JDBC sits between your application code and a database driver. Four key objects are involved in every database interaction:
| Object | Role |
|---|
DriverManager | Bootstrap class — creates Connection from a JDBC URL |
Connection | Session with the database; wraps a TCP connection |
Statement / PreparedStatement | Sends SQL to the server; PreparedStatement uses bind parameters |
ResultSet | Cursor over query results; advance with next() |
The driver is discovered automatically via the ServiceLoader mechanism since JDBC 4.0 — no Class.forName() call needed.
Add the JDBC driver for your database. PostgreSQL is used throughout this article:
<dependency>
<groupId>org.postgresql</groupId>
<artifactId>postgresql</artifactId>
<version>42.7.3</version>
</dependency>
For H2 in-memory testing: com.h2database:h2:2.2.224.
DriverManager.getConnection(url, user, password) returns a Connection. Always wrap in try-with-resources so the connection is closed even if an exception occurs:
import java.sql.*;
public class JdbcDemo {
private static final String URL = "jdbc:postgresql://localhost:5432/mydb";
private static final String USER = "myuser";
private static final String PASS = "secret";
public static void main(String[] args) throws SQLException {
// try-with-resources auto-closes the Connection
try (Connection conn = DriverManager.getConnection(URL, USER, PASS)) {
System.out.println("Connected: " + conn.getMetaData().getDatabaseProductName());
}
}
}
For production code use a connection pool (HikariCP) instead of DriverManager directly. DriverManager creates a new TCP connection on every call.
Statement is fine for SQL with no user-supplied parameters. Use executeQuery() for SELECT, executeUpdate() for INSERT/UPDATE/DELETE:
try (Connection conn = DriverManager.getConnection(URL, USER, PASS);
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT id, name, email FROM users")) {
while (rs.next()) {
long id = rs.getLong("id");
String name = rs.getString("name");
String email = rs.getString("email");
System.out.printf("%-4d %-20s %s%n", id, name, email);
}
}
Key ResultSet getter methods:
| SQL Type | Java Getter |
|---|
| INTEGER / BIGINT | getInt(col) / getLong(col) |
| VARCHAR / TEXT | getString(col) |
| NUMERIC / DECIMAL | getBigDecimal(col) |
| BOOLEAN | getBoolean(col) |
| DATE / TIMESTAMP | getDate(col) / getTimestamp(col) |
| BLOB / BYTEA | getBytes(col) / getBlob(col) |
executeUpdate() returns the number of affected rows:
try (Connection conn = DriverManager.getConnection(URL, USER, PASS);
Statement stmt = conn.createStatement()) {
// INSERT
int inserted = stmt.executeUpdate(
"INSERT INTO users (name, email) VALUES ('Alice', 'alice@example.com')");
System.out.println("Rows inserted: " + inserted);
// UPDATE
int updated = stmt.executeUpdate(
"UPDATE users SET email = 'alice2@example.com' WHERE name = 'Alice'");
System.out.println("Rows updated: " + updated);
// DELETE
int deleted = stmt.executeUpdate(
"DELETE FROM users WHERE name = 'Alice'");
System.out.println("Rows deleted: " + deleted);
}
Never concatenate user input into SQL strings passed to Statement. This opens SQL injection attacks. Use PreparedStatement with bind parameters for any user-supplied value.
Pass Statement.RETURN_GENERATED_KEYS to get the database-assigned primary key after an INSERT:
try (Connection conn = DriverManager.getConnection(URL, USER, PASS);
Statement stmt = conn.createStatement()) {
stmt.executeUpdate(
"INSERT INTO users (name, email) VALUES ('Bob', 'bob@example.com')",
Statement.RETURN_GENERATED_KEYS);
try (ResultSet keys = stmt.getGeneratedKeys()) {
if (keys.next()) {
long newId = keys.getLong(1);
System.out.println("Generated ID: " + newId);
}
}
}
DatabaseMetaData exposes driver version, supported features, and schema information without writing SQL:
try (Connection conn = DriverManager.getConnection(URL, USER, PASS)) {
DatabaseMetaData meta = conn.getMetaData();
System.out.println("DB: " + meta.getDatabaseProductName()
+ " " + meta.getDatabaseProductVersion());
System.out.println("Driver: " + meta.getDriverName()
+ " " + meta.getDriverVersion());
System.out.println("Max conn:" + meta.getMaxConnections());
// List all tables in the public schema
try (ResultSet tables = meta.getTables(null, "public", "%", new String[]{"TABLE"})) {
while (tables.next()) {
System.out.println(" Table: " + tables.getString("TABLE_NAME"));
}
}
}
Primitive getters like getInt() return 0 when the column is NULL — call wasNull() immediately after to distinguish zero from NULL:
int score = rs.getInt("score");
boolean isNull = rs.wasNull(); // true if column was SQL NULL
// Safer: use getObject() for nullable columns
Integer scoreBoxed = (Integer) rs.getObject("score"); // null if SQL NULL
| Task | API |
|---|
| Open connection | DriverManager.getConnection(url, user, pass) |
| Static SQL (no params) | conn.createStatement() |
| Parameterised SQL | conn.prepareStatement(sql) |
| SELECT | stmt.executeQuery(sql) → ResultSet |
| DML (INSERT/UPDATE/DELETE) | stmt.executeUpdate(sql) → row count |
| Any SQL | stmt.execute(sql) → boolean (true = ResultSet) |
| Auto-generated keys | stmt.getGeneratedKeys() |
| DB metadata | conn.getMetaData() |
| Resource cleanup | try-with-resources on Connection, Statement, ResultSet |