我們可以通過Spring Boot快速開發REST接口，同時也可能需要在實現接口的過程中，通過Spring Boot調用內外部REST接口完成業務邏輯。
在Spring Boot中，調用REST Api常見的一般主要有兩種方式，通過自帶的RestTemplate或者自己開發http客戶端工具實現服務調用。
RestTemplate基本功能非常強大，不過某些特殊場景，我們可能還是更習慣用自己封裝的工具類，比如上傳文件至分布式文件系統、處理帶證書的https請求等。
本文以RestTemplate來舉例，記錄幾個使用RestTemplate調用接口過程中發現的問題和解決方案。
一、RestTemplate簡介
1、什么是RestTemplate
我們自己封裝的HttpClient，通常都會有一些模板代碼，比如建立連接，構造請求頭和請求體，然后根據響應，解析響應信息，最后關閉連接。
RestTemplate是Spring中對HttpClient的再次封裝，簡化了發起HTTP請求以及處理響應的過程，抽象層級更高，減少消費者的模板代碼，使冗余代碼更少。
其實仔細想想Spring Boot下的很多XXXTemplate類，它們也提供各種模板方法，只不過抽象的層次更高，隱藏了更多細節而已。
順便提一下，Spring Cloud有一個聲明式服務調用Feign,是基于Netflix Feign實現的，整合了Spring Cloud Ribbon與 Spring Cloud Hystrix，并且實現了聲明式的Web服務客戶端定義方式。
本質上Feign是在RestTemplate的基礎上對其再次封裝，由它來幫助我們定義和實現依賴服務接口的定義。
2、RestTemplate常見方法
常見的REST服務有很多種請求方式，如GET,POST,PUT,DELETE,HEAD,OPTIONS等。RestTemplate實現了最常見的方式，用的最多的就是Get和Post了，調用API可參考源碼，這里列舉幾個方法定義（GET、POST、DELETE）：
methods
public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables) public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... uriVariables)public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType,Object... uriVariables)public <T> ResponseEntity<T> postForEntity(String url, @Nullable Object request,Class<T> responseType, Object... uriVariables)public void delete(String url, Object... uriVariables)public void delete(URI url)
同時要注意兩個較為“靈活”的方法 exchange 和 execute 。
RestTemplate暴露的exchange與其它接口的不同：
（1）允許調用者指定HTTP請求的方法（GET,POST,DELETE等）
（2）可以在請求中增加body以及頭信息，其內容通過參數‘HttpEntity<?>requestEntity'描述
（3）exchange支持‘含參數的類型'（即泛型類）作為返回類型，該特性通過‘ParameterizedTypeReference<T>responseType'描述。
RestTemplate所有的GET,POST等等方法，最終調用的都是execute方法。excute方法的內部實現是將String格式的URI轉成了java.net.URI，之后調用了doExecute方法，doExecute方法的實現如下：
doExecute
/** * Execute the given method on the provided URI. * <p>The {@link ClientHttpRequest} is processed using the {@link RequestCallback}; * the response with the {@link ResponseExtractor}. * @param url the fully-expanded URL to connect to * @param method the HTTP method to execute (GET, POST, etc.) * @param requestCallback object that prepares the request (can be {@code null}) * @param responseExtractor object that extracts the return value from the response (can be {@code null}) * @return an arbitrary object, as returned by the {@link ResponseExtractor} */ @Nullable protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException { Assert.notNull(url, "'url' must not be null"); Assert.notNull(method, "'method' must not be null"); ClientHttpResponse response = null; try { ClientHttpRequest request = createRequest(url, method); if (requestCallback != null) { requestCallback.doWithRequest(request); } response = request.execute(); handleResponse(url, method, response); if (responseExtractor != null) { return responseExtractor.extractData(response); } else { return null; } } catch (IOException ex) { String resource = url.toString(); String query = url.getRawQuery(); resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource); throw new ResourceAccessException("I/O error on " + method.name() + " request for /"" + resource + "/": " + ex.getMessage(), ex); } finally { if (response != null) { response.close(); } } }doExecute方法封裝了模板方法，比如創建連接、處理請求和應答，關閉連接等。
多數人看到這里，估計都會覺得封裝一個RestClient不過如此吧？
3、簡單調用
以一個POST調用為例：
GoodsServiceClient
