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
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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
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/** * 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} */@nullableprotected <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
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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） **/@componentpublic 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
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@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
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/* * 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
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/* * 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。
stackoverflow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
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/** * 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：
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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：
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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
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@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
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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()); // 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
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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 { // 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
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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
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/** * 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
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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
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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
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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 { // 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轮子特殊处理，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：
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内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：http://www.cnblogs.com/jeffwongishandsome/p/spring-boot-consume-rest-api-by-resttemplate.html
