Re: Throwing Constructor Exceptions and cleaning up

From:
Lew <noone@lewscanon.com>
Newsgroups:
comp.lang.java.programmer
Date:
Tue, 14 Apr 2009 09:10:37 -0400
Message-ID:
<gs220f$sh6$1@news.albasani.net>
Larry K. Wollensham wrote:

Arne Vajh??j wrote:

Richard Maher wrote:

For me, the important bits were: - finalize() is really just
belt-and-braces
and not a destructor, everything is GC'ed at process exit,


Note that finalizers will not necessarily be run at JVM exit.

It will only be guaranteed if System runFinalizersOnExit is called.

And that method is deprecated in every sense of the word.

 > and you gotta

clean-up after yourself, there's no magic-wand or guaranteed
rundown-handler.


Yep.

Try catch finally and do the right thing.


Remember that finally executes on any exit from the try block, including
success (fall out or return from inside it, or break or continue from
inside it to outside it).

Something like this will work:

public class MyClass {
    private final InputStream wrapped;

    public MyClass (int arg1) throws IOException {
        if (arg1 < 0) throw new IllegalArgumentException();
        boolean success = false;
        InputStream w = null;
        try {
            w = new FileInputStream(getFile(arg1));
            someSetupStuffThatMayThrow();
            wrapped = w;
            success = true;
        } finally {
            if (w != null && !success) w.close();
        }
    }

    private static File getFile (int arg1) { ... }
    private static void someSetupStuffThatMayThrow ()
        throws IOException { ... }
}

The finally block cleans up by closing the InputStream only if a success
flag wasn't put up at the end of the try body (and only if the stream
itself was opened successfully).


Another approach puts separate try blocks around things that can fail.

    public MyClass (int arg1) throws IOException
    {
      if (arg1 < 0) throw new IllegalArgumentException();
      InputStream w = new FileInputStream(getFile(arg1));
      assert w != null;
      try
      {
        someSetupStuffThatMayThrow();
      }
      finally
      {
        w.close();
      }
    }

--
Lew

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