package com.power.demo.restclient;import com.power.demo.common.AppConst;import com.power.demo.restclient.clientrequest.ClientGetGoodsByGoodsIdRequest;import com.power.demo.restclient.clientresponse.ClientGetGoodsByGoodsIdResponse;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.beans.factory.annotation.Value;import org.springframework.stereotype.Component;import org.springframework.web.client.RestTemplate;/** * 商品REST接口客戶端 （demo測試用） **/@Componentpublic class GoodsServiceClient { //服務消費者調用的接口URL 形如：http://localhost:9090 @Value("${spring.power.serviceurl}") private String _serviceUrl; @Autowired private RestTemplate restTemplate; public ClientGetGoodsByGoodsIdResponse getGoodsByGoodsId(ClientGetGoodsByGoodsIdRequest request) { String svcUrl = getGoodsSvcUrl() + "/getinfobyid"; ClientGetGoodsByGoodsIdResponse response = null; try { response = restTemplate.postForObject(svcUrl, request, ClientGetGoodsByGoodsIdResponse.class); } catch (Exception e) { e.printStackTrace(); response = new ClientGetGoodsByGoodsIdResponse(); response.setCode(AppConst.FAIL); response.setMessage(e.toString()); } return response; } private String getGoodsSvcUrl() { String url = ""; if (_serviceUrl == null) { _serviceUrl = ""; } if (_serviceUrl.length() == 0) { return url; } if (_serviceUrl.substring(_serviceUrl.length() - 1, _serviceUrl.length()) == "/") { url = String.format("%sapi/v1/goods", _serviceUrl); } else { url = String.format("%s/api/v1/goods", _serviceUrl); } return url; }}demo里直接RestTemplate.postForObject方法調用，反序列化實體轉換這些RestTemplate內部封裝搞定。
二、問題匯總
1、no suitable HttpMessageConverter found for request type異常
這個問題通常會出現在postForObject中傳入對象進行調用的時候。
分析RestTemplate源碼，在HttpEntityRequestCallback類的doWithRequest方法中，如果 messageConverters （這個字段后面會繼續提及）列表字段循環處理的過程中沒有滿足return跳出的邏輯（也就是沒有匹配的HttpMessageConverter），則拋出上述異常：
HttpEntityRequestCallback.doWithRequest
@Override @SuppressWarnings("unchecked") public void doWithRequest(ClientHttpRequest httpRequest) throws IOException { super.doWithRequest(httpRequest); Object requestBody = this.requestEntity.getBody(); if (requestBody == null) { HttpHeaders httpHeaders = httpRequest.getHeaders(); HttpHeaders requestHeaders = this.requestEntity.getHeaders(); if (!requestHeaders.isEmpty()) { for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) { httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue())); } } if (httpHeaders.getContentLength() < 0) { httpHeaders.setContentLength(0L); } } else { Class<?> requestBodyClass = requestBody.getClass(); Type requestBodyType = (this.requestEntity instanceof RequestEntity ? ((RequestEntity<?>)this.requestEntity).getType() : requestBodyClass); HttpHeaders httpHeaders = httpRequest.getHeaders(); HttpHeaders requestHeaders = this.requestEntity.getHeaders(); MediaType requestContentType = requestHeaders.getContentType(); for (HttpMessageConverter<?> messageConverter : getMessageConverters()) { if (messageConverter instanceof GenericHttpMessageConverter) { GenericHttpMessageConverter<Object> genericConverter = (GenericHttpMessageConverter<Object>) messageConverter; if (genericConverter.canWrite(requestBodyType, requestBodyClass, requestContentType)) { if (!requestHeaders.isEmpty()) { for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) { httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue())); } } if (logger.isDebugEnabled()) { if (requestContentType != null) { logger.debug("Writing [" + requestBody + "] as /"" + requestContentType + "/" using [" + messageConverter + "]"); } else { logger.debug("Writing [" + requestBody + "] using [" + messageConverter + "]"); } } genericConverter.write(requestBody, requestBodyType, requestContentType, httpRequest); return; } } else if (messageConverter.canWrite(requestBodyClass, requestContentType)) { if (!requestHeaders.isEmpty()) { for (Map.Entry<String, List<String>> entry : requestHeaders.entrySet()) { httpHeaders.put(entry.getKey(), new LinkedList<>(entry.getValue())); } } if (logger.isDebugEnabled()) { if (requestContentType != null) { logger.debug("Writing [" + requestBody + "] as /"" + requestContentType + "/" using [" + messageConverter + "]"); } else { logger.debug("Writing [" + requestBody + "] using [" + messageConverter + "]"); } } ((HttpMessageConverter<Object>) messageConverter).write( requestBody, requestContentType, httpRequest); return; } } String message = "Could not write request: no suitable HttpMessageConverter found for request type [" + requestBodyClass.getName() + "]"; if (requestContentType != null) { message += " and content type [" + requestContentType + "]"; } throw new RestClientException(message); } }最簡單的解決方案是，可以通過包裝http請求頭，并將請求對象序列化成字符串的形式傳參，參考示例代碼如下：
postForObject
/* * Post請求調用 * */ public static String postForObject(RestTemplate restTemplate, String url, Object params) { HttpHeaders headers = new HttpHeaders(); MediaType type = MediaType.parseMediaType("application/json; charset=UTF-8"); headers.setContentType(type); headers.add("Accept", MediaType.APPLICATION_JSON.toString()); String json = SerializeUtil.Serialize(params); HttpEntity<String> formEntity = new HttpEntity<String>(json, headers); String result = restTemplate.postForObject(url, formEntity, String.class); return result; }如果我們還想直接返回對象，直接反序列化返回的字符串即可：
postForObject
/* * Post請求調用 * */ public static <T> T postForObject(RestTemplate restTemplate, String url, Object params, Class<T> clazz) { T response = null; String respStr = postForObject(restTemplate, url, params); response = SerializeUtil.DeSerialize(respStr, clazz); return response; }其中，序列化和反序列化工具比較多，常用的比如fastjson、jackson和gson。
2、no suitable HttpMessageConverter found for response type異常
和發起請求發生異常一樣，處理應答的時候也會有問題。
StackOverflow上有人問過相同的問題，根本原因是HTTP消息轉換器HttpMessageConverter缺少 MIME Type ，也就是說HTTP在把輸出結果傳送到客戶端的時候，客戶端必須啟動適當的應用程序來處理這個輸出文檔，這可以通過多種MIME（多功能網際郵件擴充協議）Type來完成。
對于服務端應答，很多HttpMessageConverter默認支持的媒體類型（MIMEType）都不同。StringHttpMessageConverter默認支持的則是MediaType.TEXT_PLAIN，SourceHttpMessageConverter默認支持的則是MediaType.TEXT_XML，FormHttpMessageConverter默認支持的是MediaType.APPLICATION_FORM_URLENCODED和MediaType.MULTIPART_FORM_DATA，在REST服務中，我們用到的最多的還是 MappingJackson2HttpMessageConverter ，這是一個比較通用的轉化器（繼承自GenericHttpMessageConverter接口），根據分析，它默認支持的MIMEType為MediaType.APPLICATION_JSON：
MappingJackson2HttpMessageConverter
/** * Construct a new {@link MappingJackson2HttpMessageConverter} with a custom {@link ObjectMapper}. * You can use {@link Jackson2ObjectMapperBuilder} to build it easily. * @see Jackson2ObjectMapperBuilder#json() */ public MappingJackson2HttpMessageConverter(ObjectMapper objectMapper) { super(objectMapper, MediaType.APPLICATION_JSON, new MediaType("application", "*+json")); }但是有些應用接口默認的應答MIMEType不是application/json，比如我們調用一個外部天氣預報接口，如果使用RestTemplate的默認配置，直接返回一個字符串應答是沒有問題的：
String url = "http://wthrcdn.etouch.cn/weather_mini?city=上海";String result = restTemplate.getForObject(url, String.class);ClientWeatherResultVO vo = SerializeUtil.DeSerialize(result, ClientWeatherResultVO.class);
但是，如果我們想直接返回一個實體對象：
String url = "http://wthrcdn.etouch.cn/weather_mini?city=上海";ClientWeatherResultVO weatherResultVO = restTemplate.getForObject(url, ClientWeatherResultVO.class);
則直接報異常：
Could not extract response: no suitable HttpMessageConverter found for response type [class ]
and content type [application/octet-stream]
很多人碰到過這個問題，首次碰到估計大多都比較懵吧，很多接口都是json或者xml或者plain text格式返回的，什么是application/octet-stream？
查看RestTemplate源代碼，一路跟蹤下去會發現 HttpMessageConverterExtractor 類的extractData方法有個解析應答及反序列化邏輯，如果不成功，拋出的異常信息和上述一致：
