Spring Boot and AWS S3: Download file

YatinApril 1st, 2020Last Updated: March 31st, 2020

6 3,426 5 minutes read

Welcome readers, in this tutorial, we will show how to download a file from an AWS S3 bucket using the spring boot framework.

1. Introduction

Spring boot is a module that provides rapid application development feature to the spring framework including auto-configuration, standalone-code, and production-ready code
It creates applications that are packaged as jar and are directly started using embedded servlet container (such as Tomcat, Jetty or Undertow). Thus, no need to deploy the war files
It simplifies the maven configuration by providing the starter template and helps to resolve the dependency conflicts. It automatically identifies the required dependencies and imports them in the application
It helps in removing the boilerplate code, extra annotations, and XML configurations
It provides a powerful batch processing and manages the rest endpoints
It provides an efficient jpa-starter library to effectively connect the application with the relational databases
It offers a Microservice architecture and cloud configuration that manages all the application related configuration properties in a centralized manner

1.1 AWS Storage Service

AWS Storage Service or simply known as AWS S3 is an online storage facility for the users. It cheap, easy to set up and the user only pays for what they utilize. It offers,

To host static web-content and data or even the dynamic pages
Data storage for analytics
Backup and archival of data
Disaster recovery solutions

1.1.1 S3 Bucket

Amazon S3 Bucket has two primary entities i.e. Object and Bucket, where objects are stored inside the buckets. It provides high availability and durability solutions by replicating the data of one bucket in multiple data centers. Each AWS account offers 100 buckets as free, however, this count can be increased on submitting a request to the support center. Amazon S3 features –

Each object in a bucket is given a unique id
S3 allows a developer to upload/delete or read an object via the REST API
S3 offers two read-after-write and eventual consistency models to ensure that every change command committed to a system should be visible to all the participants
Objects stored in a bucket never leave it’s location unless the user transfer it out
Objects can be made private or public and rights can be granted to specific users

1.1.2 Setting up AWS S3 bucket and IAM user

If someone needs to go through the process of creating an S3 bucket and attach it to an IAM user, please watch this video.

To start with this tutorial, we are hoping that users at present have their AWS account created and know how to set up an S3 bucket and attach it to an IAM user of their choice.

2. Spring boot and AWS S3: Download file

Here is a systematic guide for implementing this tutorial.

2.1 Application pre-requisite

To start with this tutorial, I would recommend readers to go through Part 1 of Spring Boot and AWS S3 operations tutorial available at this link. The tutorial will help users to understand the code changes required to start up.

3. Creating a Spring Boot application

Since the Project structure, Maven dependencies, and Configuration level changes remain the same so we’ll directly jump start with the changes required to download the file and its content from the S3 bucket.

3.3 Java Classes

Let us write the updated java classes involved in this application.

3.3.1 Service Class

Add the download(……) method in the AWSS3ServiceImpl.java class to fetch the S3 object and its content for the given key name.

AWSS3ServiceImpl.java

package org.jcg.springboot.aws.s3.serv;
 
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.time.LocalDateTime;
 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;
import org.springframework.web.multipart.MultipartFile;
 
import com.amazonaws.AmazonServiceException;
import com.amazonaws.services.s3.AmazonS3;
import com.amazonaws.services.s3.model.PutObjectRequest;
import com.amazonaws.services.s3.model.S3Object;
import com.amazonaws.services.s3.model.S3ObjectInputStream;
import com.amazonaws.util.IOUtils;
 
@Service
public class AWSS3ServiceImpl implements AWSS3Service {
 
    private static final Logger LOGGER = LoggerFactory.getLogger(AWSS3ServiceImpl.class);
 
    @Autowired
    private AmazonS3 amazonS3;
    @Value("${aws.s3.bucket}")
    private String bucketName;
 
