Custom Iterator for Processing Large Files

I frequently work with large files and implemented a custom iterator to process these files efficiently while minimizing memory usage. The iterator reads each line from a file, parses it into a Message object, and skips invalid entries.

Are there any optimizations or best practices I can apply to this custom iterable implementation to further reduce memory usage or improve its performance? Below is the implementation for reference:

PostExceptionTasksHandler seems like an unnecessary type. It is basically a Runnable that is executed when an exception occurs. It would likely be more useful if you passed the exception that caused handler invocation to the handler itself. If you do that, the handler type could be replaced with Consumer and that already contains enough information in it's name to suggest to the caller what it is used for.

If you have access to the MessageParserService.getMessageFromRowJson(String) method source code, I would change it to throw an exception on an invalid message instead of returning a null. That way you provide the caller with more information for deciding on how to handle the error. Of if you want to use it in a context where exceptions are not desired, maybe change the Message class so that it contains information about the validity so that you can return an error status from the conversion method.

MessageParserService seems like it's more of a mapper than an actual service. The method signature suggests that it converts a JSON string to a data object. These are of course just naming conventions and thus highly opinion based, but I prefer that classes that just convert data types to follow naming pattern like MessageMapper.from(String source). This has benefits because the name restricts the class' scope to creating Message objects and the method name clearly tells that it creates a Message from a String. Only having methods named "from" in your mapper helps you think twice before you start loading responsibilities to it.

Do not call System.exit() to handle an error unless you know exactly why you want to do that. Based on the fact that you describe the code as "skipping invalid entries" I would guess that you don't know. Because the System.exit doesn't skip invalid entries. Instead it does the equivalent of yanking the power cord from the computer when it encounters a spelling error... There are a lot of related questions on Stack Overflow about this. You can start from here: https://stackoverflow.com/questions/3715967/when-should-we-call-system-exit-in-java

For what it's worth, maybe you could use Files.lines(Path, Charset) method instead of writing a special iterator for this? Something like this:

I haven't looked too deeply at this, but I notice that the Iterator only generates the next value in hasNext, and next just assumes this has already been done. This means that if there are ever two next() calls without a hasNext() in between, the second one will always return null as opposed to the value it ought return. Instead of assuming every call to next will be preceded by a hasNext call, make it so that next may itself advance the generation if needed (one way to do this could be having it call hasNext. Breaking the advancing logic into a helper method which both next and hasNext individually call is another).

Additionally, Iterator::next() should throw a NoSuchElementException when the underlying collection is exhausted. This implementation doesn't seem to do this.

Finally, it seems we expect message parsing to indicate failure in two different ways - either by throwing an IOException or by returning null. The former is handled well. The latter is handled by forcibly terminating the whole entire program with System::exit, leaving the caller no chance to recover or even clean up. Prefer handling this case in a manner more like the way you handle the other exception path.

An iterator is a very well known design pattern, and within it, next and hasNext have clearly defined roles. From the Java documentation on the Iterator class:

What this means in practice is that hasNext should not be performing any logic that actually retrieves the next value. It is a test method whose only job is to check that a next value exists. The logic for actually retrieving that value should go in next.

Sometimes this gets tricky if there's no effective way to know if there is a next value without actually getting the next value. In these situations, when you fetch the next value, you want to fetch the next next value so you can keep it in a buffer for easy reference. If that buffer is ever empty, you know there isn't a next value.

And to accomplish this...

The catch with using a buffer is that you need to populate the buffer first, and this is troublesome with an anonymous class because they don't support constructors. Using a named class will allow you to prepopulate the buffer with the initial value (and also fail fast if the underlying source has no values).

An additional benefit to using a named class is that it documents itself. Anonymous classes should be restricted to extremely simple use cases where the code can speak for itself. Otherwise, a future developer looking at your code will have a much easier time understanding this code if the classes are well-named to make their intentions clear. It also makes it easy to refactor into its own standalone class should you choose to reuse it in the future.