HttpMessageConverterExtractor.extractData
@Override @SuppressWarnings({"unchecked", "rawtypes", "resource"}) public T extractData(ClientHttpResponse response) throws IOException { MessageBodyClientHttpResponseWrapper responseWrapper = new MessageBodyClientHttpResponseWrapper(response); if (!responseWrapper.hasMessageBody() || responseWrapper.hasEmptyMessageBody()) { return null; } MediaType contentType = getContentType(responseWrapper); try { for (HttpMessageConverter<?> messageConverter : this.messageConverters) { if (messageConverter instanceof GenericHttpMessageConverter) { GenericHttpMessageConverter<?> genericMessageConverter = (GenericHttpMessageConverter<?>) messageConverter; if (genericMessageConverter.canRead(this.responseType, null, contentType)) { if (logger.isDebugEnabled()) { logger.debug("Reading [" + this.responseType + "] as /"" + contentType + "/" using [" + messageConverter + "]"); } return (T) genericMessageConverter.read(this.responseType, null, responseWrapper); } } if (this.responseClass != null) { if (messageConverter.canRead(this.responseClass, contentType)) { if (logger.isDebugEnabled()) { logger.debug("Reading [" + this.responseClass.getName() + "] as /"" + contentType + "/" using [" + messageConverter + "]"); } return (T) messageConverter.read((Class) this.responseClass, responseWrapper); } } } } catch (IOException | HttpMessageNotReadableException ex) { throw new RestClientException("Error while extracting response for type [" + this.responseType + "] and content type [" + contentType + "]", ex); } throw new RestClientException("Could not extract response: no suitable HttpMessageConverter found " + "for response type [" + this.responseType + "] and content type [" + contentType + "]"); }StackOverflow上的解決的示例代碼可以接受，但是并不準確，常見的MIMEType都應該加進去，貼一下我認為正確的代碼：
RestTemplateConfig
package com.power.demo.restclient.config;import com.fasterxml.jackson.databind.ObjectMapper;import com.google.common.collect.Lists;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.web.client.RestTemplateBuilder;import org.springframework.context.annotation.Bean;import org.springframework.http.MediaType;import org.springframework.http.converter.*;import org.springframework.http.converter.cbor.MappingJackson2CborHttpMessageConverter;import org.springframework.http.converter.feed.AtomFeedHttpMessageConverter;import org.springframework.http.converter.feed.RssChannelHttpMessageConverter;import org.springframework.http.converter.json.GsonHttpMessageConverter;import org.springframework.http.converter.json.JsonbHttpMessageConverter;import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;import org.springframework.http.converter.smile.MappingJackson2SmileHttpMessageConverter;import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;import org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter;import org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter;import org.springframework.http.converter.xml.SourceHttpMessageConverter;import org.springframework.stereotype.Component;import org.springframework.util.ClassUtils;import org.springframework.web.client.RestTemplate;import java.util.Arrays;import java.util.List;@Componentpublic class RestTemplateConfig { private static final boolean romePresent = ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", RestTemplate .class.getClassLoader()); private static final boolean jaxb2Present = ClassUtils.isPresent("javax.xml.bind.Binder", RestTemplate.class.getClassLoader()); private static final boolean