Stufe 6·Professionelles Java

JDBC & Databases

Drivers and connections, PreparedStatement and SQL injection, transactions, and pooling.

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Real applications remember data in databases. JDBC (Java Database Connectivity) is Java's standard API for talking to relational databases like PostgreSQL, MySQL, and others. It's the foundation every higher-level tool (including Hibernate and Spring Data) builds on.

The pieces

A database stores data in tables (rows and columns).
SQL is the language you use to query and modify it.
A JDBC driver is a library that lets Java speak to a specific database.
JDBC is the common API so your Java code looks the same regardless of database.

JDBC is like a universal remote

Different TVs (databases) speak different internal signals. A universal remote (JDBC) gives you one set of buttons; a small code chip for each brand (the driver) translates. You learn one interface and control any database.

Connecting and querying

String url = "jdbc:postgresql://localhost:5432/shop";

try (Connection conn = DriverManager.getConnection(url, user, password);
     Statement stmt = conn.createStatement();
     ResultSet rs = stmt.executeQuery("SELECT id, name FROM products")) {

    while (rs.next()) {
        int id = rs.getInt("id");
        String name = rs.getString("name");
        System.out.println(id + ": " + name);
    }
}   // try-with-resources closes everything automatically

ResultSet is a cursor over the returned rows; rs.next() advances to each row.

PreparedStatement - always use it for input

Never build SQL by gluing strings together with user input. That opens the door to SQL injection, one of the most dangerous security flaws. Use a PreparedStatement with ? placeholders instead:

// DANGEROUS - never do this
String sql = "SELECT * FROM users WHERE name = '" + input + "'";  // 💥 injectable

// SAFE - parameters are sent separately, never mixed into the SQL text
String sql = "SELECT * FROM users WHERE name = ?";
try (PreparedStatement ps = conn.prepareStatement(sql)) {
    ps.setString(1, input);       // safely bound
    ResultSet rs = ps.executeQuery();
}

SQL injection is a top security risk

If input like ' OR '1'='1 is concatenated into SQL, an attacker can read or destroy your whole database. A PreparedStatement keeps data separate from the query structure, making injection impossible. Always use it for any value that isn't a hard-coded constant.

Inserting and updating

String sql = "INSERT INTO products (name, price) VALUES (?, ?)";
try (PreparedStatement ps = conn.prepareStatement(sql)) {
    ps.setString(1, "Keyboard");
    ps.setDouble(2, 49.99);
    int rows = ps.executeUpdate();     // returns number of rows affected
}

Transactions - all or nothing

A transaction groups operations so they all succeed or all fail together - essential for correctness (imagine transferring money between accounts).

A transaction is like a bank transfer

Moving $100 from A to B is two steps: subtract from A, add to B. If the power fails between them, the money must not vanish. A transaction guarantees both happen or neither does - the account is never left inconsistent.

conn.setAutoCommit(false);            // start a transaction
try {
    debit(conn, accountA, 100);
    credit(conn, accountB, 100);
    conn.commit();                    // both succeeded - make it permanent
} catch (SQLException e) {
    conn.rollback();                  // something failed - undo everything
}

Connection pooling

Opening a database connection is expensive. Production apps use a connection pool (like HikariCP) that keeps a set of open connections ready to reuse - just like the thread pool idea from the concurrency stage.

You'll rarely write raw JDBC

JDBC is the foundation, but in real projects you'll usually use higher-level tools - Spring's JdbcTemplate, or an ORM like Hibernate (next lessons) - that remove the boilerplate. Understanding JDBC makes those tools far less mysterious.

Quick check

Why should you use a PreparedStatement instead of concatenating user input into SQL?

Key takeaways

JDBC is Java's standard API for relational databases; a driver adapts it to a specific database.
Use Connection, Statement/PreparedStatement, and ResultSet; try-with-resources closes them safely.
ALWAYS use PreparedStatement with ? placeholders for input to prevent SQL injection.
executeQuery reads (ResultSet); executeUpdate writes (returns affected row count).
Transactions (setAutoCommit(false), commit, rollback) make grouped operations all-or-nothing.
Production apps use a connection pool (e.g. HikariCP) to reuse expensive connections.

Before building bigger apps, you need to know they work. Next: testing.