More "semantic-versioning" compatible check for back-end schema

Summary

This MR refactors the checks for the mysql schema version.

Test Environment

Just put different values for MYSQL_SCHEMA_VERSION.

Check List for the Author

• All automated tests pass
• Reference related Issues
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.

src/main/java/org/caosdb/server/database/backend/implementation/MySQL/DatabaseConnectionPool.java

@@ -175,6 +175,15 @@ class DatabaseConnectionPool {
     logger.debug("####################");
   }
 
+  /**
+   * Check the version of the SQL server's database.
+   *
+   * <p>Current behaviour: Major versions must match and the database's minor version must be at
+   * least the expected minor version as defined in {@link
+   * ServerProperties.KEY_MYSQL_SCHEMA_VERSION}.
+   *
+   * <p>@todo Patch versions are not handled yet.
+   */
   private static void checkVersion(final Connection con)
       throws SQLException, ConnectionException, CaosDBException {
     try {
@@ -186,9 +195,9 @@ class DatabaseConnectionPool {
       final String expected_minor = expected_version[1];
 
       con.setReadOnly(false);
-      final PreparedStatement prepareStatement = con.prepareStatement("SELECT CaosDBVersion()");
+      final PreparedStatement prepared = con.prepareStatement("SELECT CaosDBVersion()");
       try {
-        final ResultSet executeQuery = prepareStatement.executeQuery();
+        final ResultSet executeQuery = prepared.executeQuery();
         if (executeQuery.next()) {
 
           final String actual_version = executeQuery.getString(1).toLowerCase();
@@ -200,13 +209,13 @@ class DatabaseConnectionPool {
             if (Integer.parseInt(actual_major.replace("v", ""))
                 < Integer.parseInt(expected_major.replace("v", ""))) {
               logger.error(
-                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the mysql backend.\n\n",
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the MySQL backend.\n\n",
                   expected_major,
                   expected_minor,
                   actual_version);
             } else {
               logger.error(
-                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the server.\n\n",
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the CaosDB server.\n\n",
                   expected_major,
                   expected_minor,
                   actual_version);
@@ -216,7 +225,7 @@ class DatabaseConnectionPool {
           if (!Objects.equal(actual_minor, expected_minor)) {
             if (Integer.parseInt(actual_minor) < Integer.parseInt(expected_minor)) {
               logger.error(
-                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the mysql backend.\n\n",
+                  "Version of the MySQL/MariaDB schema is incompatible.\n\tExpected: {}.{}\n\tActual: {}\nPlease upgrade the MySQL backend.\n\n",
                   expected_major,
                   expected_minor,
                   actual_version);
@@ -225,7 +234,7 @@ class DatabaseConnectionPool {
           }
         } else {
           logger.error(
-              "Version of the MySQL schema could not be determined. This probably means, that the version is too old.\n\tExpected: {}.{}\n\tActual: unknown\nPlease upgrade the mysql backend.\n\n",
+              "Version of the MySQL schema could not be determined. This probably means that the version is too old.\n\tExpected: {}.{}\n\tActual: unknown\nPlease upgrade the MySQL backend.\n\n",
               expected_major,
               expected_minor);
           System.exit(1);