RestTemplate在连接失败的时候，会直接抛出异常，我们可以通过设置

restTemplate.setErrorHandler(new CustomErrorHandler());

来处理异常。但是这种灵活度不够，不能在业务发生点灵活处理。

还有一种思路就是捕获各种情况下的异常，之后根据异常类型就知道对应的异常了。
HttpClientErrorException - 在HTTP状态为4xx的情况下
HttpServerErrorException - 在HTTP状态为5xx的情况下
UnknownHttpStatusCodeException - 如果HTTP状态未知
ResourceAccessException - 请求超时，Spring将抛出该实例下的底层异常将是java.net.SocketTimeoutException。

代码实现

        @Autowired
    private RestTemplate restTemplate;

    @RequestMapping("/netTest")
    public String netTest() {
        try {
            //String url = "http://127.0.0.1:8080/statusCode3xx";
            //String url = "http://127.0.0.1:8080/statusCode4xx";
            //String url = "http://127.0.0.1:8080/statusCode5xx";
            String url = "http://127.0.0.1:8080/timeout";
            String result = restTemplate.getForObject(url, String.class);
            logger.info(" result = " + result);
        } catch (HttpClientErrorException e) {// 4xx
            logger.error(" HttpClientErrorException , e = " + e.getMessage());
            e.printStackTrace();
        } catch (HttpServerErrorException e) {// 5xx
            logger.error(" HttpServerErrorException , e = " + e.getMessage());
            e.printStackTrace();
        } catch (UnknownHttpStatusCodeException e) {
            logger.error(" UnknownHttpStatusCodeException , e = " + e.getMessage());
            e.printStackTrace();
        }catch(HttpStatusCodeException  e){
            logger.error(" HttpStatusCodeException , e = " + e.getMessage());
            e.printStackTrace();
        }catch(ResourceAccessException e){//timeout
            logger.error(" ResourceAccessException , e = " + e.getMessage());
            e.printStackTrace();
        }
        return "netOK";
    }

    @RequestMapping("/statusCode3xx")
    public ResponseEntity<String> returnStatusCode3XX() {
        return new ResponseEntity<String>(" error ", HttpStatus.USE_PROXY);
    }
    
    @RequestMapping("/statusCode4xx")
    public ResponseEntity<String> returnStatusCode4XX() {
        return new ResponseEntity<String>(" error ", HttpStatus.UNAUTHORIZED);
    }

    @RequestMapping("/statusCode5xx")
    public ResponseEntity<String> returnStatusCode5XX() {
        return new ResponseEntity<String>(" error ", HttpStatus.SERVICE_UNAVAILABLE);
    }

    @RequestMapping("/timeout")
    public String returnTimeout() {
        try {
            Thread.sleep(9*1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return " ok ";
    }