try-with-resources

Key points

• Anything implementing AutoCloseable can be declared in try (...) and Java closes it for you.
• Closing happens whether the block finishes normally or throws — and before any catch or finally runs.
• Multiple resources close in reverse order of declaration.
• Resource variables are implicitly final. Since Java 9 you can also list an existing effectively-final variable.
• If the body and close() both throw, the close failure is attached as a suppressed exception rather than lost.
• Use it for streams, readers, JDBC connections and sockets — anything you would otherwise close in finally.

Example

import java.util.Scanner;

public class Main {

    static class Connection implements AutoCloseable {
        private final String name;

        Connection(String name) {
            this.name = name;
            System.out.println("open  " + name);
        }

        void run(String sql) { System.out.println("  run " + sql + " on " + name); }

        @Override
        public void close() { System.out.println("close " + name); }
    }

    public static void main(String[] args) {
        try (Connection a = new Connection("db-1");
             Connection b = new Connection("db-2")) {
            a.run("select 1");
            b.run("select 2");
        }

        try (Connection c = new Connection("db-3")) {
            c.run("select boom");
            throw new IllegalStateException("query failed");
        } catch (IllegalStateException e) {
            System.out.println("close ran before this catch: " + e.getMessage());
        }

        try (Scanner sc = new Scanner("10 20 30")) {
            int total = 0;
            while (sc.hasNextInt()) total += sc.nextInt();
            System.out.println("scanner total: " + total);
        }
    }
}

If it is closeable, declare it in the try and never close it by hand.