在 try-with-resources 中关闭动态数量的 AutoCloseable 对象

Question

I am creating a variable amount of AutoCloseable objects in a try-with-resources block. At any point of exit, I want all of the allocated resources closed.

I can imagine writing something myself to do this, but is there an existing utility similar to Python's contextlib.ExitStack that will close allocated resources? I would expect it to look like this:

try (ExitStack exitStack = new ExitStack()) {
    List<Widget> widgets = new ArrayList<>();
    for (...) {
        widgets.add(exitStack.add(new Widget()));
    }
    // use widgets
}

(Note: this is not this question because I don't know how many resources I'll have ahead of time.

Hey close voters I'm not asking for a library, I'm asking how you would accomplish the task of safely closing a dynamic number of AutoCloseable s, if there's a language feature for it, great, if there's a standard library function, also great, if I have to write my own, fine. If you'd like to recommend a third-party library in common use that has this in it then sure.

Answer 1

Given that this utility does not appear to exist, I wrote one. It wraps up any thrown exceptions and then only throws if a resource's close() threw. Always closes everything before returning.

public class ClosingException extends Exception { }

And

import java.util.Deque;
import java.util.ArrayDeque;

public final class ClosingStack implements AutoCloseable {
  public void close() throws ClosingException {
    ClosingException allClosingExceptions = new ClosingException();
    while (!resources.isEmpty()) {
      try {
        resources.removeLast().close();
      } catch (Throwable e) {
        allClosingExceptions.addSuppressed(e);
      }
    }
    if (allClosingExceptions.getSuppressed().length != 0) {
      throw allClosingExceptions;
    }
  }

  public <T extends AutoCloseable> T add(T resource) {
    resources.addLast(resource);
    return resource;
  }


  private Deque<AutoCloseable> resources = new ArrayDeque<>();
}

And use:

try (ClosingStack closingStack = new ClosingStack()) {
    List<Widget> widgets = new ArrayList<>();
    for (...) {
        widgets.add(closingStack.add(new Widget()));
    }
    // use widgets
}

Answer 2

I think you'll find Guava's Closer class to be what you need here:

try (Closer closer = Closer.create()) {
   InputStream in1 = closer.register(new FileInputStream("foo"));
   InputStream in2 = closer.register(new FileInputStream("bar"));
   // use in1 and in2
}
// in2 and in1 closed in that order

The class is still marked as Beta mind you, but has appeared to stick around. The original intent was to provide a try-with-resources experience without Java 7 language feature support, however a useful side effect is that it should work with a dynamic number of resources.

Answer 3

Perhaps you could do something like this:

<T extends AutoCloseable> void recursively(
    List<T> things,
    Iterator<? extends Supplier<? extends T>> thingSuppliers,
    Consumer<List<T>> whenEmpty) {
  if (!thingSuppliers.hasNext()) {
    // No more to create. Pass all the things to the consumer.
    whenEmpty.accept(things);
    return;
  }

  // Create a new thing, and make a recursive call. This thing gets
  // closed as the stack unwinds.
  try (T thing = thingSuppliers.next().get()) {
    things.add(thing);
    recursively(things, thingSuppliers, whenEmpty);
  }
}

// Some means of starting the recursion.
<T extends AutoCloseable> void recursively(
    Iterable<? extends Supplier<? extends T>> thingSuppliers,
    Consumer<List<T>> whenEmpty) {
  recursively(new ArrayList<>(), thingSuppliers.iterator(), whenEmpty);
}

Example invocation:

recursively(
    Arrays.asList(Widget::new, Widget::new), 
    System.out::println);