jackson2Present = ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", RestTemplate.class.getClassLoader()) && ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", RestTemplate.class.getClassLoader()); private static final boolean jackson2XmlPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", RestTemplate.class.getClassLoader()); private static final boolean jackson2SmilePresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.smile.SmileFactory", RestTemplate.class.getClassLoader()); private static final boolean jackson2CborPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.cbor.CBORFactory", RestTemplate.class.getClassLoader()); private static final boolean gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", RestTemplate.class.getClassLoader()); private static final boolean jsonbPresent = ClassUtils.isPresent("javax.json.bind.Jsonb", RestTemplate.class.getClassLoader()); // 啟動的時候要注意，由于我們在服務中注入了RestTemplate，所以啟動的時候需要實例化該類的一個實例 @Autowired private RestTemplateBuilder builder; @Autowired private ObjectMapper objectMapper; // 使用RestTemplateBuilder來實例化RestTemplate對象，spring默認已經注入了RestTemplateBuilder實例 @Bean public RestTemplate restTemplate() { RestTemplate restTemplate = builder.build(); List<HttpMessageConverter<?>> messageConverters = Lists.newArrayList(); MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter(); converter.setObjectMapper(objectMapper); //不加會出現異常 //Could not extract response: no suitable HttpMessageConverter found for response type [class ] MediaType[] mediaTypes = new MediaType[]{ MediaType.APPLICATION_JSON, MediaType.APPLICATION_OCTET_STREAM, MediaType.APPLICATION_JSON_UTF8, MediaType.TEXT_HTML, MediaType.TEXT_PLAIN, MediaType.TEXT_XML, MediaType.APPLICATION_STREAM_JSON, MediaType.APPLICATION_ATOM_XML, MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_PDF, }; converter.setSupportedMediaTypes(Arrays.asList(mediaTypes)); //messageConverters.add(converter); if (jackson2Present) { messageConverters.add(converter); } else if (gsonPresent) { messageConverters.add(new GsonHttpMessageConverter()); } else if (jsonbPresent) { messageConverters.add(new JsonbHttpMessageConverter()); } messageConverters.add(new FormHttpMessageConverter()); messageConverters.add(new ByteArrayHttpMessageConverter()); messageConverters.add(new StringHttpMessageConverter()); messageConverters.add(new ResourceHttpMessageConverter(false)); messageConverters.add(new SourceHttpMessageConverter()); messageConverters.add(new AllEncompassingFormHttpMessageConverter()); if (romePresent) { messageConverters.add(new AtomFeedHttpMessageConverter()); messageConverters.add(new RssChannelHttpMessageConverter()); } if (jackson2XmlPresent) { messageConverters.add(new MappingJackson2XmlHttpMessageConverter()); } else if (jaxb2Present) { messageConverters.add(new Jaxb2RootElementHttpMessageConverter()); } if (jackson2SmilePresent) { messageConverters.add(new MappingJackson2SmileHttpMessageConverter()); } if (jackson2CborPresent) { messageConverters.add(new MappingJackson2CborHttpMessageConverter()); } restTemplate.setMessageConverters(messageConverters); return restTemplate; }}看到上面的代碼，再對比一下RestTemplate內部實現，就知道我參考了RestTemplate的源碼，有潔癖的人可能會說這一坨代碼有點啰嗦，上面那一堆static final的變量和messageConverters填充數據方法，暴露了RestTemplate的實現，如果RestTemplate修改了，這里也要改，非常不友好，而且看上去一點也不OO。
經過分析，RestTemplateBuilder.build()構造了RestTemplate對象，只要將內部MappingJackson2HttpMessageConverter修改一下支持的MediaType即可，RestTemplate的messageConverters字段雖然是private final的，我們依然可以通過反射修改之，改進后的代碼如下：
RestTemplateConfig
