Spring boot 3.1.0 RestTemplateBuilder with apache HTTP client in classpath (org.apache.httpcomponents.client5:httpclient5) ignore java system HTTP proxy command line options like -Dhttp.proxyHost=127.0.0.1 -Dhttp.proxyPort=8888.

Spring boot before 3.1 respects such settings.

For simple RestTemplate restTemplate = restTemplateBuilder.build() spring boot 2.7 instantiate HttpComponentsClientHttpRequestFactory via default constructor, which create HTTP client as HttpClients.createSystem(), i.e. with system proxy support.

Spring boot 3.0 (if readTimeout duration is not set) also create HttpComponentsClientHttpRequestFactory via default constructor and HTTP client as HttpClients.createSystem().

But spring boot 3.1 create HTTP client as HttpClientBuilder.create().setConnectionManager(connectionManager).build(). Without useSystemProperties() as HttpClients.createSystem() makes.

Suggestion: add useSystemProperties() to createHttpClient method like this

return HttpClientBuilder.create()
    .useSystemProperties()
    .setConnectionManager(connectionManager)
    .build();

For compatibility with spring boot 2.x, 3.0, and core spring HttpComponentsClientHttpRequestFactory.