How to close resources automatically

Short answer

Declare the resource in parentheses after try. Java calls its close() when leaving the block — even if an exception happened:

void main() throws IOException {
    Files.writeString(Path.of("data.txt"), "line 1\nline 2\n");
 
    try (BufferedReader reader = Files.newBufferedReader(Path.of("data.txt"))) {
        IO.println(reader.readLine());
    }
}

Output:

line 1

Several resources

List the resources separated by semicolons. They are closed in reverse order:

void main() throws IOException {
    Files.writeString(Path.of("in.txt"), "hello\n");
 
    try (var reader = Files.newBufferedReader(Path.of("in.txt"));
         var writer = Files.newBufferedWriter(Path.of("out.txt"))) {
        writer.write(reader.readLine().toUpperCase());
    }
    IO.println(Files.readString(Path.of("out.txt")));
}

Output:

HELLO

Your own resource

Any class that implements AutoCloseable can be used in try:

class Connection implements AutoCloseable {
    Connection() {
        IO.println("open");
    }
 
    @Override
    public void close() {
        IO.println("close");
    }
}
 
void main() {
    try (var connection = new Connection()) {
        IO.println("work");
        throw new IllegalStateException("failed");
    } catch (IllegalStateException e) {
        IO.println("caught: " + e.getMessage());
    }
}

Output:

open
work
close
caught: failed

Note the order: the resource is closed before the catch block runs.

Common mistake

Closing the resource manually at the end of try. If an exception happens earlier, close() is never called:

class Connection implements AutoCloseable {
    @Override
    public void close() {
        IO.println("close");
    }
}
 
void main() {
    try {
        var connection = new Connection();
        Integer.parseInt("x");
        connection.close();
    } catch (NumberFormatException e) {
        IO.println("error, connection left open");
    }
}

Output:

error, connection left open

Thanks for reading!

Short answer

Several resources

Your own resource

Common mistake

Functions used in this recipe

Thanks for reading!