发行说明解释了为什么没有Content-Length。
下面是我的Springboot 3.2.0的RestTemplate，它设置了Content-Length：

@Bean
@SneakyThrows
public RestTemplate restTemplate() {
    SSLContext sslContext = SSLContextBuilder.create()
            .loadTrustMaterial((X509Certificate[] certificateChain, String authType) -> true)  // <--- accepts each certificate
            .build();
    Registry<ConnectionSocketFactory> socketRegistry = RegistryBuilder.<ConnectionSocketFactory>create()
            .register(URIScheme.HTTPS.getId(), new SSLConnectionSocketFactory(sslContext, NoopHostnameVerifier.INSTANCE))
            .register(URIScheme.HTTP.getId(), new PlainConnectionSocketFactory())
            .build();
    HttpClient httpClient = HttpClientBuilder.create()
            .setConnectionManager(new PoolingHttpClientConnectionManager(socketRegistry))
            .setConnectionManagerShared(true)
            .build();
    // Use BufferingClientHttpRequestFactory to ensure Content-Length is added to the request.
    // See https://github.com/spring-projects/spring-framework/wiki/Upgrading-to-Spring-Framework-6.x#web-applications
    // at the end.
    ClientHttpRequestFactory requestFactory = new BufferingClientHttpRequestFactory(new HttpComponentsClientHttpRequestFactory(httpClient));
    return new RestTemplate(requestFactory);
}

字符串
添加BufferingClientHttpRequestFactory允许Content-Length作为头文件添加。