我們可以通過spring boot快速開發rest接口，同時也可能需要在實現接口的過程中，通過spring boot調用內外部rest接口完成業務邏輯。
在spring boot中，調用rest api常見的一般主要有兩種方式，通過自帶的resttemplate或者自己開發http客戶端工具實現服務調用。
resttemplate基本功能非常強大，不過某些特殊場景，我們可能還是更習慣用自己封裝的工具類，比如上傳文件至分布式文件系統、處理帶證書的https請求等。
本文以resttemplate來舉例，記錄幾個使用resttemplate調用接口過程中發現的問題和解決方案。
一、resttemplate簡介
1、什么是resttemplate
我們自己封裝的httpclient，通常都會有一些模板代碼，比如建立連接，構造請求頭和請求體，然后根據響應，解析響應信息，最后關閉連接。
resttemplate是spring中對httpclient的再次封裝，簡化了發起http請求以及處理響應的過程，抽象層級更高，減少消費者的模板代碼，使冗余代碼更少。
其實仔細想想spring boot下的很多xxxtemplate類，它們也提供各種模板方法，只不過抽象的層次更高，隱藏了更多細節而已。
順便提一下，spring cloud有一個聲明式服務調用feign,是基于netflix feign實現的，整合了spring cloud ribbon與 spring cloud hystrix，并且實現了聲明式的web服務客戶端定義方式。
本質上feign是在resttemplate的基礎上對其再次封裝，由它來幫助我們定義和實現依賴服務接口的定義。
2、resttemplate常見方法
常見的rest服務有很多種請求方式，如get,post,put,delete,head,options等。resttemplate實現了最常見的方式，用的最多的就是get和post了，調用api可參考源碼，這里列舉幾個方法定義（get、post、delete）：
methods
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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) |
同時要注意兩個較為“靈活”的方法 exchange 和 execute 。
resttemplate暴露的exchange與其它接口的不同：
（1）允許調用者指定http請求的方法（get,post,delete等）
（2）可以在請求中增加body以及頭信息，其內容通過參數‘httpentity<?>requestentity'描述
（3）exchange支持‘含參數的類型'（即泛型類）作為返回類型，該特性通過‘parameterizedtypereference<t>responsetype'描述。
resttemplate所有的get,post等等方法，最終調用的都是execute方法。excute方法的內部實現是將string格式的uri轉成了java.net.uri，之后調用了doexecute方法，doexecute方法的實現如下：
doexecute
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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方法封裝了模板方法，比如創建連接、處理請求和應答，關閉連接等。
多數人看到這里，估計都會覺得封裝一個restclient不過如此吧？
3、簡單調用
以一個post調用為例：
goodsserviceclient
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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接口客戶端 （demo測試用） **/@componentpublic class goodsserviceclient { //服務消費者調用的接口url 形如：http://localhost:9090 @value("${spring.power.serviceurl}") private string _serviceurl; @autowired private resttemplate resttemplate; public clientgetgoodsbygoodsidresponse getgoodsbygoodsid(clientgetgoodsbygoodsidrequest request) { string svcurl = getgoodssvcurl() + "/getinfobyid"; clientgetgoodsbygoodsidresponse response = null; try { response = resttemplate.postforobject(svcurl, request, clientgetgoodsbygoodsidresponse.class); } catch (exception e) { e.printstacktrace(); response = new clientgetgoodsbygoodsidresponse(); response.setcode(appconst.fail); response.setmessage(e.tostring()); } return response; } private string getgoodssvcurl() { string url = ""; if (_serviceurl == null) { _serviceurl = ""; } if (_serviceurl.length() == 0) { return url; } if (_serviceurl.substring(_serviceurl.length() - 1, _serviceurl.length()) == "/") { url = string.format("%sapi/v1/goods", _serviceurl); } else { url = string.format("%s/api/v1/goods", _serviceurl); } return url; }} |
demo里直接resttemplate.postforobject方法調用，反序列化實體轉換這些resttemplate內部封裝搞定。
二、問題匯總
1、no suitable httpmessageconverter found for request type異常
這個問題通常會出現在postforobject中傳入對象進行調用的時候。
分析resttemplate源碼，在httpentityrequestcallback類的dowithrequest方法中，如果 messageconverters （這個字段后面會繼續提及）列表字段循環處理的過程中沒有滿足return跳出的邏輯（也就是沒有匹配的httpmessageconverter），則拋出上述異常：
httpentityrequestcallback.dowithrequest
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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); }} |
最簡單的解決方案是，可以通過包裝http請求頭，并將請求對象序列化成字符串的形式傳參，參考示例代碼如下：
postforobject