package com.power.demo.restclient.config;import com.fasterxml.jackson.databind.ObjectMapper;import com.google.common.collect.Lists;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.web.client.RestTemplateBuilder;import org.springframework.context.annotation.Bean;import org.springframework.http.MediaType;import org.springframework.http.converter.HttpMessageConverter;import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;import org.springframework.stereotype.Component;import org.springframework.web.client.RestTemplate;import java.lang.reflect.Field;import java.util.Arrays;import java.util.List;import java.util.Optional;import java.util.stream.Collectors;@Componentpublic class RestTemplateConfig { // 啟動的時候要注意，由于我們在服務中注入了RestTemplate，所以啟動的時候需要實例化該類的一個實例 @Autowired private RestTemplateBuilder builder; @Autowired private ObjectMapper objectMapper; // 使用RestTemplateBuilder來實例化RestTemplate對象，spring默認已經注入了RestTemplateBuilder實例 @Bean public RestTemplate restTemplate() { RestTemplate restTemplate = builder.build(); List<HttpMessageConverter<?>> messageConverters = Lists.newArrayList(); MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter(); converter.setObjectMapper(objectMapper); //不加可能會出現異常 //Could not extract response: no suitable HttpMessageConverter found for response type [class ] MediaType[] mediaTypes = new MediaType[]{ MediaType.APPLICATION_JSON, MediaType.APPLICATION_OCTET_STREAM, MediaType.TEXT_HTML, MediaType.TEXT_PLAIN, MediaType.TEXT_XML, MediaType.APPLICATION_STREAM_JSON, MediaType.APPLICATION_ATOM_XML, MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_JSON_UTF8, MediaType.APPLICATION_PDF, }; converter.setSupportedMediaTypes(Arrays.asList(mediaTypes)); try { //通過反射設置MessageConverters Field field = restTemplate.getClass().getDeclaredField("messageConverters"); field.setAccessible(true); List<HttpMessageConverter<?>> orgConverterList = (List<HttpMessageConverter<?>>) field.get(restTemplate); Optional<HttpMessageConverter<?>> opConverter = orgConverterList.stream() .filter(x -> x.getClass().getName().equalsIgnoreCase(MappingJackson2HttpMessageConverter.class .getName())) .findFirst(); if (opConverter.isPresent() == false) { return restTemplate; } messageConverters.add(converter);//添加MappingJackson2HttpMessageConverter //添加原有的剩余的HttpMessageConverter List<HttpMessageConverter<?>> leftConverters = orgConverterList.stream() .filter(x -> x.getClass().getName().equalsIgnoreCase(MappingJackson2HttpMessageConverter.class .getName()) == false) .collect(Collectors.toList()); messageConverters.addAll(leftConverters); System.out.println(String.format("【HttpMessageConverter】原有數量：%s，重新構造后數量：%s" , orgConverterList.size(), messageConverters.size())); } catch (Exception e) { e.printStackTrace(); } restTemplate.setMessageConverters(messageConverters); return restTemplate; }}除了一個messageConverters字段，看上去我們不再關心RestTemplate那些外部依賴包和內部構造過程，果然干凈簡潔好維護了很多。
3、亂碼問題
這個也是一個非常經典的問題。解決方案非常簡單，找到HttpMessageConverter，看看默認支持的Charset。AbstractJackson2HttpMessageConverter是很多HttpMessageConverter的基類，默認編碼為UTF-8：
AbstractJackson2HttpMessageConverter
public abstract class AbstractJackson2HttpMessageConverter extends AbstractGenericHttpMessageConverter<Object> { public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;}而StringHttpMessageConverter比較特殊，有人反饋過發生亂碼問題由它默認支持的編碼 ISO-8859-1 引起：
StringHttpMessageConverter
