automatically closing a resource passed as an argument

Question

If I want to automatically close a resource passed as an argument, is there a more elegant solution than this?

void doSomething(OutputStream out) {

  try (OutputStream closeable = out) {
    // do something with the OutputStream
  }
}

Ideally, I'd like to have this resource closed automatically, without declaring another variable closeable that refers to the same object as out.

Aside

I realise that closing out within doSomething is considered a bad practice

I think the `doSomething` method should not close the input stream itselft. The caller should take care of it. — Mạnh Quyết Nguyễn, Aug 14 '18 at 09:19
I believe java 9 allows that (the `out` variable is effectively final). But there's a risk for method parameters, I think. — ernest_k, Aug 14 '18 at 09:21
@oleg.cherednik It does implement `AutoCloseable`, from the [Javadoc](https://docs.oracle.com/javase/10/docs/api/java/io/OutputStream.html): _"All Implemented Interfaces: Closeable, Flushable, AutoCloseable". It directly implements `Closeable` which extends `AutoCloseable`. — Mark Rotteveel, Aug 14 '18 at 09:39
@MarkRotteveel I can see another picture in my sources. Probably it depends on JVM version. — Oleg Cherednik, Aug 14 '18 at 09:40
@oleg.cherednik Only if you're using Java 6 or lower. `AutoCloseable` was introduced in Java 7. Or maybe you've overlooked that `Closeable` is a sub-interface of `AutoCloseable`). — Mark Rotteveel, Aug 14 '18 at 09:41

score 5 · Accepted Answer · answered Aug 14 '18 at 09:37

5

With Java 9 and higher, you can do

void doSomething(OutputStream out) {
  try (out) {
    // do something with the OutputStream
  }
}

This is only allowed if out is final or effectively final. See also the Java Language Specification version 10 14.20.3. try-with-resources.

answered Aug 14 '18 at 09:37

Mark Rotteveel

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I'm using Java 8 at the moment, but good to know there's a more elegant solution in the next version – Dónal Aug 14 '18 at 09:55
@Dónal The only thing in Java 8 that would make your current code (as shown) shorter is replacing `OutputStream closeable` by `Closeable closeable`, but I think that is grasping at straws and makes the intent even less obvious. – Mark Rotteveel Aug 14 '18 at 10:16

Oleg Cherednik · Answer 2 · 2018-08-14T10:23:01.643

3

I use Java 8 and it does not support Resource Reference. What about create universal method that accepts Closable and payload:

public static <T extends Closeable> void doAndClose(T out, Consumer<T> payload) throws Exception {
    try {
        payload.accept(out);
    } finally {
        out.close();
    }
}

Client code could look like this:

OutputStream out = null;

doAndClose(out, os -> {
    // do something with the OutputStream
});

InputStream in = null;

doAndClose(in, is -> {
    // do something with the InputStream
});

edited Aug 14 '18 at 10:23

answered Aug 14 '18 at 09:48

Oleg Cherednik

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4
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Can you include an example how this would be applied (eg as a replacement in the `doSomething` method)? – Mark Rotteveel Aug 14 '18 at 10:18
This removes the potential confusion that might arise from 2 references to the closeable resource being accessible within the payload body. It should probably accept a `Consumer super T>`, though. – Hulk Aug 14 '18 at 10:57

score -4 · Answer 3 · answered Aug 14 '18 at 09:28

-4

void doSomething(OutputStream out) {
  try {
    // do something with the OutputStream
  }
  finally {
    org.apache.commons.io.IOUtils.closeQuietly(out);
  }
}

answered Aug 14 '18 at 09:28

M.F

403
2
10

From the [JavaDocs of org.apache.commons.io.IOUtils.ArrayUtils.closeQuietly](https://commons.apache.org/proper/commons-io/javadocs/api-release/org/apache/commons/io/IOUtils.html#closeQuietly-java.io.OutputStream-): "**Deprecated.** As of 2.6 removed without replacement. Please use the try-with-resources statement or handle suppressed exceptions manually." – Hulk Aug 14 '18 at 10:50

automatically closing a resource passed as an argument

Aside

3 Answers3