try-with-resource-internals.adoc

Try-with-resource internals

Basic try-with-resource without catch and finally blocks

Original code

try (AutoCloseable resource = new ClosableResource()) {
    // ...
}

Compiled code equivalent

AutoCloseable resource = new ClosableResource();
Throwable $exception = null;

try {
    // ...
} catch (Throwable $e) {
    $exception = $e;
    throw $e;
} finally {
    if (resource != null) {
        if ($exception != null) {
            try {
                resource.close();
            } catch (Throwable $e) {
                $exception.addSuppressed($e);
            }
        } else {
            resource.close();
        }
    }
}

Note	Local variables starting with '$' in the above code are only named for clarity. These variables are otherwise unnamed, referenced only by slot number.

Try-with-resource with catch and finally blocks

Catch and finally block are compiled as parts of a regular try-catch-finally block wrapping the try-with-resource block.

Original code

try (AutoCloseable resource = new ClosableResource()) {
    // ...
} catch (Exception1 e) {
    // ...
} catch (Exception2 e) {
    // ...
} finally {
    // ...
}

Compiled code equivalent (using basic try-with-resource for code brevity)

try {
    try (AutoCloseable resource = new ClosableResource()) {
        // ...
    }
} catch (Exception1 e) {
    // ...
} catch (Exception2 e) {
    // ...
} finally {
    // ...
}

Try-with-resource on multiple resources

Multiple resources are compiled as nested try-with-resource code blocks.

Original code

try (AutoCloseable resource1 = new ClosableResource();
     AutoCloseable resource2 = new ClosableResource()) {
    // ...
}

Compiled code equivalent (using basic try-with-resource for code brevity)

try (AutoCloseable resource1 = new ClosableResource()) {
    try (AutoCloseable resource2 = new ClosableResource()) {
        // ...
    }
}