📘 Java Resource Management: Closeable vs AutoCloseable

🎯 Objective

Understand the difference between Closeable and AutoCloseable in Java, when to use each, and how they behave in real-world scenarios like file I/O and JDBC.

---

🔍 Overview

---

✅ Best Practices

---

🧪 Real-World Examples

📁 1. File I/O with Closeable

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class FileReaderExample {
    public static void main(String[] args) {
        try (BufferedReader reader = new BufferedReader(new FileReader("data.txt"))) {
            System.out.println(reader.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

🔍 Why Closeable?

• FileReader and BufferedReader are designed to throw IOException, so Closeable fits naturally.

• Clean and restricted exception handling.

---

🧬 2. JDBC with AutoCloseable

import java.sql.*;

public class JdbcAutoCloseableExample {
    public static void main(String[] args) {
        String url = "jdbc:h2:mem:testdb";
        try (
            Connection conn = DriverManager.getConnection(url);
            PreparedStatement stmt = conn.prepareStatement("SELECT 1");
            ResultSet rs = stmt.executeQuery()
        ) {
            while (rs.next()) {
                System.out.println("Result: " + rs.getInt(1));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

🔍 Why AutoCloseable?

• JDBC resources throw SQLException, not IOException.

• Allows full flexibility of throws Exception.

---

🛠️ 3. Custom Resource with AutoCloseable

class MyResource implements AutoCloseable {
    public void use() {
        System.out.println("Using MyResource...");
    }

    @Override
    public void close() throws Exception {
        System.out.println("Cleaning up MyResource...");
    }
}

public class CustomResourceDemo {
    public static void main(String[] args) {
        try (MyResource res = new MyResource()) {
            res.use();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

🔍 When to use this?

• Any object that needs deterministic cleanup (e.g., releasing a lock, stopping a timer, rolling back a transaction).

---

💡 Did You Know?

🔂 Try-With-Resources Order

• Resources are closed in reverse order of declaration.

try (A a = new A(); B b = new B()) {
    // use resources
}
// b.close() is called first, then a.close()

⚠️ Suppressed Exceptions

• If both the main try block and close() throw exceptions, the one from close() is suppressed, and the first one is thrown.

Throwable primary = new IOException("Primary failure");
Throwable suppressed = new IOException("Suppressed during close");
primary.addSuppressed(suppressed);

---

🧠 Summary Table

---

📝 Interview Tip

“What happens if both the try block and close() throw exceptions?”

• The exception in close() is suppressed and stored inside the primary exception via Throwable.addSuppressed().