One common task is handling file uploads—let’s explore how to implement this in Javalin with easy-to-follow examples.
Example 1: Basic File Upload with Javalin
This example uses Javalin’s built-in upload handler to save files to a local directory.
Step 1: Add Dependencies
<!-- Maven pom.xml -->
<dependencies>
<dependency>
<groupId>io.javalin</groupId>
<artifactId>javalin</artifactId>
<version>6.0.0</version>
</dependency>
</dependencies>Step 2: Create the Upload Endpoint
import io.javalin.Javalin;
import io.javalin.http.UploadedFile;
public class FileUploadExample {
public static void main(String[] args) {
Javalin app = Javalin.create().start(7070);
app.post("/upload", ctx -> {
UploadedFile file = ctx.uploadedFile("file"); // "file" matches HTML input name
if (file != null) {
String savePath = "/tmp/uploads/" + file.getFilename();
file.saveTo(savePath); // Save to disk
ctx.result("File uploaded: " + file.getFilename());
} else {
ctx.status(400).result("No file received");
}
});
System.out.println("Server running at http://localhost:7070");
}
}Step 3: Create an HTML Form
<!-- upload.html -->
<!DOCTYPE html>
<html>
<body>
<form action="http://localhost:7070/upload" method="post" enctype="multipart/form-data">
<input type="file" name="file" />
<button type="submit">Upload</button>
</form>
</body>
</html>Output
- Start the server and open
upload.htmlin a browser. - Select a file and click Upload.
- The server responds with:
File uploaded: example.txt - Check
/tmp/uploads/on your server—the file will be there!
Example 2: Advanced Upload with File Validation
Let’s add size limits and file type checks using Apache Commons FileUpload (for more control).
Step 1: Add Dependencies
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.5</version>
</dependency>Step 2: Create the Endpoint
import io.javalin.Javalin;
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;
import javax.servlet.MultipartConfigElement;
import java.io.File;
import java.util.List;
public class AdvancedFileUpload {
public static void main(String[] args) {
Javalin app = Javalin.create(config -> {
config.register(MultipartConfigElement::new); // Enable multipart support
}).start(7070);
app.post("/advanced-upload", ctx -> {
// Check if request is multipart
if (!ServletFileUpload.isMultipartContent(ctx.req())) {
ctx.status(400).result("Not a multipart request");
return;
}
// Configure upload settings
DiskFileItemFactory factory = new DiskFileItemFactory();
ServletFileUpload upload = new ServletFileUpload(factory);
upload.setFileSizeMax(1024 * 1024); // 1MB limit
try {
List<FileItem> items = upload.parseRequest(ctx.req());
for (FileItem item : items) {
if (!item.isFormField()) { // Skip form fields, process files only
String fileName = item.getName();
String fileType = fileName.substring(fileName.lastIndexOf("."));
// Validate file type
if (!fileType.matches(".(jpg|png|txt)")) {
ctx.status(400).result("Invalid file type: " + fileType);
return;
}
// Save file
String savePath = "/tmp/uploads/" + fileName;
item.write(new File(savePath));
ctx.result("Uploaded: " + fileName);
}
}
} catch (Exception e) {
ctx.status(500).result("Upload failed: " + e.getMessage());
}
});
}
}Output
- Upload a file larger than 1MB or an unsupported type (e.g.,
.exe). - The server responds with:
orInvalid file type: .exeUpload failed: File size limit exceeded
Key Takeaways
- Javalin’s
UploadedFileis the simplest way to handle uploads. - For advanced needs (e.g., size limits, type checks), use Apache Commons FileUpload.
- Always validate files before saving them to disk!
Happy coding! 🚀