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/* * post請求調用 * */public static string postforobject(resttemplate resttemplate, string url, object params) { httpheaders headers = new httpheaders(); mediatype type = mediatype.parsemediatype("application/json; charset=utf-8"); headers.setcontenttype(type); headers.add("accept", mediatype.application_json.tostring()); string json = serializeutil.serialize(params); httpentity<string> formentity = new httpentity<string>(json, headers); string result = resttemplate.postforobject(url, formentity, string.class); return result;} |
如果我們還想直接返回對象，直接反序列化返回的字符串即可：
postforobject
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/* * post請求調用 * */public static <t> t postforobject(resttemplate resttemplate, string url, object params, class<t> clazz) { t response = null; string respstr = postforobject(resttemplate, url, params); response = serializeutil.deserialize(respstr, clazz); return response;} |
其中，序列化和反序列化工具比較多，常用的比如fastjson、jackson和gson。
2、no suitable httpmessageconverter found for response type異常
和發起請求發生異常一樣，處理應答的時候也會有問題。
stackoverflow上有人問過相同的問題，根本原因是http消息轉換器httpmessageconverter缺少 mime type ，也就是說http在把輸出結果傳送到客戶端的時候，客戶端必須啟動適當的應用程序來處理這個輸出文檔，這可以通過多種mime（多功能網際郵件擴充協議）type來完成。
對于服務端應答，很多httpmessageconverter默認支持的媒體類型（mimetype）都不同。stringhttpmessageconverter默認支持的則是mediatype.text_plain，sourcehttpmessageconverter默認支持的則是mediatype.text_xml，formhttpmessageconverter默認支持的是mediatype.application_form_urlencoded和mediatype.multipart_form_data，在rest服務中，我們用到的最多的還是 mappingjackson2httpmessageconverter ，這是一個比較通用的轉化器（繼承自generichttpmessageconverter接口），根據分析，它默認支持的mimetype為mediatype.application_json：
mappingjackson2httpmessageconverter
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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：
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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，如果我們想直接返回一個實體對象：
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string url = "http://wthrcdn.etouch.cn/weather_mini?city=上海";clientweatherresultvo weatherresultvo = resttemplate.getforobject(url, clientweatherresultvo.class); |
則直接報異常：
could not extract response: no suitable httpmessageconverter found for response type [class ]
and content type [application/octet-stream]
很多人碰到過這個問題，首次碰到估計大多都比較懵吧，很多接口都是json或者xml或者plain text格式返回的，什么是application/octet-stream？
查看resttemplate源代碼，一路跟蹤下去會發現 httpmessageconverterextractor 類的extractdata方法有個解析應答及反序列化邏輯，如果不成功，拋出的異常信息和上述一致：
httpmessageconverterextractor.extractdata
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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上的解決的示例代碼可以接受，但是并不準確，常見的mimetype都應該加進去，貼一下我認為正確的代碼：
resttemplateconfig
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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()); // 啟動的時候要注意，由于我們在服務中注入了resttemplate，所以啟動的時候需要實例化該類的一個實例 @autowired private resttemplatebuilder builder; @autowired private objectmapper objectmapper; // 使用resttemplatebuilder來實例化resttemplate對象，spring默認已經注入了resttemplatebuilder實例 @bean public resttemplate resttemplate() { resttemplate resttemplate = builder.build(); list<httpmessageconverter<?