🚨 Addressing "Insufficient Data Written" Error in RestTemplate or WebClient

Are you encountering the "Insufficient Data Written" error while using RestTemplate or WebClient in your Spring application? This error might occur due to various reasons, but one common root cause is passing incorrect or overridden headers, especially the Content-Length header.

In the latest versions of Spring, specifically Spring 3.x and onwards, there's a change in behavior regarding how the Content-Length header is handled when making HTTP requests. Previously, in Spring 2.x, the library would automatically recalculate the Content-Length based on the size of the request body, even if you bypassed or modified the original headers. However, this behavior is no longer supported in Spring 3.x.

If you're experiencing this issue, it's essential to understand that bypassing original headers or modifying them while making subsequent requests is not a recommended practice. Specifically, passing a manually set Content-Length header that doesn't match the actual size of the request body can lead to the "Insufficient Data Written" error.

Moreover, be cautious when manipulating headers such as the "Host" header. Incorrectly setting this header might result in sending your request to the wrong server, leading to unexpected behavior or errors.

To mitigate this issue:

1. Avoid Bypassing Original Headers: Instead of manually setting headers, rely on the RestTemplate or WebClient to handle headers automatically whenever possible.
2. Ensure Correct Content-Length: If you need to modify headers, ensure that the Content-Length header accurately reflects the size of your request body.
3. Update to Spring 3.x Best Practices: If you're migrating from Spring 2.x to 3.x or later, review and update your code to adhere to the new best practices regarding header handling and request construction.

By following these practices, you can prevent the "Insufficient Data Written" error and ensure smooth communication between your Spring application and external services.

e.g. sample code:

HttpHeaders newHeaders = new HttpHeaders();
newHeaders.setContentType(MediaType.APPLICATION_JSON);
// by passing the headers required from original request headers
// avoid the Host and Content-Length headers
HttpEntity<> requestEntity = new HttpEntity<>(body, newHeaders);
ResponseEntity<...> responseEntity = restTemplate
        .exchange(url, HttpMethod.POST, requestEntity, ...);