Spring Boot使用RestTemplate消费REST服务的几个问题记录

我们可以通过spring boot快速开发rest接口，同时也可能需要在实现接口的过程中，通过spring boot调用内外部rest接口完成业务逻辑。

在spring boot中，调用rest api常见的一般主要有两种方式，通过自带的resttemplate或者自己开发http客户端工具实现服务调用。

resttemplate基本功能非常强大，不过某些特殊场景，我们可能还是更习惯用自己封装的工具类，比如上传文件至分布式文件系统、处理带证书的https请求等。

本文以resttemplate来举例，记录几个使用resttemplate调用接口过程中发现的问题和解决方案。

一、resttemplate简介

1、什么是resttemplate

我们自己封装的httpclient，通常都会有一些模板代码，比如建立连接，构造请求头和请求体，然后根据响应，解析响应信息，最后关闭连接。

resttemplate是spring中对httpclient的再次封装，简化了发起http请求以及处理响应的过程，抽象层级更高，减少消费者的模板代码，使冗余代码更少。

其实仔细想想spring boot下的很多xxxtemplate类，它们也提供各种模板方法，只不过抽象的层次更高，隐藏了更多细节而已。

顺便提一下，spring cloud有一个声明式服务调用feign,是基于netflix feign实现的，整合了spring cloud ribbon与 spring cloud hystrix，并且实现了声明式的web服务客户端定义方式。

本质上feign是在resttemplate的基础上对其再次封装，由它来帮助我们定义和实现依赖服务接口的定义。

2、resttemplate常见方法

常见的rest服务有很多种请求方式，如get,post,put,delete,head,options等。resttemplate实现了最常见的方式，用的最多的就是get和post了，调用api可参考源码，这里列举几个方法定义（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)

同时要注意两个较为“灵活”的方法 exchange 和 execute 。

resttemplate暴露的exchange与其它接口的不同：

（1）允许调用者指定http请求的方法（get,post,delete等）

（2）可以在请求中增加body以及头信息，其内容通过参数‘httpentity<?>requestentity'描述

（3）exchange支持‘含参数的类型'（即泛型类）作为返回类型，该特性通过‘parameterizedtypereference<t>responsetype'描述。

resttemplate所有的get,post等等方法，最终调用的都是execute方法。excute方法的内部实现是将string格式的uri转成了java.net.uri，之后调用了doexecute方法，doexecute方法的实现如下：

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方法封装了模板方法，比如创建连接、处理请求和应答，关闭连接等。

多数人看到这里，估计都会觉得封装一个restclient不过如此吧？

3、简单调用

以一个post调用为例：

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接口客户端 （demo测试用）
 **/
@component
public class goodsserviceclient {

 //服务消费者调用的接口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方法调用，反序列化实体转换这些resttemplate内部封装搞定。

二、问题汇总

1、no suitable httpmessageconverter found for request type异常

这个问题通常会出现在postforobject中传入对象进行调用的时候。

分析resttemplate源码，在httpentityrequestcallback类的dowithrequest方法中，如果 messageconverters （这个字段后面会继续提及）列表字段循环处理的过程中没有满足return跳出的逻辑（也就是没有匹配的httpmessageconverter），则抛出上述异常：

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);
   }
  }

最简单的解决方案是，可以通过包装http请求头，并将请求对象序列化成字符串的形式传参，参考示例代码如下：

postforobject

 /*
  * post请求调用
  * */
 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;
 }

如果我们还想直接返回对象，直接反序列化返回的字符串即可：

postforobject

 /*
  * post请求调用
  * */
 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较多，常用的比如fastjson、jackson和gson。

2、no suitable httpmessageconverter found for response type异常

和发起请求发生异常一样，处理应答的时候也会有问题。

*上有人问过相同的问题，根本原因是http消息转换器httpmessageconverter缺少 mime type ，也就是说http在把输出结果传送到客户端的时候，客户端必须启动适当的应用程序来处理这个输出文档，这可以通过多种mime（多功能网际邮件扩充协议）type来完成。

对于服务端应答，很多httpmessageconverter默认支持的媒体类型（mimetype）都不同。stringhttpmessageconverter默认支持的则是mediatype.text_plain，sourcehttpmessageconverter默认支持的则是mediatype.text_xml，formhttpmessageconverter默认支持的是mediatype.application_form_urlencoded和mediatype.multipart_form_data，在rest服务中，我们用到的最多的还是 mappingjackson2httpmessageconverter ，这是一个比较通用的转化器（继承自generichttpmessageconverter接口），根据分析，它默认支持的mimetype为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"));
 }

但是有些应用接口默认的应答mimetype不是application/json，比如我们调用一个外部天气预报接口，如果使用resttemplate的默认配置，直接返回一个字符串应答是没有问题的：

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，如果我们想直接返回一个实体对象：

string url = "http://wthrcdn.etouch.cn/weather_mini?city=上海";

clientweatherresultvo weatherresultvo = resttemplate.getforobject(url, clientweatherresultvo.class);

则直接报异常：

could not extract response: no suitable httpmessageconverter found for response type [class ]

and content type [application/octet-stream]

很多人碰到过这个问题，首次碰到估计大多都比较懵吧，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源代码，一路跟踪下去会发现 httpmessageconverterextractor 类的extractdata方法有个解析应答及反序列化逻辑，如果不成功，抛出的异常信息和上述一致：

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 + "]");
 }

*上的解决的示例代码可以接受，但是并不准确，常见的mimetype都应该加进去，贴一下我认为正确的代码：

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;

@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());

 // 启动的时候要注意，由于我们在服务中注入了resttemplate，所以启动的时候需要实例化该类的一个实例
 @autowired
 private resttemplatebuilder builder;

