springboot中的RestTemplate使用详解
作者:morris131
RestTemplate介绍
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; } }
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