Using Thumbnailator and SpringBoot Thread Pool for Image Watermarking and Asynchronous Upload

The article introduces a backend solution for handling image uploads, adding watermarks, generating thumbnails, and storing them in FastDFS using SpringBoot.

Environment : SpringBoot + FastDFS + Thumbnailator. The FastDFS environment must be set up separately.

Maven dependency for Thumbnailator :

<dependency>
    <groupId>net.coobird</groupId>
    <artifactId>thumbnailator</artifactId>
    <version>0.4.8</version>
</dependency>

Utility class (PictureUtil) provides three methods:

public void photoMark(File sourceFile, File toFile) throws IOException { /* adds watermark */ }
public void photoSmaller(File sourceFile, File toFile) throws IOException { /* creates thumbnail */ }
public void photoSmallerForVedio(File sourceFile, File toFile) throws IOException { /* video thumbnail */ }

Both methods use Thumbnails.of(...).size(...).watermark(...).outputQuality(...).toFile(...) to process images.

SpringBoot thread pool configuration :

@Configuration
@EnableAsync
public class PoolConfig {
    @Bean
    public TaskExecutor taskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.initialize();
        executor.setCorePoolSize(4);
        executor.setMaxPoolSize(32);
        executor.setQueueCapacity(512);
        executor.setKeepAliveSeconds(60);
        executor.setThreadNamePrefix("ThreadPool-");
        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
        executor.setWaitForTasksToCompleteOnShutdown(true);
        return executor;
    }
}

Note the required annotations @Configuration and @EnableAsync on the configuration class, and @Async on the service methods that should run in the pool.

Controller example for receiving an image:

@ApiOperation("上传业务图片")
@PostMapping("/push/photo/{id}/{name}")
public R pushHousingPhotoMethod(@PathVariable Integer id,
                               @PathVariable(required = false) String name,
                               @RequestParam MultipartFile file) throws Exception {
    String fileName = file.getOriginalFilename();
    String ext = StringUtils.substring(fileName, fileName.lastIndexOf('.'));
    File tempPhoto = File.createTempFile(UUIDUtil.make32BitUUID(), ext);
    file.transferTo(tempPhoto);
    service.pushPhoto(id, name, tempPhoto);
    return new R();
}

The controller saves the uploaded MultipartFile as a temporary file before delegating to the service; this prevents the file from being deleted before asynchronous processing completes.

Service layer (asynchronous) :

@Async
public void pushHousingPhoto(Integer id, String name, File file) throws Exception {
    Long startTime = System.currentTimeMillis();
    Integer[] nums = fastDfsService.upLoadPhoto(StringUtils.isBlank(name) ? file.getName() : name, file).get();
    SourcePhotosContext context = new SourcePhotosContext();
    context.setSourceId(id);
    context.setNumber(nums[0]);
    context.setNumber2(nums[1]);
    sourcePhotosContextService.insertNew(context);
    Long endTime = System.currentTimeMillis();
    LogUtil.info(this.getClass(), "source ["+id+"] bind photo ["+name+"] success, cost " + (endTime-startTime) + "ms");
}

The service uploads the original image, adds a watermark, creates a thumbnail, stores both in FastDFS, and records their IDs in a database table.

FastDFS upload helper shows overloaded methods handling both MultipartFile and File , generating temporary files, applying watermark/thumbnail via PictureUtil , uploading, and cleaning up temporary files.

Common pitfalls highlighted include:

Ensuring @EnableAsync is present on a configuration class and @Async on the service method.

Saving MultipartFile to a temporary file before async processing, because the original temporary file is deleted when the controller returns.

Avoiding unnecessary .get() on Future when using a properly configured thread pool.

Overall, the guide provides a complete, production‑ready pattern for asynchronous image handling in a SpringBoot backend.