>> messageconverters = lists.newarraylist(); mappingjackson2httpmessageconverter converter = new mappingjackson2httpmessageconverter(); converter.setobjectmapper(objectmapper); //不加會出現異常 //could not extract response: no suitable httpmessageconverter found for response type [class ] mediatype[] mediatypes = new mediatype[]{ mediatype.application_json, mediatype.application_octet_stream, mediatype.application_json_utf8, mediatype.text_html, mediatype.text_plain, mediatype.text_xml, mediatype.application_stream_json, mediatype.application_atom_xml, mediatype.application_form_urlencoded, mediatype.application_pdf, }; converter.setsupportedmediatypes(arrays.aslist(mediatypes)); //messageconverters.add(converter); if (jackson2present) { messageconverters.add(converter); } else if (gsonpresent) { messageconverters.add(new gsonhttpmessageconverter()); } else if (jsonbpresent) { messageconverters.add(new jsonbhttpmessageconverter()); } messageconverters.add(new formhttpmessageconverter()); messageconverters.add(new bytearrayhttpmessageconverter()); messageconverters.add(new stringhttpmessageconverter()); messageconverters.add(new resourcehttpmessageconverter(false)); messageconverters.add(new sourcehttpmessageconverter()); messageconverters.add(new allencompassingformhttpmessageconverter()); if (romepresent) { messageconverters.add(new atomfeedhttpmessageconverter()); messageconverters.add(new rsschannelhttpmessageconverter()); } if (jackson2xmlpresent) { messageconverters.add(new mappingjackson2xmlhttpmessageconverter()); } else if (jaxb2present) { messageconverters.add(new jaxb2rootelementhttpmessageconverter()); } if (jackson2smilepresent) { messageconverters.add(new mappingjackson2smilehttpmessageconverter()); } if (jackson2cborpresent) { messageconverters.add(new mappingjackson2cborhttpmessageconverter()); } resttemplate.setmessageconverters(messageconverters); return resttemplate; }} |
看到上面的代碼，再對比一下resttemplate內部實現，就知道我參考了resttemplate的源碼，有潔癖的人可能會說這一坨代碼有點啰嗦，上面那一堆static final的變量和messageconverters填充數據方法，暴露了resttemplate的實現，如果resttemplate修改了，這里也要改，非常不友好，而且看上去一點也不oo。
經過分析，resttemplatebuilder.build()構造了resttemplate對象，只要將內部mappingjackson2httpmessageconverter修改一下支持的mediatype即可，resttemplate的messageconverters字段雖然是private final的，我們依然可以通過反射修改之，改進后的代碼如下：
resttemplateconfig
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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 { // 啟動的時候要注意，由于我們在服務中注入了resttemplate，所以啟動的時候需要實例化該類的一個實例 @autowired private resttemplatebuilder builder; @autowired private objectmapper objectmapper; // 使用resttemplatebuilder來實例化resttemplate對象，spring默認已經注入了resttemplatebuilder實例 @bean public resttemplate resttemplate() { resttemplate resttemplate = builder.build(); list<httpmessageconverter<?>> messageconverters = lists.newarraylist(); mappingjackson2httpmessageconverter converter = new mappingjackson2httpmessageconverter(); converter.setobjectmapper(objectmapper); //不加可能會出現異常 //could not extract response: no suitable httpmessageconverter found for response type [class ] mediatype[] mediatypes = new mediatype[]{ mediatype.application_json, mediatype.application_octet_stream, mediatype.text_html, mediatype.text_plain, mediatype.text_xml, mediatype.application_stream_json, mediatype.application_atom_xml, mediatype.application_form_urlencoded, mediatype.application_json_utf8, mediatype.application_pdf, }; converter.setsupportedmediatypes(arrays.aslist(mediatypes)); try { //通過反射設置messageconverters field field = resttemplate.getclass().getdeclaredfield("messageconverters"); field.setaccessible(true); list<httpmessageconverter<?>> orgconverterlist = (list<httpmessageconverter<?>>) field.get(resttemplate); optional<httpmessageconverter<?