Initialization cleanup: use provider classes for Settings and DataSource

- Separate provider methods from other tasks performed on startup
  - No longer requires BukkitService to be instantiated manually
- Merge MetricsManager and Initializer into OnStartUpTasks

src/main/java/fr/xephi/authme/initialization/DataSourceProvider.java

@@ -0,0 +1,123 @@
+package fr.xephi.authme.initialization;
+
+import fr.xephi.authme.ConsoleLogger;
+import fr.xephi.authme.datasource.CacheDataSource;
+import fr.xephi.authme.datasource.DataSource;
+import fr.xephi.authme.datasource.DataSourceType;
+import fr.xephi.authme.datasource.FlatFile;
+import fr.xephi.authme.datasource.MySQL;
+import fr.xephi.authme.datasource.SQLite;
+import fr.xephi.authme.datasource.converter.ForceFlatToSqlite;
+import fr.xephi.authme.service.BukkitService;
+import fr.xephi.authme.settings.Settings;
+import fr.xephi.authme.settings.properties.DatabaseSettings;
+
+import javax.inject.Inject;
+import javax.inject.Provider;
+import java.io.File;
+import java.io.IOException;
+import java.sql.SQLException;
+
+/**
+ * Creates the AuthMe data source.
+ */
+public class DataSourceProvider implements Provider<DataSource> {
+
+    private static final String FLATFILE_FILENAME = "auths.db";
+    private static final int SQLITE_MAX_SIZE = 4000;
+
+    @Inject
+    @DataFolder
+    private File dataFolder;
+    @Inject
+    private Settings settings;
+    @Inject
+    private BukkitService bukkitService;
+
+    DataSourceProvider() {
+    }
+
+    @Override
+    public DataSource get() {
+        try {
+            return createDataSource();
+        } catch (Exception e) {
+            ConsoleLogger.logException("Could not create data source:", e);
+            throw new IllegalStateException("Error during initialization of data source", e);
+        }
+    }
+
+    /**
+     * Sets up the data source.
+     *
+     * @return the constructed datasource
+     * @throws ClassNotFoundException if no driver could be found for the datasource
+     * @throws SQLException           when initialization of a SQL datasource failed
+     * @throws IOException            if flat file cannot be read
+     */
+    private DataSource createDataSource() throws ClassNotFoundException, SQLException, IOException {
+        DataSourceType dataSourceType = settings.getProperty(DatabaseSettings.BACKEND);
+        DataSource dataSource;
+        switch (dataSourceType) {
+            case FILE:
+                File source = new File(dataFolder, FLATFILE_FILENAME);
+                dataSource = new FlatFile(source);
+                break;
+            case MYSQL:
+                dataSource = new MySQL(settings);
+                break;
+            case SQLITE:
+                dataSource = new SQLite(settings);
+                break;
+            default:
+                throw new UnsupportedOperationException("Unknown data source type '" + dataSourceType + "'");
+        }
+
+        dataSource = convertFlatfileToSqlite(dataSource);
+
+        if (settings.getProperty(DatabaseSettings.USE_CACHING)) {
+            dataSource = new CacheDataSource(dataSource);
+        }
+        if (DataSourceType.SQLITE.equals(dataSourceType)) {
+            checkDataSourceSize(dataSource, bukkitService);
+        }
+        return dataSource;
+    }
+
+    private void checkDataSourceSize(final DataSource dataSource, BukkitService bukkitService) {
+        bukkitService.runTaskAsynchronously(() -> {
+            int accounts = dataSource.getAccountsRegistered();
+            if (accounts >= SQLITE_MAX_SIZE) {
+                ConsoleLogger.warning("YOU'RE USING THE SQLITE DATABASE WITH "
+                    + accounts + "+ ACCOUNTS; FOR BETTER PERFORMANCE, PLEASE UPGRADE TO MYSQL!!");
+            }
+        });
+    }
+
+    /**
+     * Converts the data source from the deprecated FLATFILE type to SQLITE.
+     *
+     * @param dataSource the data source to convert if necessary
+     * @return the data source to use: the converted datasource (SQLite),
+     *         or the same data source if no conversion was performed
+     */
+    private DataSource convertFlatfileToSqlite(DataSource dataSource) {
+        if (DataSourceType.FILE == settings.getProperty(DatabaseSettings.BACKEND)) {
+            ConsoleLogger.warning("FlatFile backend has been detected and is now deprecated; it will be changed "
+                + "to SQLite... Connection will be impossible until conversion is done!");
+            FlatFile flatFile = (FlatFile) dataSource;
+            try {
+                SQLite sqlite = new SQLite(settings);
+                ForceFlatToSqlite converter = new ForceFlatToSqlite(flatFile, sqlite);
+                converter.execute(null);
+                settings.setProperty(DatabaseSettings.BACKEND, DataSourceType.SQLITE);
+                settings.save();
+                return sqlite;
+            } catch (Exception e) {
+                ConsoleLogger.logException("Error during conversion from Flatfile to SQLite", e);
+                throw new IllegalStateException(e);
+            }
+        }
+        return dataSource;
+    }
+}