What is the best way to emulate try-with-resources in Java 6?

Question

It turns out that almost nobody closes resources in Java correctly. Programmers either do not use try-finally block at all, or just put resource.close() in finally which is also incorrect (because Throwable from close() can shadow Throwable from try block). Sometimes they put something like IOUtils.closeQuietly() with is only correct for InputStream, but not for OutputStream. try-with-resources solves all of these problems but there are still huge number of projects written in Java 6.

What is the best way to emulate try-with-resources in Java 6? Now I use Guava Closer, which is better than nothing but still much uglier than try-with-resources. Also, there is a pattern called a loan-pattern, but the absence of lambdas in Java makes this pattern very cumbersome. Is there a better way?

Guava Closer seems to be as elegant as it can get with Java 6. — Thomas, Oct 21 '13 at 14:45
FYI `IOUtils.closeQuietly` in newer versions of `commons-io` is defined for `Closable` interface. So you can close what you want with it (not just streams). http://commons.apache.org/proper/commons-io/javadocs/api-2.4/org/apache/commons/io/IOUtils.html#closeQuietly(java.io.Closeable) — Pavel Horal, Oct 21 '13 at 14:48

score 3 · Accepted Answer · answered Oct 24 '13 at 11:50

I've found a good replacement for try-with-resources. It uses Lombok library with annotation processing:

 @Cleanup InputStream in = new FileInputStream(args[0]);
 @Cleanup OutputStream out = new FileOutputStream(args[1]);
 byte[] b = new byte[10000];
 while (true) {
   int r = in.read(b);
   if (r == -1) break;
   out.write(b, 0, r);
 }

However, it doesn't handle exception correctly. This bug is more than 1 year old and still is not closed: https://code.google.com/p/projectlombok/issues/detail?id=384

score 0 · Answer 2 · answered Oct 21 '13 at 15:04

0

Though anonymous class is quite verbose, it's still acceptable in java land

    new TryWithResource<InputStream>(){
        protected InputStream init() throws Exception {
            return new FileInputStream("abc.txt");
        }
        protected void use(InputStream input) throws Exception{
            input.read();
        }
    };

----

abstract class TryWithResource<R>
{
    abstract protected R init() throws Exception;
    abstract protected void use(R resource) throws Exception;

    // caution: invoking virtual methods in constructor!
    TryWithResource() throws Exception
    {
        // ... code before
        R r = init();
        use(r);
        // ... code after
    }
}

answered Oct 21 '13 at 15:04

ZhongYu

19,446
5
33
61

2

Where is the close happening? – Thomas Jungblut Oct 21 '13 at 15:10
That's wrong, if `use` throws an exception then the stream will not be closed. You would need to wrap a try/finally block anyway, thus you're solution is just more code for the same job. – Thomas Jungblut Oct 21 '13 at 15:35
`code before/after` involves try/finally. I thought that is self evident for anyone interested in this question. – ZhongYu Oct 21 '13 at 16:54

score 0 · Answer 3 · answered Oct 21 '13 at 15:07

If your only problem with IOUtils.closeQuietly is that it ignores exceptions on OutputStreams, then you can either simply call close() on them, or create your own utility class which automatically treats the two differently, like this:

public static void close(Closeable resource)
{
    try
    {
        resource.close();
    }
    catch(Exception e)
    {
        //swallow exception
    }
}

public static void close(OutputStream o)
{
    //throw any exceptions
    o.close();
}

The correct overloaded method will be selected at compile time in all common situations, although if you're passing OutputStreams around as Closeables then you'll have to change this to do a dynamic instanceof check to make sure OutputStreams always throw exceptions.

The code required to properly deal with cleanup exceptions is just plain nasty without try-with-resources, since such exceptions should propagate if the `try` block succeeds, but be suppressed if it fails. — supercat, Feb 04 '14 at 23:34

What is the best way to emulate try-with-resources in Java 6?

3 Answers3

Linked