>> opconverter = orgconverterlist.stream() .filter(x -> x.getclass().getname().equalsignorecase(mappingjackson2httpmessageconverter.class .getname())) .findfirst(); if (opconverter.ispresent() == false) { return resttemplate; } messageconverters.add(converter);//添加mappingjackson2httpmessageconverter //添加原有的剩余的httpmessageconverter list<httpmessageconverter<?>> leftconverters = orgconverterlist.stream() .filter(x -> x.getclass().getname().equalsignorecase(mappingjackson2httpmessageconverter.class .getname()) == false) .collect(collectors.tolist()); messageconverters.addall(leftconverters); system.out.println(string.format("【httpmessageconverter】原有數量：%s，重新構造后數量：%s" , orgconverterlist.size(), messageconverters.size())); } catch (exception e) { e.printstacktrace(); } resttemplate.setmessageconverters(messageconverters); return resttemplate; }} |
除了一個messageconverters字段，看上去我們不再關心resttemplate那些外部依賴包和內部構造過程，果然干凈簡潔好維護了很多。
3、亂碼問題
這個也是一個非常經典的問題。解決方案非常簡單，找到httpmessageconverter，看看默認支持的charset。abstractjackson2httpmessageconverter是很多httpmessageconverter的基類，默認編碼為utf-8：
abstractjackson2httpmessageconverter
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public abstract class abstractjackson2httpmessageconverter extends abstractgenerichttpmessageconverter<object> { public static final charset default_charset = standardcharsets.utf_8;} |
而stringhttpmessageconverter比較特殊，有人反饋過發生亂碼問題由它默認支持的編碼 iso-8859-1 引起：
stringhttpmessageconverter
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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); }} |
如果在使用過程中發生亂碼，我們可以通過方法設置httpmessageconverter支持的編碼，常用的有utf-8、gbk等。
4、反序列化異常
這是開發過程中容易碰到的又一個問題。因為java的開源框架和工具類非常之多，而且版本更迭頻繁，所以經常發生一些意想不到的坑。
以joda time為例，joda time是流行的java時間和日期框架，但是如果你的接口對外暴露joda time的類型，比如datetime，那么接口調用方（同構和異構系統）可能會碰到序列化難題，反序列化時甚至直接拋出如下異常：
org.springframework.http.converter.httpmessageconversionexception: type definition error: [simple type, class org.joda.time.chronology]; nested exception is com.fasterxml.jackson.databind.exc.invaliddefinitionexception: cannot construct instance of `org.joda.time.chronology` (no creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
at [source: (pushbackinputstream);
我在前廠就碰到過，后來為了調用方便，改回直接暴露java的date類型。
當然解決的方案不止這一種，可以使用jackson支持自定義類的序列化和反序列化的方式。在精度要求不是很高的系統里，實現簡單的datetime自定義序列化：
datetimeserializer
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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;/** * 在默認情況下，jackson會將joda time序列化為較為復雜的形式，不利于閱讀，并且對象較大。 * <p> * jodatime 序列化的時候可以將datetime序列化為字符串，更容易讀 **/public class datetimeserializer extends jsonserializer<datetime> { private static datetimeformatter dateformatter = datetimeformat.forpattern("yyyy-mm-dd hh:mm:ss"); @override public void serialize(datetime value, jsongenerator jgen, serializerprovider provider) throws ioexception, jsonprocessingexception { jgen.writestring(value.tostring(dateformatter)); }} |
以及datetime反序列化：
datetimedeserializer
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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 反序列化將字符串轉化為datetime **/public class datetimedeserializer extends jsondeserializer<datetime> { private static datetimeformatter dateformatter = datetimeformat.forpattern("yyyy-mm-dd hh:mm:ss"); @override public datetime deserialize(jsonparser jp, deserializationcontext context) throws ioexception, jsonprocessingexception { jsonnode node = jp.getcodec().readtree(jp); string s = node.astext(); datetime parse = datetime.parse(s, dateformatter); return parse; }} |
