We upgraded spring boot from 3.1.11 to 3.3.3. Post upgrade we are seeing issues with Rest template when we are trying to post a pdf file.

The API call fails abruptly with 400 BAD_REQUEST and the detail says
Stream ended unexpectedly

http call has following headers

1. contentType : multipart/form-data
2. contentLength :

Http Entity is built as below

  headers.set(HttpHeaders.CONTENT_LENGTH, String.valueOf(bytes.length));

  MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();
        ByteArrayResource byteArrayResource = new ByteArrayResource(bytes,
                "Pdf name") {
            @Override
            public String getFilename() {
                return filename;
            }
        };
        parts.add("data", byteArrayResource);
        HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(parts, headers);

And exchange is done with help of rest template as below

   exchange(uri, "POST", requestEntity, String.class);

The same piece of code works well with spring boot 3.1.11. So the server side issue is eliminated.

exception that we see is

at org.springframework.web.client.HttpClientErrorException.create(HttpClientErrorException.java:103) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:183) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:137) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.ResponseErrorHandler.handleError(ResponseErrorHandler.java:63) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.RestTemplate.handleResponse(RestTemplate.java:942) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:891) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:790) ~[spring-web-6.1.12.jar:6.1.12]
    at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:672) ~[spring-web-6.1.12.jar:6.1.12]

Our rest template instance is built the following way:

    SSLContext sslContext = sslContextBuilder.build();
    KeyManagerFactory keyManagerFactory = KeyManagerFactory.getInstance("****", "****");
    keyManagerFactory.init(clientStore, *****);
    sslContext.init(keyManagerFactory.getKeyManagers(), null, ****);


    SSLConnectionSocketFactory socketFactory = new SSLConnectionSocketFactory(sslContext);

    HttpClientConnectionManager connectionManager = PoolingHttpClientConnectionManagerBuilder.create().setSSLSocketFactory(socketFactory).build();

    CloseableHttpClient httpClient = HttpClients.custom().setConnectionManager(connectionManager).build();

    RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory(httpClient));

sslContextBuilder is excluded for brevity and masked a few things for security reasons. But it is configured with tls protocol and necessary certificates.