    @Override
    // @Async annotation ensures that the method is executed in a different background thread 
    // but not consume the main thread.
    @Async
    public void uploadFile(final MultipartFile multipartFile) {
        LOGGER.info("File upload in progress.");
        try {
            final File file = convertMultiPartFileToFile(multipartFile);
            uploadFileToS3Bucket(bucketName, file);
            LOGGER.info("File upload is completed.");
            file.delete();  // To remove the file locally created in the project folder.
        } catch (final AmazonServiceException ex) {
            LOGGER.info("File upload is failed.");
            LOGGER.error("Error= {} while uploading file.", ex.getMessage());
        }
    }
 
    private File convertMultiPartFileToFile(final MultipartFile multipartFile) {
        final File file = new File(multipartFile.getOriginalFilename());
        try (final FileOutputStream outputStream = new FileOutputStream(file)) {
            outputStream.write(multipartFile.getBytes());
        } catch (final IOException ex) {
            LOGGER.error("Error converting the multi-part file to file= ", ex.getMessage());
        }
        return file;
    }
 
    private void uploadFileToS3Bucket(final String bucketName, final File file) {
        final String uniqueFileName = LocalDateTime.now() + "_" + file.getName();
        LOGGER.info("Uploading file with name= " + uniqueFileName);
        final PutObjectRequest putObjectRequest = new PutObjectRequest(bucketName, uniqueFileName, file);
        amazonS3.putObject(putObjectRequest);
    }
 
    @Override
    // @Async annotation ensures that the method is executed in a different background thread 
    // but not consume the main thread.
    @Async
    public byte[] downloadFile(final String keyName) {
        byte[] content = null;
        LOGGER.info("Downloading an object with key= " + keyName);
        final S3Object s3Object = amazonS3.getObject(bucketName, keyName);
        final S3ObjectInputStream stream = s3Object.getObjectContent();
        try {
            content = IOUtils.toByteArray(stream);
            LOGGER.info("File downloaded successfully.");
            s3Object.close();
        } catch(final IOException ex) {
            LOGGER.info("IO Error Message= " + ex.getMessage());
        }
        return content;
    }
}

3.3.2 Controller Class

Add the downloadFile(……) method in the AWSS3Ctrl.java class to download the file from the S3 bucket and return the response back to the user. This method reads the filename query parameter coming in the GET request.

AWSS3Ctrl.java

package org.jcg.springboot.aws.s3.ctrl;
 
import org.jcg.springboot.aws.s3.serv.AWSS3Service;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.io.ByteArrayResource;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RequestPart;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.multipart.MultipartFile;
 
@RestController
@RequestMapping(value= "/s3")
public class AWSS3Ctrl {
 
    @Autowired
    private AWSS3Service service;
 
    @PostMapping(value= "/upload")
    public ResponseEntity<String> uploadFile(@RequestPart(value= "file") final MultipartFile multipartFile) {
        service.uploadFile(multipartFile);
        final String response = "[" + multipartFile.getOriginalFilename() + "] uploaded successfully.";
        return new ResponseEntity<>(response, HttpStatus.OK);
    }
 
    @GetMapping(value= "/download")
    public ResponseEntity<ByteArrayResource> downloadFile(@RequestParam(value= "fileName") final String keyName) {
        final byte[] data = service.downloadFile(keyName);
        final ByteArrayResource resource = new ByteArrayResource(data);
        return ResponseEntity
                .ok()
                .contentLength(data.length)
                .header("Content-type", "application/octet-stream")
                .header("Content-disposition", "attachment; filename=\"" + keyName + "\"")
                .body(resource);
    }
}

4. Run the Application

To execute the application, compile the project and right-click on the SpringbootS3tutorial.java class, Run As -> Java Application.

AWS S3 Download file - Run the Application — Fig. 1: Run the Application

5. Project Demo

Once the application is started successfully, open the Postman tool. Hit the following URL (HTTP GET request) to download the file content from the S3 bucket.

http://localhost:9098//s3/download?fileName=2020-03-26T21:29:41.957_Some text.txt

AWS S3 Download file - HTTP GET request — Fig. 2: HTTP GET request

Once the file is download successfully from the S3 bucket, the API will return the 200 OK response as shown in Fig. 3.

AWS S3 Download file - File content — Fig. 3: File content

That is all for this tutorial and I hope the article served you whatever you were looking for. Happy Learning and do not forget to share!

6. Conclusion

In the above sections, developers learned how to download a file from an S3 bucket via a spring boot application. Developers can download the sample application from the Download section.