Direct memory is not released when downloading files

[Dzmitry-yarik]

I have encountered a memory management problem in a Spring Boot application, and would like to get advice on how to properly clean up direct memory.

Configuration:

JDK: OpenJDK Runtime Environment (build 21.0.8+9-Ubuntu-0ubuntu122.04.1), OpenJDK 64-Bit Server VM
Spring Boot: Versions 3.2.6 - 3.5.4

I have the following controller and service:

@RequiredArgsConstructor
@RestController
public class MyEntityController {
    private final MyEntityService service;

    @PostMapping("/file")
    public ResponseEntity<String> addedFileInVersion(MultipartHttpServletRequest request) throws IOException {
        return service.addFileVersion(request);
    }
}

@Service
@Slf4j
@RequiredArgsConstructor
public class MyEntityService {

    @Transactional
    public ResponseEntity<String> addFileVersion(MultipartHttpServletRequest request) throws IOException {
        try {
            log.info("Before reading : {} bytes", getDirectMemoryUsed());
            MultipartFile file = request.getFile("file");
            if (file == null) {
                return ResponseEntity.badRequest().body("The file has not been transferred or the file is empty.");
            }
            byte[] array = file.getBytes();
            String fileName = file.getOriginalFilename();
            storeFile(array, fileName);
            log.info("After reading : {} bytes", getDirectMemoryUsed());
            return ResponseEntity.ok("ok");
        } catch (IOException e) {
            log.error("Error processing file", e);
            return ResponseEntity.badRequest().body("Error when uploading file: " + e.GetMessage());
        } finally {
            log.info("Finally reading : {} bytes", getDirectMemoryUsed());
        }
    }

    private void storeFile(byte[] array, String path) throws IOException {
        String decode = UriUtils.decode(path, StandardCharsets.UTF_8.toString());
        synchronized (this) {
            Path savedPath = Paths.get("/storage/tmp", decode);
            if (Files.notExists(savedPath.getParent())) {
                Files.createDirectories(savedPath.getParent());
            }
            Files.write(savedPath, array);
        }
    }

    private double getDirectMemoryUsed() {
        List<BufferPoolMXBean> bufferPoolMXBeans = ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class);
        for (BufferPoolMXBean pool : bufferPoolMXBeans) {
            if ("direct".equals(pool.getName())) {
                return pool.getMemoryUsed();
            }
        }
        return 0;
    }
}

When sending files, I observe an increase in the use of direct memory, which does not decrease after processing the file. The logs show the following:

Before reading : 24576.0 bytes
After reading : 9.58496462E8 bytes
Finally reading : 9.58496462E8 bytes
Before reading : 9.58512846E8 bytes
After reading : 1.916984732E9 bytes
Finally reading : 1.916984732E9 bytes

It can be seen that after processing the file, the direct memory is not released.

Shouldn't the direct memory be released over time? Because it has been accumulating for several days and has not been cleared and an OutOfMemoryError is being thrown.

[wilkinsona]

It's impossible to say based on the information provided thus far. For example, we don't know what servlet container (embedded or otherwise) you're using. Spring MVC's support for multipart files builds on top of the Servlet API and it's the servlet container that's responsible for allocating memory related to multipart requests.

Regardless of this specific problem, I would not recommend reading the entire uploaded file into memory (as you are doing by calling file.getBytes()). Instead, I would handle it using getInputStream() or, probably more simply, transferTo(File). If making this change does not help and you would like us to spend some more time investigating, please spend some time providing a complete yet minimal sample that reproduces the problem. You can share it with us by pushing it to a separate repository on GitHub or by zipping it up and attaching it to this issue.

[Dzmitry-yarik]

На основании предоставленной на данный момент информации сказать невозможно. Например, мы не знаем, какой контейнер сервлетов (встроенный или нет) вы используете. Поддержка многокомпонентных файлов в Spring MVC основана на API сервлетов, и именно контейнер сервлетов отвечает за выделение памяти для многокомпонентных запросов.

Независимо от конкретной проблемы, я бы не рекомендовал читать весь загруженный файл в память (как вы это делаете, вызывая file.getBytes()). Вместо этого я бы использовал getInputStream()или, что, возможно, проще, transferTo(File). Если это изменение не поможет и вы хотите, чтобы мы потратили больше времени на изучение проблемы, пожалуйста, предоставьте полный, но минимальный пример, воспроизводящий проблему. Вы можете поделиться им с нами, отправив его в отдельный репозиторий на GitHub или заархивировав и прикрепив к этой проблеме.

Thanks for the reply.
Yes, I've redesigned my project using getInputStream(), but I'd still like to figure out the current implementation to figure out if it's a bug or if I'm doing something wrong. In my opinion, this should not be the case.
I provide a link to the project:
https://github.com/Dzmitry-yarik/DirectMemory

[wilkinsona]

That link 404s for me. Perhaps you need to make the repository public?

[Dzmitry-yarik]

my fault. fixed it.

[wilkinsona]

The memory isn't released due to the use of a thread-local cache in sun.nio.ch.Util.

The cache is used by sun.nio.ch.ChannelInputStream's implementation of readAllBytes(). If you want to read the entire file into memory, you need to tune this cache so that the maximum size of a cached buffer multiplied by the number of threads that Tomcat uses for request handling doesn't exceed the available direct memory.

You can set the jdk.nio.maxCachedBufferSize system property to limit the size of buffers that will be cached.