/** * Implementation of {@link HttpMessageConverter} that can read and write strings. * * <p>By default, this converter supports all media types ({@code }), * and writes with a {@code Content-Type} of {@code text/plain}. This can be overridden * by setting the {@link #setSupportedMediaTypes supportedMediaTypes} property. * * @author Arjen Poutsma * @author Juergen Hoeller * @since 3.0 */public class StringHttpMessageConverter extends AbstractHttpMessageConverter<String> { public static final Charset DEFAULT_CHARSET = StandardCharsets.ISO_8859_1; /** * A default constructor that uses {@code "ISO-8859-1"} as the default charset. * @see #StringHttpMessageConverter(Charset) */ public StringHttpMessageConverter() { this(DEFAULT_CHARSET); }}如果在使用過程中發生亂碼，我們可以通過方法設置HttpMessageConverter支持的編碼，常用的有UTF-8、GBK等。
4、反序列化異常
這是開發過程中容易碰到的又一個問題。因為Java的開源框架和工具類非常之多，而且版本更迭頻繁，所以經常發生一些意想不到的坑。
以joda time為例，joda time是流行的java時間和日期框架，但是如果你的接口對外暴露joda time的類型，比如DateTime，那么接口調用方（同構和異構系統）可能會碰到序列化難題，反序列化時甚至直接拋出如下異常：
org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class org.joda.time.Chronology]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `org.joda.time.Chronology` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
at [Source: (PushbackInputStream);
我在前廠就碰到過，后來為了調用方便，改回直接暴露Java的Date類型。
當然解決的方案不止這一種，可以使用jackson支持自定義類的序列化和反序列化的方式。在精度要求不是很高的系統里，實現簡單的DateTime自定義序列化：
DateTimeSerializer
package com.power.demo.util;import com.fasterxml.jackson.core.JsonGenerator;import com.fasterxml.jackson.core.JsonProcessingException;import com.fasterxml.jackson.databind.JsonSerializer;import com.fasterxml.jackson.databind.SerializerProvider;import org.joda.time.DateTime;import org.joda.time.format.DateTimeFormat;import org.joda.time.format.DateTimeFormatter;import java.io.IOException;/** * 在默認情況下，jackson會將joda time序列化為較為復雜的形式，不利于閱讀，并且對象較大。 * <p> * JodaTime 序列化的時候可以將datetime序列化為字符串，更容易讀 **/public class DateTimeSerializer extends JsonSerializer<DateTime> { private static DateTimeFormatter dateFormatter = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss"); @Override public void serialize(DateTime value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException { jgen.writeString(value.toString(dateFormatter)); }}以及DateTime反序列化：
DatetimeDeserializer
package com.power.demo.util;import com.fasterxml.jackson.core.JsonParser;import com.fasterxml.jackson.core.JsonProcessingException;import com.fasterxml.jackson.databind.DeserializationContext;import com.fasterxml.jackson.databind.JsonDeserializer;import com.fasterxml.jackson.databind.JsonNode;import org.joda.time.DateTime;import org.joda.time.format.DateTimeFormat;import org.joda.time.format.DateTimeFormatter;import java.io.IOException;/** * JodaTime 反序列化將字符串轉化為datetime **/public class DatetimeDeserializer extends JsonDeserializer<DateTime> { private static DateTimeFormatter dateFormatter = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss"); @Override public DateTime deserialize(JsonParser jp, DeserializationContext context) throws IOException, JsonProcessingException { JsonNode node = jp.getCodec().readTree(jp); String s = node.asText(); DateTime parse = DateTime.parse(s, dateFormatter); return parse; }}最后可以在RestTemplateConfig類中對常見調用問題進行匯總處理，可以參考如下：
RestTemplateConfig
package com.power.demo.restclient.config;import com.fasterxml.jackson.databind.ObjectMapper;import com.fasterxml.jackson.databind.module.SimpleModule;import com.google.common.collect.Lists;import com.power.demo.util.DateTimeSerializer;import com.power.demo.util.DatetimeDeserializer;import org.joda.time.DateTime;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.web.client.RestTemplateBuilder;import org.springframework.context.annotation.Bean;import org.springframework.http.MediaType;import org.springframework.http.converter.HttpMessageConverter;import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;import