try-with-resources & AutoCloseable

Automatic resource cleanup

Before Java 7, releasing resources (files, connections, streams) required fragile nested try/finally blocks. try-with-resources guarantees close() is called automatically:

try (var reader = new BufferedReader(new FileReader("data.txt"))) {
    String line;
    while ((line = reader.readLine()) != null) {
        System.out.println(line);
    }
}  // reader.close() is called here, even if an exception is thrown

Any class that implements AutoCloseable (or its sub-interface Closeable) can be declared in the resource list.

Multiple resources — reverse-close order: resources are closed in reverse order of declaration:

try (var conn = openConnection();   // opened first, closed LAST
     var stmt = conn.createStatement(); // opened second, closed FIRST
     var rs   = stmt.executeQuery("...")) { // opened third, closed FIRST of all — wait:
    // close order: rs first, then stmt, then conn
}

The close order is: last declared → first closed.

Suppressed exceptions: if the try body throws AND close() also throws, Java preserves both. The close() exception is suppressed — attached to the primary exception:

try {
    primary.cause();
} catch (Exception e) {
    Throwable[] suppressed = e.getSuppressed(); // the close() exception
}

Contrast with the old try/finally: if both the try body and finally throw, the finally exception replaces the original — the first exception is silently lost. try-with-resources fixes this.