Data - JDBC

Why this step matters

JDBC is the low-level foundation behind many higher-level data libraries. Understanding it helps debug connection issues, SQL behavior, and transaction boundaries.

Core components

• DataSource: connection factory/pool entry point
• Connection: DB session
• PreparedStatement: parameterized SQL
• ResultSet: query result rows

Basic query example

String sql = "SELECT id, email FROM users WHERE id = ?";

try (Connection con = dataSource.getConnection();
     PreparedStatement ps = con.prepareStatement(sql)) {

    ps.setLong(1, 42L);

    try (ResultSet rs = ps.executeQuery()) {
        if (rs.next()) {
            long id = rs.getLong("id");
            String email = rs.getString("email");
            System.out.println(id + " " + email);
        }
    }
}

Why prepared statements matter

Prepared statements:

• prevent SQL injection by separating query and values
• improve reuse of execution plans

Never build SQL with user values through string concatenation.

Transactions with JDBC

try (Connection con = dataSource.getConnection()) {
    con.setAutoCommit(false);
    try {
        // execute multiple statements
        con.commit();
    } catch (Exception e) {
        con.rollback();
        throw e;
    }
}

Mapping results

Map each row to a domain object or DTO. Keep mapping code explicit and testable.

record UserRow(long id, String email) {}

Common mistakes

• not closing resources (Connection, Statement, ResultSet)
• dynamic SQL concatenation with unsafe values
• weak transaction handling and missing rollback
• silently swallowing SQL exceptions

Takeaway

1. JDBC gives you precise control and debugging clarity
2. Use PreparedStatement by default
3. Manage transactions and resource closing carefully
4. Keep result mapping explicit and maintainable