sql refactoring

db/src/main/java/org/zhdev/varioutil/sql/ConnectionProvider.java

@@ -0,0 +1,23 @@
+package org.zhdev.varioutil.sql;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+
+public interface ConnectionProvider {
+    ConnectionProvider NOT_ESTABLISHED = new NullProvider("Connection not established");
+    ConnectionProvider CLOSED = new NullProvider("Connection closed");
+
+    Connection getConnection();
+
+    default boolean isClosed() {
+        return true;
+    }
+
+    default void close() {
+        try {
+            getConnection().close();
+        } catch (SQLException e) {
+            throw new SqlException(e);
+        }
+    }
+}