Jetty 12: MultiPart Cleanup And Temp File Handling

Hey guys! Today, we're diving into a crucial topic for those of you using Jetty 12 and beyond: automatic MultiPart cleanup. Specifically, we're looking at how Jetty handles temporary files created when dealing with file uploads. In previous versions of Jetty, there was a nifty mechanism that automatically deleted these temporary files. However, this feature was removed in Jetty 12.0 for EE10. Let's explore why this change happened, its implications, and what we can do about it.

The Shift in Jetty 12: What Changed?

In the good old days of Jetty (and even in EE8/EE9), Jetty automatically took care of deleting temporary files associated with MultiPart uploads. This was super convenient because it prevented your server from accumulating a bunch of junk files, potentially leading to disk space issues and performance degradation. The magic happened behind the scenes; Jetty handled the cleanup, so you didn't have to worry about it. Specifically, any Parts backed by a temporary file were automatically deleted by Jetty.

However, with the move to Jetty 12.0 and EE10, this automatic cleanup mechanism vanished. Now, you might be wondering, "Why the change?" Well, the Jetty developers likely had their reasons, perhaps related to architectural changes or adherence to specifications. But the bottom line is that the responsibility for cleaning up temporary files now falls squarely on your shoulders, the developer. This means that if you're not explicitly calling part.delete() on the temporary parts, those files will stick around until your system's temporary file cleanup process kicks in (which might be never, depending on your setup).

This change might seem minor, but it can have significant implications, especially for applications that handle a large volume of file uploads. Imagine a scenario where your application receives hundreds or thousands of file uploads daily. If you forget to delete the temporary files, they'll accumulate over time, potentially filling up your disk space and causing your server to grind to a halt. Nobody wants that, right? Therefore, it's extremely important to understand this change and implement proper cleanup strategies in your Jetty 12 applications.

Why Was Automatic Cleanup Removed?

One key point to consider is the Servlet specification itself. As far as we can tell, the spec doesn't explicitly mandate that the container must have an automatic cleanup mechanism for temporary files. This means that while Jetty's previous behavior was a nice convenience, it wasn't strictly required by the spec. It's possible that the Jetty team decided to align more closely with the specification by removing this feature. Or maybe this is to promote a better design on the developer part for handling resources.

However, just because the spec doesn't require it doesn't mean it's not a good idea. The automatic cleanup provided by previous versions of Jetty was a helpful safeguard against resource leaks. So, even though it's gone in Jetty 12, it's worth considering adding it back in some form, either within Jetty itself or as part of your application's code. This is to help reduce temporary file accumulation if someone is not calling part.delete() on the temporary parts.

The Good Old Days: EE8 & EE9's Cleanup Mechanism

It's worth noting that Jetty versions supporting EE8 and EE9 weren't affected by this change. They had a cleanup mechanism in place within the org.eclipse.jetty.ee9.nested.Request#onCompleted class. This mechanism ensured that temporary files were automatically deleted when a request completed, providing a safety net against resource leaks. This is a really nice function, because it handles the complete event.

So, if you're still using EE8 or EE9, you don't need to worry about this issue (at least not yet). But if you're planning to upgrade to Jetty 12 or later, you'll need to be aware of this change and take appropriate action. This functionality makes a lot of difference in legacy systems.

The Problem: Temp File Accumulation

So, what's the big deal about temporary files sticking around? Well, as we touched on earlier, it can lead to a few problems.

• Disk Space Issues: The most obvious problem is that temporary files consume disk space. If you're constantly uploading files and not cleaning up the temporary files, your disk can fill up pretty quickly. This can lead to all sorts of issues, including application crashes and system instability.
• Performance Degradation: A full disk can also impact your server's performance. When the operating system runs out of space to write temporary data, it can start swapping memory to disk, which is a slow operation. This can cause your application to slow down significantly.
• Security Concerns: In some cases, temporary files might contain sensitive information. If these files aren't properly cleaned up, they could potentially be accessed by unauthorized users.

Therefore, you must avoid those risks and ensure the security and performance of your application.

The Solution: Explicitly Deleting Parts

The solution to this problem is straightforward: you need to explicitly delete the temporary files associated with MultiPart uploads by calling part.delete(). This method tells Jetty (and the underlying operating system) that you're done with the temporary file and that it can be safely deleted.

However, simply knowing that you need to call part.delete() isn't enough. You also need to make sure you're calling it in the right place and at the right time. Here are a few best practices to keep in mind:

• Call part.delete() in a finally Block: The best way to ensure that part.delete() is always called, even if an exception occurs, is to put it in a finally block. This guarantees that the cleanup code will be executed regardless of whether the request processing was successful or not.
• Handle Exceptions: When calling part.delete(), you need to be prepared to handle exceptions. The delete() method can throw an IOException if there's a problem deleting the file (e.g., the file doesn't exist, the process doesn't have permission to delete it). You should catch this exception and log it appropriately.
• Consider Using a Utility Method: To make your code cleaner and more maintainable, you might consider creating a utility method that handles the deletion of parts. This method can encapsulate the try-finally block and the exception handling logic, making it easier to reuse across your application.

Here's an example of how you might implement the explicit deletion of parts in your code:

import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.Part;

import java.io.IOException;

public class UploadServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        for (Part part : request.getParts()) {
            try {
                // Process the part
                // ...
            } finally {
                try {
                    part.delete();
                } catch (IOException e) {
                    // Log the exception
                    e.printStackTrace();
                }
            }
        }
    }
}

In this example, we iterate over the parts in the request and process each part within a try block. The finally block ensures that part.delete() is called, even if an exception occurs during the processing of the part. The catch block handles any IOException that might be thrown by part.delete().

Should Jetty Reintroduce Automatic Cleanup?

Now, let's get back to the question of whether Jetty should reintroduce the automatic cleanup mechanism. There are arguments to be made on both sides.

Arguments for Automatic Cleanup

• Convenience: Automatic cleanup is undeniably convenient. It takes the burden off developers to remember to call part.delete(), reducing the risk of resource leaks.
• Backward Compatibility: Reintroducing automatic cleanup would make Jetty 12 behave more like previous versions, which could simplify migration for existing applications.
• Safety Net: Automatic cleanup provides a safety net against accidental resource leaks. Even if a developer forgets to call part.delete() in some cases, the temporary files will still be cleaned up.

Arguments Against Automatic Cleanup

• Spec Compliance: As we discussed earlier, the Servlet specification doesn't mandate automatic cleanup. Removing the feature aligns Jetty more closely with the spec.
• Control and Flexibility: Automatic cleanup can sometimes be too aggressive. In some cases, you might want to keep the temporary files around for a longer period of time (e.g., for debugging purposes). Automatic cleanup would prevent this.
• Complexity: Reintroducing automatic cleanup would add complexity to Jetty's codebase. This could potentially make it harder to maintain and debug.

The Verdict

So, should Jetty reintroduce automatic cleanup? There's no easy answer. It's a trade-off between convenience and control. Ultimately, the decision rests with the Jetty developers. But hopefully, this discussion has given you a better understanding of the issue and the arguments on both sides. But we should consider adding it back to reduce temp file accumulation if someone is not calling part.delete() on the temporary parts.

Conclusion

The removal of automatic MultiPart cleanup in Jetty 12 is a significant change that developers need to be aware of. While it might seem like a small detail, it can have a big impact on your application's performance and stability. By understanding the implications of this change and implementing proper cleanup strategies, you can ensure that your Jetty 12 applications run smoothly and efficiently. Remember to always call part.delete() in a finally block, handle exceptions, and consider using a utility method to simplify your code. Keep those temporary files in check, guys!