最后可以在resttemplateconfig類中對常見調用問題進行匯總處理，可以參考如下：
resttemplateconfig
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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 { // 啟動的時候要注意，由于我們在服務中注入了resttemplate，所以啟動的時候需要實例化該類的一個實例 @autowired private resttemplatebuilder builder; @autowired private objectmapper objectmapper; // 使用resttemplatebuilder來實例化resttemplate對象，spring默認已經注入了resttemplatebuilder實例 @bean public resttemplate resttemplate() { resttemplate resttemplate = builder.build(); //注冊model,用于實現jackson joda time序列化和反序列化 simplemodule module = new simplemodule(); module.addserializer(datetime.class, new datetimeserializer()); module.adddeserializer(datetime.class, new datetimedeserializer()); objectmapper.registermodule(module); list<httpmessageconverter<?>> messageconverters = lists.newarraylist(); mappingjackson2httpmessageconverter converter = new mappingjackson2httpmessageconverter(); converter.setobjectmapper(objectmapper); //不加會出現異常 //could not extract response: no suitable httpmessageconverter found for response type [class ] mediatype[] mediatypes = new mediatype[]{ mediatype.application_json, mediatype.application_octet_stream, mediatype.text_html, mediatype.text_plain, mediatype.text_xml, mediatype.application_stream_json, mediatype.application_atom_xml, mediatype.application_form_urlencoded, mediatype.application_json_utf8, mediatype.application_pdf, }; converter.setsupportedmediatypes(arrays.aslist(mediatypes)); try { //通過反射設置messageconverters field field = resttemplate.getclass().getdeclaredfield("messageconverters"); field.setaccessible(true); list<httpmessageconverter<?>> orgconverterlist = (list<httpmessageconverter<?>>) field.get(resttemplate); optional<httpmessageconverter<?>> opconverter = orgconverterlist.stream() .filter(x -> x.getclass().getname().equalsignorecase(mappingjackson2httpmessageconverter.class .getname())) .findfirst(); if (opconverter.ispresent() == false) { return resttemplate; } messageconverters.add(converter);//添加mappingjackson2httpmessageconverter //添加原有的剩余的httpmessageconverter list<httpmessageconverter<?>> leftconverters = orgconverterlist.stream() .filter(x -> x.getclass().getname().equalsignorecase(mappingjackson2httpmessageconverter.class .getname()) == false) .collect(collectors.tolist()); messageconverters.addall(leftconverters); system.out.println(string.format("【httpmessageconverter】原有數量：%s，重新構造后數量：%s" , orgconverterlist.size(), messageconverters.size())); } catch (exception e) { e.printstacktrace(); } resttemplate.setmessageconverters(messageconverters); return resttemplate; }} |
目前良好地解決了resttemplate常用調用問題，而且不需要你寫resttemplate幫助工具類了。
上面列舉的這些常見問題，其實.net下面也有，有興趣大家可以搜索一下微軟的httpclient常見使用問題，用過的人都深有體會。更不用提 restsharp 這個開源類庫，幾年前用的過程中發現了非常多的bug，到現在還有一個反序列化數組的問題困擾著我們，我只好自己造個簡單輪子特殊處理，給我最深刻的經驗就是，很多看上去簡單的功能，真的碰到了依然會花掉不少的時間去排查和解決，甚至要翻看源碼。所以，我們寫代碼要認識到，越是通用的工具，越需要考慮到特例，可能你需要花80%以上的精力去處理20%的特殊情況，這估計也是滿足常見的二八定律吧。
參考：
https://stackoverflow.com/questions/10579122/resttemplate-no-suitable-httpmessageconverter
http://forum.spring.io/forum/spring-projects/android/126794-no-suitable-httpmessageconverter-found
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持服務器之家。
原文鏈接：http://www.cnblogs.com/jeffwongishandsome/p/spring-boot-consume-rest-api-by-resttemplate.html
