Chapter 95: Try-with-resources in Java

finally blocks (Chapter 49) can close handles—but easy to mistype, return early, or mask exceptions. try-with-resources asks the compiler to close() each AutoCloseable resource in a predictable order, even when try throws.

Reading and writing chapters assumed this syntax—here it gets a first-class explanation.

1. Topic title

try-with-resources: declare handles in parens; compiler inserts safe close choreography

2. What it means

try (BufferedReader br = Files.newBufferedReader(path)) {
    use(br);
}

close runs automatically at the end—normal finish, return, or throw. If try threw and close throws, the close exception is suppressed and attached to the primary throwable—getSuppressed() lists them.

Multiple resources: semicolon-separated, closed reverse order of declaration (last opened closes first).

3. Why it is used

Fewer leaked sockets, temp file cleanup, clearer code than finally { if (x != null) x.close(); }.

4. Mental sketch

Classic finally is you locking the door on the way out no matter what. try-with-resources is a smart lock that knows which keys you carried in and turns each without you narrating every step.

5. Java code example

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
 
public class TryWithResourcesDemo {
    static class LoudClose implements AutoCloseable {
        private final String name;
 
        LoudClose(String name) {
            this.name = name;
        }
 
        @Override
        public void close() {
            System.out.println("close " + name);
        }
    }
 
    public static void main(String[] args) throws IOException {
        try (LoudClose a = new LoudClose("outer");
                LoudClose b = new LoudClose("inner")) {
            System.out.println("work");
        }
 
        byte[] data = "hi".getBytes(StandardCharsets.UTF_8);
        try (InputStream in = new ByteArrayInputStream(data)) {
            System.out.println(in.read());
        }
    }
}

Prints work, then close inner, then close outer—reverse declaration order.

6. Explanation of code

InputStream extends AutoCloseable—byte streams participate too, not only Reader/Writer.

7. Common mistakes

Swallowing suppressed exceptions—log Arrays.toString(ex.getSuppressed()) when diagnosing close failures.

Assigning resource in try without final-ish semantics—resource variables are effectively final inside the block; you cannot reassign them mid-try.

8. Best practices

Keep try bodies short—if logic grows, extract private void runWith(BufferedReader br).

Add AutoCloseable to your own types when they wrap native or external resources.

9. Small practice task

Write a try with two LoudClose objects where try throws RuntimeException—implement close to throw too and print getSuppressed().length from the caught outer exception.

Beginner tip

If your IDE offers “surround with try-with-resources”, accept it for BufferedReader, PreparedStatement, HttpResponse<InputStream> bodies—boring safety beats heroic finally.