Detect HTTP interruption in Java

Question

I'm making a REST call to an external system from Java. If there are any connection interruptions like if the external system goes offline, I need a listener to detect that and perform corresponding actions. Kindly let me know if I can achieve this in Java. Preferably in Java 8.

Currently i'm not getting any exception if I get into any such situation. I having the below code currently

    Client client = ClientBuilder.newClient();
    WebTarget target = client.target(HOSTNAME);
    Invocation.Builder requestBuilder;

    requestBuilder = target.path(URL).request(MediaType.APPLICATION_JSON)
                                                .header(HEADER_AUTHORIZATION, AUTH_TOKEN);      
    Response response = null;
    if (HTTP_POST.equalsIgnoreCase(method)) {
        try{
        response = requestBuilder.post(Entity.entity(message, MediaType.APPLICATION_JSON_TYPE));
        }catch(Exception ex ){
            ex.printStackTrace();
        }
    } else if (HTTP_GET.equalsIgnoreCase(method)) {
        response = requestBuilder.get();
    } 

    String executeMessage = null;
    if(response != null){
        if (response.getStatus() == 200) {
            executeMessage = response.readEntity(String.class);         
            return new JSONObject(executeMessage);
        } else {
            executeMessage = response.readEntity(String.class);
            final JSONObject status = new JSONObject();
            status.put(STATUS_CODE, response.getStatus());
            status.put(STATUS_INFO, response.getStatusInfo());
            status.put("response", executeMessage);
            final JSONObject error = new JSONObject();
            error.put(ERROR, status);
            return error;
        }
    }

Answer 1

If you use Spring Boot you can try a declarative REST client called Feign:

@FeignClient(url = "example.com", fallback = YourTestFeignClient.YourTestFeignClientFallback.class)
public interface YourTestFeignClient {

    @RequestMapping(method = RequestMethod.POST, value = "/example/path/to/resource/{id}")
    YourTestResponse requestExternalResource(@RequestParam("id") Long id, SomeRequestDTO requestDTO);

    @Component
    public static class YourTestFeignClientFallback implements YourTestFeignClient {

        @Override
        public YourTestResponse requestExternalResource(Long id, SomeRequestDTO requestDTO) {
            YourTestResponse testResponse = new YourTestResponse();
            testResponse.setMessage("Service unavailable");
            return testResponse;
        }

    }
}

All you need to do here is to inject YourTestFeignClient in your code and invoke method requestExternalResource on it. This will call POST example.com/example/path/to/resource/34with JSON body taken from SomeRequestDTO. In case a request fails a fallback method inside YourTestFeignClientFallback will get called, which then returns some default data.