 @autowired
 private objectmapper objectmapper;

 // 使用resttemplatebuilder来实例化resttemplate对象，spring默认已经注入了resttemplatebuilder实例
 @bean
 public resttemplate resttemplate() {

  resttemplate resttemplate = builder.build();

  list<httpmessageconverter<?>> messageconverters = lists.newarraylist();
  mappingjackson2httpmessageconverter converter = new mappingjackson2httpmessageconverter();
  converter.setobjectmapper(objectmapper);

  //不加会出现异常
  //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;
 }

}

看到上面的代码，再对比一下resttemplate内部实现，就知道我参考了resttemplate的源码，有洁癖的人可能会说这一坨代码有点啰嗦，上面那一堆static final的变量和messageconverters填充数据方法，暴露了resttemplate的实现，如果resttemplate修改了，这里也要改，非常不友好，而且看上去一点也不oo。

经过分析，resttemplatebuilder.build()构造了resttemplate对象，只要将内部mappingjackson2httpmessageconverter修改一下支持的mediatype即可，resttemplate的messageconverters字段虽然是private final的，我们依然可以通过反射修改之，改进后的代码如下：

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;

@component
public class resttemplateconfig {

 // 启动的时候要注意，由于我们在服务中注入了resttemplate，所以启动的时候需要实例化该类的一个实例
 @autowired
 private resttemplatebuilder builder;

 @autowired
 private objectmapper objectmapper;

 // 使用resttemplatebuilder来实例化resttemplate对象，spring默认已经注入了resttemplatebuilder实例
 @bean
 public resttemplate resttemplate() {

  resttemplate resttemplate = builder.build();

  list<httpmessageconverter<?>> messageconverters = lists.newarraylist();
  mappingjackson2httpmessageconverter converter = new mappingjackson2httpmessageconverter();
  converter.setobjectmapper(objectmapper);

  //不加可能会出现异常
  //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 {
   //通过反射设置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】原有数量：%s，重新构造后数量：%s"
     , orgconverterlist.size(), messageconverters.size()));

  } catch (exception e) {
   e.printstacktrace();
  }

  resttemplate.setmessageconverters(messageconverters);

  return resttemplate;
 }
}

除了一个messageconverters字段，看上去我们不再关心resttemplate那些外部依赖包和内部构造过程，果然干净简洁好维护了很多。

3、乱码问题

这个也是一个非常经典的问题。解决方案非常简单，找到httpmessageconverter，看看默认支持的charset。abstractjackson2httpmessageconverter是很多httpmessageconverter的基类，默认编码为utf-8：

abstractjackson2httpmessageconverter

public abstract class abstractjackson2httpmessageconverter extends abstractgenerichttpmessageconverter<object> {

 public static final charset default_charset = standardcharsets.utf_8;

}

而stringhttpmessageconverter比较特殊，有人反馈过发生乱码问题由它默认支持的编码 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);
 }

}

如果在使用过程中发生乱码，我们可以通过方法设置httpmessageconverter支持的编码，常用的有utf-8、gbk等。

4、反序列化异常

这是开发过程中容易碰到的又一个问题。因为java的开源框架和工具类非常之多，而且版本更迭频繁，所以经常发生一些意想不到的坑。

以joda time为例，joda time是流行的java时间和日期框架，但是如果你的接口对外暴露joda time的类型，比如datetime，那么接口调用方（同构和异构系统）可能会碰到序列化难题，反序列化时甚至直接抛出如下异常：

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);

我在前厂就碰到过，后来为了调用方便，改回直接暴露java的date类型。

当然解决的方案不止这一种，可以使用jackson支持自定义类的序列化和反序列化的方式。在精度要求不是很高的系统里，实现简单的datetime自定义序列化：

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;

/**
 * 在默认情况下，jackson会将joda time序列化为较为复杂的形式，不利于阅读，并且对象较大。
 * <p>
 * jodatime 序列化的时候可以将datetime序列化为字符串，更容易读
 **/
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 反序列化将字符串转化为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类中对常见调用问题进行汇总处理，可以参考如下：

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;

@component
public class resttemplateconfig {

 // 启动的时候要注意，由于我们在服务中注入了resttemplate，所以启动的时候需要实例化该类的一个实例
 @autowired
 private resttemplatebuilder builder;

 @autowired
 private objectmapper objectmapper;

 // 使用resttemplatebuilder来实例化resttemplate对象，spring默认已经注入了resttemplatebuilder实例
 @bean
 public resttemplate resttemplate() {

  resttemplate resttemplate = builder.build();

  //注册model,用于实现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);

  //不加会出现异常
  //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 {
   //通过反射设置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】原有数量：%s，重新构造后数量：%s"
     , orgconverterlist.size(), messageconverters.size()));

  } catch (exception e) {
   e.printstacktrace();
  }

  resttemplate.setmessageconverters(messageconverters);

  return resttemplate;
 }
}

目前良好地解决了resttemplate常用调用问题，而且不需要你写resttemplate帮助工具类了。

上面列举的这些常见问题，其实.net下面也有，有兴趣大家可以搜索一下微软的httpclient常见使用问题，用过的人都深有体会。更不用提 restsharp 这个开源类库，几年前用的过程中发现了非常多的bug，到现在还有一个反序列化数组的问题困扰着我们，我只好自己造个简单*特殊处理，给我最深刻的经验就是，很多看上去简单的功能，真的碰到了依然会花掉不少的时间去排查和解决，甚至要翻看源码。所以，我们写代码要认识到，越是通用的工具，越需要考虑到特例，可能你需要花80%以上的精力去处理20%的特殊情况，这估计也是满足常见的二八定律吧。

参考：

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。