org.springframework.stereotype.Component;import org.springframework.web.client.RestTemplate;import java.lang.reflect.Field;import java.util.Arrays;import java.util.List;import java.util.Optional;import java.util.stream.Collectors;@Componentpublic class RestTemplateConfig { // 啟動的時候要注意，由于我們在服務中注入了RestTemplate，所以啟動的時候需要實例化該類的一個實例 @Autowired private RestTemplateBuilder builder; @Autowired private ObjectMapper objectMapper; // 使用RestTemplateBuilder來實例化RestTemplate對象，spring默認已經注入了RestTemplateBuilder實例 @Bean public RestTemplate restTemplate() { RestTemplate restTemplate = builder.build(); //注冊model,用于實現jackson joda time序列化和反序列化 SimpleModule module = new SimpleModule(); module.addSerializer(DateTime.class, new DateTimeSerializer()); module.addDeserializer(DateTime.class, new DatetimeDeserializer()); objectMapper.registerModule(module); List<HttpMessageConverter<?>> messageConverters = Lists.newArrayList(); MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter(); converter.setObjectMapper(objectMapper); //不加會出現異常 //Could not extract response: no suitable HttpMessageConverter found for response type [class ] MediaType[] mediaTypes = new MediaType[]{ MediaType.APPLICATION_JSON, MediaType.APPLICATION_OCTET_STREAM, MediaType.TEXT_HTML, MediaType.TEXT_PLAIN, MediaType.TEXT_XML, MediaType.APPLICATION_STREAM_JSON, MediaType.APPLICATION_ATOM_XML, MediaType.APPLICATION_FORM_URLENCODED, MediaType.APPLICATION_JSON_UTF8, MediaType.APPLICATION_PDF, }; converter.setSupportedMediaTypes(Arrays.asList(mediaTypes)); try { //通過反射設置MessageConverters Field field = restTemplate.getClass().getDeclaredField("messageConverters"); field.setAccessible(true); List<HttpMessageConverter<?>> orgConverterList = (List<HttpMessageConverter<?>>) field.get(restTemplate); Optional<HttpMessageConverter<?>> opConverter = orgConverterList.stream() .filter(x -> x.getClass().getName().equalsIgnoreCase(MappingJackson2HttpMessageConverter.class .getName())) .findFirst(); if (opConverter.isPresent() == false) { return restTemplate; } messageConverters.add(converter);//添加MappingJackson2HttpMessageConverter //添加原有的剩余的HttpMessageConverter List<HttpMessageConverter<?>> leftConverters = orgConverterList.stream() .filter(x -> x.getClass().getName().equalsIgnoreCase(MappingJackson2HttpMessageConverter.class .getName()) == false) .collect(Collectors.toList()); messageConverters.addAll(leftConverters); System.out.println(String.format("【HttpMessageConverter】原有數量：%s，重新構造后數量：%s" , orgConverterList.size(), messageConverters.size())); } catch (Exception e) { e.printStackTrace(); } restTemplate.setMessageConverters(messageConverters); return restTemplate; }}目前良好地解決了RestTemplate常用調用問題，而且不需要你寫RestTemplate幫助工具類了。
上面列舉的這些常見問題，其實.NET下面也有，有興趣大家可以搜索一下微軟的HttpClient常見使用問題，用過的人都深有體會。更不用提 RestSharp 這個開源類庫，幾年前用的過程中發現了非常多的Bug，到現在還有一個反序列化數組的問題困擾著我們，我只好自己造個簡單輪子特殊處理，給我最深刻的經驗就是，很多看上去簡單的功能，真的碰到了依然會花掉不少的時間去排查和解決，甚至要翻看源碼。所以，我們寫代碼要認識到，越是通用的工具，越需要考慮到特例，可能你需要花80%以上的精力去處理20%的特殊情況，這估計也是滿足常見的二八定律吧。
參考：
https://stackoverflow.com/questions/21854369/no-suitable-httpmessageconverter-found-for-response-type
https://stackoverflow.com/questions/40726145/rest-templatecould-not-extract-response-no-suitable-httpmessageconverter-found
https://stackoverflow.com/questions/10579122/resttemplate-no-suitable-httpmessageconverter
http://forum.spring.io/forum/spring-projects/android/126794-no-suitable-httpmessageconverter-found
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持VeVb武林網。
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