Spring Boot使用RestTemplate消費REST服務的幾個問題記錄

本文轉載自查看原文 2018-05-17 09:56 45733 spring/ 經驗/ java/ 服務/ 工具/ 系統

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,是基於Netflix Feign實現的，整合了Spring Cloud Ribbon與 Spring Cloud Hystrix，並且實現了聲明式的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）：

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)

methods

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：

    /**
     * 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

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：

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）
 **/
@Component
public 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;
    }

}

GoodsServiceClient

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：

        @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);
            }
        }

HttpEntityRequestCallback.doWithRequest

最簡單的解決方案是，可以通過包裝http請求頭，並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：

    /*
     * 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;
    }

postForObject

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：

    /*
     * 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;
    }

postForObject

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。

對於服務端應答，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：

    /**
     * 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"));
    }

MappingJackson2HttpMessageConverter

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：

    @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 + "]");
    }

HttpMessageConverterExtractor.extractData

StackOverflow上的解決的示例代碼可以接受，但是並不准確，常見的MIMEType都應該加進去，貼一下我認為正確的代碼：

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;

@Component
public 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，由於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; } }

RestTemplateConfig

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：

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;

@Component
public class RestTemplateConfig {

    // 啟動的時候要注意，由於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;
    }

}

RestTemplateConfig

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：

public abstract class AbstractJackson2HttpMessageConverter extends AbstractGenericHttpMessageConverter<Object> {

    public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

}

AbstractJackson2HttpMessageConverter

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：

/**
 * Implementation of {@link HttpMessageConverter} that can read and write strings.
 *
 * <p>By default, this converter supports all media types ({@code &#42;&#47;&#42;}),
 * 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);
    }

}

StringHttpMessageConverter

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，可以參考這里，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：

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，不利於閱讀，並且對象較大。
 * <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));
    }
}

DateTimeSerializer

以及DateTime反序列化：

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;
    }
}

DatetimeDeserializer

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：

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;

@Component
public class RestTemplateConfig {

    // 啟動的時候要注意，由於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,用於實現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;
    }

}

RestTemplateConfig

目前良好地解決了RestTemplate常見調用問題，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着我們，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://www.cnblogs.com/rollenholt/p/3934649.html

http://forum.spring.io/forum/spring-projects/android/126794-no-suitable-httpmessageconverter-found

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