Your loop when you are fetching the next value is redundant:

Either next is null and you forcibly exit the program or it is not null and you return a value. In both cases, the loop will never iterate more than a single time, so there's no reason to have a loop at all.

And on that note...

The only reason to call System.exit in the middle of a program's execution is if something has gone very very wrong and you can neither elegantly handle nor meaningfully clean up the issue. Your case here doesn't seem like either of these cases, so you should decide how you want to handle the case of your iterator getting invalid input - either report and skip it or abort the entire iteration.

For the remainder of the answer, I will assume the latter.

From your example, it is unclear what purpose the PostExceptionTasksHandler class is supposed to have. It seems to just be some generic code that performs some logic in the event of an exception, but in that case, it isn't doing anything that can't be accomplished with a simple Consumer.

Furthermore, if you are going to be executing code in response to an exception, you should be passing a reference to what the exception was. That way, you can centralize your general purpose exception handling (logging, etc.) and you can reference the exception if you need to perform specific tasks depending on what actually went wrong.

And finally, your iterator class shouldn't be concerning itself with this kind of business logic. If something exceptional happens, throw the exception and let the caller handle it.

With these, here is a proposed refactor of your iterator class:

However, there is one issue that has yet to be addressed...

A major problem with this implementation is its reliance on the BufferedReader. Generally speaking, readers in Java are consumed on first use, and there is no easy way of reusing them once they have been consumed. This means that once you've iterated through the MessageIterable, iterating through it again will give an empty result because the BufferedReader will have reached the end of its internal stream.

What's more, there is no logical place to clean up the BufferedReader and its stream, so you would be forced to perform the cleanup after the fact. This would result in the iterable's dependency no longer being valid, leaving the iterable in a broken state.

It's quite possible that none of this will matter to you in practice as you aren't intending the iterable to be used more than once for any given resource anyway. But the fact is that this is a clunky solution from the get-go, and when that is the case, it's usually because you are using the wrong approach. Which leads me to...

Breaking down your goals, you are essentially doing the following:

This flow doesn't require a custom iterator. You can accomplish this with a simple loop:

Alternatively, you can use Files.lines as @TorbenPutkonen suggests which returns a Stream. Streams already expose an iterable-like API, which would let you reduce the entire thing to a functional chain:

Which approach you find more agreeable is ultimately up to you.

Disclaimer: I'm not a Java programmer (last time was ~14 years ago)

Buffer lines FileIO is slow. While you mention you want to reduce memory, you also mention you want to improve performance ™. In order to speed things up and assumming one line doesn't not occupy several hundred megabytes of memory, you should consider reading multiple lines at once to reduce the bottleneck of accessing the file. I suggest adding a parameter to the constructor which specifies how many lines (or bytes) should be read so the user can fine-tune it. I don't know too much about the specifics of BufferedReader though, maybe it already takes care apropriately.

Don't terminate on non-critical errors I would never expect that a function that's called hasNext has the ability to nuke the running application (System.exit(-1)). That's an absolute no-go. Definitely prefer exceptions over program termination.

Don't loop if you don't want a loop It looks like this is just copy/pasted because why is there a while loop here? Either you terminate the program or you return from the function.

If you decide to implement the buffer system, use a for loop instead (for reading lines) or a while loop (for reading byte sizes).

User expectation Typically the hasNext implementation should be very light. In your case it contains FileIO. Initially I found this weird but decided not to raise a comment about it, since you can't know if there is a nextValue unless when you read the file. Except that you can. The only way that there is no further data is when the file reader has encountered EOF. Otherwise there is another line to be read. If the line read is invalid, you can still throw an exception. That is, move the file reading code into the next function and just check if another read will succeed. Potentially couple this with the buffering (check if there are still lines on the buffer first).

Typos "Unable to parse messaeg row" contains a typo.