disposing streams in Java

Question

In C#, I almost always use the using pattern when working with stream objects. For example:

        using (Stream stream = new MemoryStream())
        {
            // do stuff
        }

By using the using block, we ensure that dispose is called on the stream immediately after that code bock executes.

I know Java doesn't have the equivalent of a using keyword, but my question is that when working with an object like a FileOutputStream in Java, do we need to do any housekeeping to make sure it gets disposed? I was looking at this code example, and I noticed they don't do any.

I just wondered what the best practice was for Java in handling disposing streams, or if it's good enough to let the garbage collector handle it.

Answer 1

generally, you have to do the following:

InputStream stream = null;
try {
   // IO stuff - create the stream and manipulate it
} catch (IOException ex){
  // handle exception
} finally {
  try {
     stream.close();
  } catch (IOException ex){}
}

But apache commons-io provides IOUtils.closeQuietly(stream); which is put in the finally clause to make it a little less-ugly. I think there will be some improvement on that in Java 7.

Update: Jon Skeet made a very useful comment, that the actual handling of the exception is rarely possible to happen in the class itself (unless it is simply to log it, but that's not actually handling it). So you'd better declare your method throw that exception up, or wrap it in a custom exception (except for simple, atomic operations).

Answer 2

There's (sadly) no equivalent of the using statement in Java, although there have been some thoughts about including something similar in Java 7. (I think last time I looked they were "out", but I find it hard to keep up with the status of features in Java 7.)

Baically you need a try/finally block:

InputStream stream = new FileInputStream(...);
try { 
    ...
} finally {
    stream.close();
}

There's then the problem of what to do with the IOException in the case of the close() failing, and the problem of an exception there "overwriting" any exception thrown by the main body of the code - although the latter is a problem in .NET too.

Guava makes this slightly easier with the Closeables class, with static close and closeQuietly methods which will deal with stream being null (in the case where you declare the variable before the block but assign the value within the try block).

Answer 3

In case anybody is curious the new syntax in Java 7 could be:

do (BufferedInputStream bis = ...; BufferedOutputStream bos = ...) {
   ... // Perform action with bis and bos
}

(See proposal by Joshua Bloch)

Here is a related thread on Stack Overflow.