.java

@@ -0,0 +1,57 @@
+package fr.xephi.authme.message.updater;
+
+import ch.jalu.configme.properties.Property;
+import fr.xephi.authme.ConsoleLogger;
+import fr.xephi.authme.util.FileUtils;
+import org.bukkit.configuration.file.FileConfiguration;
+import org.bukkit.configuration.file.YamlConfiguration;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+
+/**
+ * Returns messages from the JAR's message files. Favors a local JAR (e.g. messages_nl.yml)
+ * before falling back to the default language (messages_en.yml).
+ */
+public class JarMessageSource {
+
+    private final FileConfiguration localJarConfiguration;
+    private final FileConfiguration defaultJarConfiguration;
+
+    /**
+     * Constructor.
+     *
+     * @param localJarPath path to the messages file of the language the plugin is configured to use (may not exist)
+     * @param defaultJarPath path to the default messages file in the JAR (must exist)
+     */
+    public JarMessageSource(String localJarPath, String defaultJarPath) {
+        localJarConfiguration = localJarPath.equals(defaultJarPath) ? null : loadJarFile(localJarPath);
+        defaultJarConfiguration = loadJarFile(defaultJarPath);
+
+        if (defaultJarConfiguration == null) {
+            throw new IllegalStateException("Default JAR file '" + defaultJarPath + "' could not be loaded");
+        }
+    }
+
+    public String getMessageFromJar(Property<String> property) {
+        String key = property.getPath();
+        String message = localJarConfiguration == null ? null : localJarConfiguration.getString(key);
+        return message == null ? defaultJarConfiguration.getString(key) : message;
+    }
+
+    private static YamlConfiguration loadJarFile(String jarPath) {
+        try (InputStream stream = FileUtils.getResourceFromJar(jarPath)) {
+            if (stream == null) {
+                ConsoleLogger.debug("Could not load '" + jarPath + "' from JAR");
+                return null;
+            }
+            try (InputStreamReader isr = new InputStreamReader(stream)) {
+                return YamlConfiguration.loadConfiguration(isr);
+            }
+        } catch (IOException e) {
+            ConsoleLogger.logException("Exception while handling JAR path '" + jarPath + "'", e);
+        }
+        return null;
+    }
+}