【springboot】RestTemplate的使用、格式转换、异常处理、拦截器，
RestTemplate提供了一个基于Http客户端库（HttpClient，OkHttp等）的高层次API，并不是重复制造轮子。
RestTemplate提供了常见的REST请求方案的模版，例如GET请求、POST请求、PUT请求、DELETE请求以及一些通用的请求执行方法exchange以及execute。
RestTemplate已处于维护状态，官方推荐使用WebClient。
RestTemplate继承自InterceptingHttpAccessor并且实现了RestOperations接口，其中RestOperations接口定义了基本的RESTful操作，这些操作在RestTemplate中都得到了实现。
RestTemplate的核心API
- getForObject：发送GET请求，直接返回一个对象
- getForEntity：发送GET请求，返回ResponseEntity (包含status, headers, and body)
- headForHeaders：指定header，返回HttpHeaders对象
- postForLocation：返回URI
- postForObject：发送POST请求，直接返回一个对象
- postForEntity：发送POST请求，返回ResponseEntity (包含status, headers, and body)
- put：发送PUT请求
- delete：发送Delete请求
- optionsForAllow：发送OPTIONS请求
- exchange：提供更灵活的方式去发送请求，请求参数RequestEntity（包含HTTP方式，请求头，请求体），返回ResponseEntity，支持泛型
- execute：在exchange的基础上，可以通过callback来对请求和响应进行处理
RestTemplate的创建
RestTemplate底层默认使用JDK自带的HttpURLConnection来处理请求的。我们可以使用其他HTTP客户端库来处理请求，只需要实现ClientHttpRequestFactory。
目前官方支持的HTTP客户端库有：
- Apache HttpComponents：HttpComponentsClientHttpRequestFactory
- Netty：Netty4ClientHttpRequestFactory
- OkHttp：OkHttp3ClientHttpRequestFactory
例如使用Apache HttpComponents，可以这样创建RestTemplate：
RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory());
URIs
Restful风格的请求URL上面通常会带有变量。
RestTemplate可以使用可变数组来处理URL上面的变量：
String result = restTemplate.getForObject(
"https://example.com/hotels/{hotel}/bookings/{booking}", String.class, "42", "21");
也可以使用MAP来处理URL上面的变量，MAP中的key需要与URL中的占位符的变量一致：
Map<String, String> vars = Collections.singletonMap("hotel", "42");
String result = restTemplate.getForObject(
"https://example.com/hotels/{hotel}/rooms/{hotel}", String.class, vars);
注意URI会自动进行编码，例如：
restTemplate.getForObject("https://example.com/hotel list", String.class);
// Results in request to "https://example.com/hotel%20list"
可以指定RestTemplate的uriTemplateHandler属性来自定义URI如何进行编码，另外RestTemplate的方法也支持传入URI类型的参数。
RestTemplate的简单使用
GET
发送简单的GET请求，URL支持带参数。
@GetMapping("get")
public String getUser() {
String url = "http://localhost:8888/rest/user/{id}";
RestTemplate restTemplate = new RestTemplate();
String result = restTemplate.getForObject(url, String.class, 1);
log.info("getForObject: {}", result);
HashMap<String, Long> map = new HashMap<>();
map.put("id", 1L);
String result2 = restTemplate.getForObject(url, String.class, map);
log.info("getForObject2: {}", result2);
URI uri = UriComponentsBuilder.fromUriString(url).build(1);
ResponseEntity<String> responseEntity = restTemplate.getForEntity(uri, String.class);
log.info("getForEntity status: {}", responseEntity.getStatusCode());
log.info("getForEntity header: {}", responseEntity.getHeaders());
log.info("getForEntity body: {}", responseEntity.getBody());
return result;
}
POST
发送POST请求。
@GetMapping("post")
public R<Void> postUser() {
String url = "http://localhost:8888/rest/user";
RestTemplate restTemplate = new RestTemplate();
User user = new User();
user.setName("jj");
user.setAge(16);
String result = restTemplate.postForObject(url, user, String.class);
log.info("postForObject: {}", result);
ResponseEntity<String> responseEntity = restTemplate.postForEntity(url, user, String.class);
log.info("postForEntity: {}", responseEntity);
return R.ok();
}
PUT
发送PUT请求。
@GetMapping("put")
public R<Void> putUser() {
String url = "http://localhost:8888/rest/user";
RestTemplate restTemplate = new RestTemplate();
User user = new User();
user.setName("jj");
user.setAge(16);
restTemplate.put(url, user);
log.info("put: {}", user);
return R.ok();
}
DELETE
发送DELETE请求
