存储1 MB字节数组作为会话属性

Question

I am running a Java web app.

A user uploads a file (max 1 MB) and I would like to store that file until the user completes an entire process (which consists of multiple requests).

Is it ok to store the file as a byte array in the session until the user completes the entire process? Or is this expensive in terms of resources used?

The reason I am doing this is because I ultimately store the file on an external server (eg aws s3) but I only want to send it to that server if the whole process is completed.

Another option would be to just write the file to a temporary file on my server. However, this means I would need to remove the file in case the user exits the website. But it seems excessive for me to add code to the SessionDestroyed method in my SessionListener which removes the file if it's just for this one particular case (ie: sessions are created throughout my entire application where I don't need to check for temp files).

Thanks.

Answer 1

Maybe Yes, maybe No

Certainly it is reasonable to store such data in memory in a session if that fits your deployment constraints.

Remember that each user has their own session. So if all of your users have such a file in their session , then you must multiply to calculate the approximate impact on memory usage.

If you exceed the amount of memory available at runtime, there will be consequences. Your Servlet container may serialize less-used sessions to storage, which is a problem if you've not programmed all of your objects to support serialization. The JVM and OS may use a swap file to move contents out of real memory as part of the virtual memory system. That swapping may impact or even cripple performance.

You must consider your runtime deployment constraints, which you did not disclose. Are you running on a Raspberry Pi or inexpensive little cloud server with little memory available? Or will you run on an enterprise-class server with half a terabyte of RAM? Do you have 3 users, 300, or 30,000? You need to crunch the numbers and determine your needs, and maybe do some runtime profiling to see actual usage.

For example… I write web apps using the Vaadin Framework , a sophisticated package for creating desktop-style apps within a web browser. Being Servlet -based, Vaadin maintains a complete representation of each user's entire work data on the server-side in the Servlet session. Multiplied by the number of users, and depending on the complexity of the app, this may require much memory. So I need to account for this and run my server on sufficient hardware with 64-bit Java tuned to run with a large amount of memory. Or take other approaches such load-balancing across multiple servers with sticky sessions .

Fortunately, RAM is quite cheap nowadays. And 64-bit hardware with large physical support for RAM modules , 64-bit operating systems, and 64-bit JVM implementations ( Azul , others ) are all readily available.