How to Set Content-Length Header in ResponseEntity in Spring MVC

1. Overview

In Spring MVC applications, we commonly return HTTP responses using the ResponseEntity class. Spring usually determines and sets HTTP headers automatically based on the response body and our configured message converters.

However, certain situations still require explicit Content-Length header settings. This typically occurs when we know the response size in advance and must communicate it accurately to the client to ensure correctness or compatibility.

Throughout this article, we explain what the Content-Length header represents, when explicit configuration makes sense, and how to define it correctly in ResponseEntity. All examples follow established Spring MVC best practices.

2. Understanding the Content-Length Header

The Content-Length header specifies the exact size of the HTTP response body in bytes. By sending this header, we inform clients of the amount of data they will receive before the transfer begins.

When we omit Content-Length, Spring may use chunked transfer encoding instead. In this mode, the server sends the response in multiple chunks rather than as a single fixed-length payload. Chunked encoding suits streaming or dynamic content well, but it doesn’t fit every use case.

We typically see Transfer-Encoding: chunked when we stream data or cannot determine the content length in advance. Common scenarios include endpoints that generate content on-the-fly, stream large datasets, or implement Server-Sent Events (SSE).

However, file downloads and certain HTTP clients need a predefined content size to track progress, verify completeness, or meet protocol expectations. The HTTP specification also forbids sending both the Content-Length header and the Transfer-Encoding: chunked header together. When we use chunked transfer encoding, the total response size remains unknown upfront, so we cannot declare it in advance.

By understanding how Content-Length affects response handling, we can decide when to set it explicitly and when to rely on Spring MVC’s default behavior.

3. Setting Content-Length for a Simple Text Response

When we return text-based responses, we can set Content-Length by calculating the byte length of the response body using the correct character encoding:

@GetMapping("/hello")
public ResponseEntity<String> hello() {
    String body = "Hello Spring MVC!";
    byte[] bytes = body.getBytes(StandardCharsets.UTF_8);

    return ResponseEntity.ok()
      .contentLength(bytes.length)
      .body(body);
}

We explicitly pass StandardCharsets.UTF_8 to getBytes() to eliminate platform-dependent variations in byte length. This step is crucial because an accurate Content-Length depends on knowing the exact byte count that will be sent, and that count must be the same in development, testing, and production environments.

4. Setting Content-Length for Binary Data

Binary responses such as images, PDFs, or serialized objects require accurate Content-Length values to ensure reliable transmission:

@GetMapping("/binary")
public ResponseEntity<byte[]> binary() {
    byte[] data = {1, 2, 3, 4, 5};

    return ResponseEntity.ok()
      .contentLength(data.length)
      .contentType(MediaType.APPLICATION_OCTET_STREAM)
      .body(data);
}

In this example, we know the response size upfront, which makes explicit configuration safe and appropriate.

5. Setting Content-Length for File Downloads

File downloads represent the most common case where we should explicitly set Content-Length. Browsers and HTTP clients use this value to display download progress and confirm that the file transfer completed successfully:

@GetMapping("/download")
public ResponseEntity<Resource> download() throws IOException {
    Path filePath = Paths.get("example.pdf"); // For tests, this file should exist
    Resource resource = new UrlResource(filePath.toUri());

    long fileSize = Files.size(filePath);

    return ResponseEntity.ok()
      .contentLength(fileSize)
      .contentType(MediaType.APPLICATION_PDF)
      .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"example.pdf\"")
      .body(resource);
}

Explicitly setting Content-Length in this context improves client compatibility and provides a better user experience. We should also verify that the resource exists and is readable before setting the header, as an incorrect file size may cause client-side errors.

6. Using HttpHeaders to Set Content-Length

We can also set Content-Length manually by using the HttpHeaders class. This approach helps when we need to build more customized responses:

@GetMapping("/manual")
public ResponseEntity<String> manual() {
    String body = "Manual Content-Length";
    byte[] bytes = body.getBytes(StandardCharsets.UTF_8);

    HttpHeaders headers = new HttpHeaders();
    headers.setContentLength(bytes.length);

    return ResponseEntity.ok()
      .headers(headers)
      .body(body);
}

While this method gives us full control over the response headers, it should be used with care to ensure consistency between the declared content length and the actual response body.

7. Best Practices and Common Pitfalls

When working with the Content-Length header, the most important consideration is to set it explicitly only when the response size is known and fixed in advance. The value must always represent the exact number of bytes written to the response body, which means character encoding must be taken into account for textual content.

Explicitly defining the Content-Length header for streaming or dynamically generated responses should generally be avoided, as an incorrect value may lead to truncated responses, stalled clients, or protocol-level errors.

We should avoid explicitly setting Content-Length in several scenarios:

• Streaming responses, such as those using StreamingResponseBody
• Server-Sent Events (SSE)
• Any endpoint that generates response content incrementally or depends on runtime processing

In most cases, using the contentLength() method provided by ResponseEntity is preferable, as it clearly expresses intent and integrates naturally with Spring’s response-building API. For typical REST endpoints, allowing Spring to manage this header automatically remains the safest approach, reducing the risk of inconsistencies while keeping the code simple and maintainable.

8. Conclusion

Setting the Content-Length header in Spring MVC’s ResponseEntity is straightforward, but it should be done explicitly and only when required. Spring automatically manages this header for most use cases, reducing boilerplate code and minimizing the risk of calculation errors.

Manual configuration is recommended for scenarios such as file downloads, where the exact content size is known upfront and must be communicated accurately to the client. By ensuring correct byte-length calculation and understanding how Content-Length interacts with other HTTP headers, we can build more reliable and maintainable Spring MVC applications.

As always, the complete source code for the tutorial is available over on GitHub.