@GetMapping("delete")
public R<Void> deleteUser() {
String url = "http://localhost:8888/rest/user/{id}";
RestTemplate restTemplate = new RestTemplate();
restTemplate.delete(url, 1);
return R.ok();
}
指定Header
可以使用HttpEntity来指定请求Header头中的参数，也可以使用RequestEntity，RequestEntity是HttpEntity的子类。
可以从ResponseEntity中获取响应的Header头中的参数。
@GetMapping("header")
public R<Void> header() {
String url = "http://localhost:8888/rest/user";
RestTemplate restTemplate = new RestTemplate();
User user = new User();
user.setName("jj");
user.setAge(16);
MultiValueMap<String, Object> paramMap = new LinkedMultiValueMap<>();
paramMap.add("token", "2yy01x90x2xxx25");
// set headers
HttpHeaders headers = new HttpHeaders();
HttpEntity<User> httpEntity = new HttpEntity<>(user, headers);
ResponseEntity<String> response2 = restTemplate.postForEntity(url, httpEntity, String.class);
log.info("postForEntity: {}", response2.getBody());
// 使用exchange请求接口
ResponseEntity<String> response3 = restTemplate.exchange(url, HttpMethod.POST, httpEntity, String.class);
log.info("exchange: {}", response3.getBody());
return R.ok();
}
返回值支持泛型
可以使用ParameterizedTypeReference来支持泛型。
@GetMapping("generic")
public R<User> generic() {
String url = "http://localhost:8888/rest/user/{id}";
ParameterizedTypeReference<R<User>> parameterizedTypeReference = new ParameterizedTypeReference<R<User>>() {
};
RestTemplate template = new RestTemplate();
ResponseEntity<R<User>> responseEntity = template.exchange(url, HttpMethod.GET, null, parameterizedTypeReference, 1);
return responseEntity.getBody();
}
消息内容的转换
RestTemplate的方法中能够直接传入对象或者返回对象，这是因为底层使用了HttpMessageConverter进行了转换。
常用的HttpMessageConverter有以下几个：
- StringHttpMessageConverter：将请求和响应当成字符串来处理，支持
Content-Type为text/plain。 - MappingJackson2HttpMessageConverter：使用Jackson的ObjectMapper将请求和响应当成JSON对象来处理。
如果想修改MappingJackson2HttpMessageConverter的序列化或者反序列化方式，可以自定义ObjectMapper来实现：
RestTemplate template = new RestTemplate();
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setPropertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
template.setMessageConverters(Collections.singletonList(new MappingJackson2HttpMessageConverter(objectMapper)));
异常处理
RestTemplate发起请求时如果发生错误默认就会直接抛出异常，而不会往下执行，所以无法在代码中直接根据ResponseEntity的status来判断接口是否响应成功，例如下面的代码就会直接抛出异常：
@GetMapping("exception")
public String exception() {
String url = "http://localhost:8888/rest/user11";
RestTemplate template = new RestTemplate();
ResponseEntity<String> responseEntity = template.getForEntity(url, String.class);
log.info("responseEntity status: {}", responseEntity.getStatusCode());
return responseEntity.getBody();
}
运行结果如下：
2023-07-19 15:50:49.425 ERROR 15268 --- [nio-8888-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.client.HttpClientErrorException$NotFound: 404 : "{"timestamp":"2023-07-19T07:50:49.225+00:00","status":404,"error":"Not Found","path":"/rest/user11"}"] with root cause
RestTemplate支持设置ResponseErrorHandler来处理异常，默认是使用DefaultResponseErrorHandler，DefaultResponseErrorHandler的处理就是状态码不是20X都会抛出异常：
protected boolean hasError(int unknownStatusCode) {
HttpStatus.Series series = HttpStatus.Series.resolve(unknownStatusCode);
return (series == HttpStatus.Series.CLIENT_ERROR || series == HttpStatus.Series.SERVER_ERROR);
}
@Override
public void handleError(ClientHttpResponse response) throws IOException {
HttpStatus statusCode = HttpStatus.resolve(response.getRawStatusCode());
if (statusCode == null) {
byte[] body = getResponseBody(response);
String message = getErrorMessage(response.getRawStatusCode(),
response.getStatusText(), body, getCharset(response));
throw new UnknownHttpStatusCodeException(message,
response.getRawStatusCode(), response.getStatusText(),
response.getHeaders(), body, getCharset(response));
}
handleError(response, statusCode);
}
private String getErrorMessage(
int rawStatusCode, String statusText, @Nullable byte[] responseBody, @Nullable Charset charset) {
String preface = rawStatusCode + " " + statusText + ": ";
if (ObjectUtils.isEmpty(responseBody)) {
return preface + "[no body]";
}
charset = (charset != null ? charset : StandardCharsets.UTF_8);
String bodyText = new String(responseBody, charset);
bodyText = LogFormatUtils.formatValue(bodyText, -1, true);
return preface + bodyText;
}
protected void handleError(ClientHttpResponse response, HttpStatus statusCode) throws IOException {
String statusText = response.getStatusText();
HttpHeaders headers = response.getHeaders();
byte[] body = getResponseBody(response);
Charset charset = getCharset(response);
String message = getErrorMessage(statusCode.value(), statusText, body, charset);
switch (statusCode.series()) {
case CLIENT_ERROR:
throw HttpClientErrorException.create(message, statusCode, statusText, headers, body, charset);
case SERVER_ERROR:
throw HttpServerErrorException.create(message, statusCode, statusText, headers, body, charset);
default:
throw new UnknownHttpStatusCodeException(message, statusCode.value(), statusText, headers, body, charset);
}
}
自定义ResponseErrorHandler进行异常处理：
@GetMapping("exception2")
public R<Void> exception2() {
String url = "http://localhost:8888/rest/user11";
RestTemplate template = new RestTemplate();
template.setErrorHandler(new ResponseErrorHandler() {
@Override
public boolean hasError(ClientHttpResponse response) throws IOException {
log.info("hasError: {}", response.getStatusCode());
return false;
}
@Override
public void handleError(ClientHttpResponse response) throws IOException {
// hasError
log.info("handleError: {}", response.getStatusCode());
}
});
ResponseEntity<String> responseEntity = template.getForEntity(url, String.class);
log.info("responseEntity status: {}", responseEntity.getStatusCode());
if(responseEntity.getStatusCode().is2xxSuccessful()) {
return R.ok();
}
return R.fail();
}
拦截请求
RestTemplate支持添加拦截器ClientHttpRequestInterceptor在请求前对请求参数进行处理，在请求后对响应进行处理。
例如可以使用ClientHttpRequestInterceptor对URL进行重写，从注册中心拉取配置实现负载均衡。可以使用ClientHttpRequestInterceptor对Header增加全局TraceId参数，对请求进行链路跟踪定位。
下面通过拦截器来实现负载均衡。
String[] loabancerhost = {"https://www.baidu.com", "https://www.sina.com.cn"};
AtomicInteger times = new AtomicInteger(0);
@GetMapping("intercept")
public String intercept() {
String url = "http://test";
RestTemplate restTemplate = new RestTemplate();
restTemplate.setInterceptors(Collections.singletonList((request, body, execution) -> {
log.info("请求开始");
int length = loabancerhost.length;
int i = times.getAndIncrement() % length;
// 因为 HttpRequest 和 URI 不提供修改功能，因此需要借助 HttpRequestWrapper 对request进行包装
Request newRequest = new Request(request, loabancerhost[i]);
return execution.execute(newRequest, body);
}));
ResponseEntity<String> exchange = restTemplate.exchange(url, HttpMethod.GET, null, String.class);
log.info("intercept: {}", exchange);
return exchange.getBody();
}
private static class Request extends HttpRequestWrapper {
private String url;
public Request(HttpRequest request,String url) {
super(request);
this.url = url;
}
@Override
public URI getURI() {
try {
return new URI(url);
} catch (URISyntaxException e) {
}
return null;
}
}