一篇文章教你弄懂springmvc中的httpmessageconverter -尊龙游戏旗舰厅官网

文章目录

• • • 一、httpmessageconverter介绍
    • 二、自定义httpmessageconverter

写在前面：
我是「境里婆娑」。我还是从前那个少年，没有一丝丝改变，时间只不过是考验，种在心中信念丝毫未减，眼前这个少年，还是最初那张脸，面前再多艰险不退却。
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导语：相信使用过spring的开发人员都用过@requestbody、@responsebody注解，可以直接将输入解析成json、将输出解析成json，但http 请求和响应是基于文本的，意味着浏览器和服务器通过交换原始文本进行通信，而这里其实就是httpmessageconverter发挥着作用。

一、httpmessageconverter介绍

http请求响应报文其实都是字符串，当请求报文到java程序会被封装为一个servletinputstream流，开发人员再读取报文，响应报文则通过servletoutputstream流，来输出响应报文。

从流中只能读取到原始的字符串报文，同样输出流也是。那么在报文到达springmvc / springboot出去，都存在一个字符串到java对象的转化问题。这一过程，在springmvc / springboot中，是通过httpmessageconverter来解决的。

httpmessageconverter接口提供了5个方法：

• canread：判断该转换器是否能将请求内容转换成java对象
• canwrite：判断该转换器是否可以将java对象转换成返回内容
• getsupportedmediatypes：获得该转换器支持的mediatype类型
• read：读取请求内容并转换成java对象
• write：将java对象转换后写入返回内容
  其中read和write方法的参数分别有有httpinputmessage和httpoutputmessage对象，这两个对象分别代表着一次http通讯中的请求和响应部分，可以通过getbody方法获得对应的输入流和输出流。
  其中read和write方法的参数分别有有httpinputmessage和httpoutputmessage对象，这两个对象分别代表着一次http通讯中的请求和响应部分，可以通过getbody方法获得对应的输入流和输出流。

public interface httpmessageconverter<t> {boolean canread(class<?> clazz, mediatype mediatype);boolean canwrite(class<?> clazz, mediatype mediatype);list<mediatype> getsupportedmediatypes();t read(class<? extends t> clazz, httpinputmessage inputmessage)throws ioexception, httpmessagenotreadableexception;void write(t t, mediatype contenttype, httpoutputmessage outputmessage)throws ioexception, httpmessagenotwritableexception; }

缺省配置
平常我们在发送http请求时，没有配置任何 messageconverter，但是数据前后传递依旧好用，是因为 springmvc 启动时会自动配置一些httpmessageconverter，在 webmvcconfigurationsupport 类中添加了缺省 messageconverter：

protected final void adddefaulthttpmessageconverters(list<httpmessageconverter<?>> messageconverters) {stringhttpmessageconverter stringconverter = new stringhttpmessageconverter();stringconverter.setwriteacceptcharset(false);messageconverters.add(new bytearrayhttpmessageconverter());messageconverters.add(stringconverter);messageconverters.add(new resourcehttpmessageconverter());messageconverters.add(new sourcehttpmessageconverter<source>());messageconverters.add(new allencompassingformhttpmessageconverter());if (romepresent) {messageconverters.add(new atomfeedhttpmessageconverter());messageconverters.add(new rsschannelhttpmessageconverter());}if (jackson2xmlpresent) {objectmapper objectmapper = jackson2objectmapperbuilder.xml().applicationcontext(this.applicationcontext).build();messageconverters.add(new mappingjackson2xmlhttpmessageconverter(objectmapper));}else if (jaxb2present) {messageconverters.add(new jaxb2rootelementhttpmessageconverter());}if (jackson2present) {objectmapper objectmapper = jackson2objectmapperbuilder.json().applicationcontext(this.applicationcontext).build();messageconverters.add(new mappingjackson2httpmessageconverter(objectmapper));}else if (gsonpresent) {messageconverters.add(new gsonhttpmessageconverter());}}

httpmessageconverter匹配过程
当使用 @requestbody和 @responsebody注解时，requestmappinghandleradapter就使用它们来进行读取或者写入相应格式的数据。

@requestbody 据request对象header部分的content-type类型，逐一匹配合适的httpmessageconverter来读取数据。 底层调用的是requestresponsebodymethodprocessor readwithmessageconverters方法

protected <t> object readwithmessageconverters(httpinputmessage inputmessage, methodparameter parameter,type targettype) throws ioexception, httpmediatypenotsupportedexception, httpmessagenotreadableexception {mediatype contenttype;boolean nocontenttype = false;try {contenttype = inputmessage.getheaders().getcontenttype();}catch (invalidmediatypeexception ex) {throw new httpmediatypenotsupportedexception(ex.getmessage());}if (contenttype == null) {nocontenttype = true;contenttype = mediatype.application_octet_stream;}class<?> contextclass = parameter.getcontainingclass();class<t> targetclass = (targettype instanceof class ? (class<t>) targettype : null);if (targetclass == null) {resolvabletype resolvabletype = resolvabletype.formethodparameter(parameter);targetclass = (class<t>) resolvabletype.resolve();}httpmethod httpmethod = (inputmessage instanceof httprequest ? ((httprequest) inputmessage).getmethod() : null);object body = no_value;emptybodycheckinghttpinputmessage message;try {message = new emptybodycheckinghttpinputmessage(inputmessage);for (httpmessageconverter<?> converter : this.messageconverters) {class<httpmessageconverter<?>> convertertype = (class<httpmessageconverter<?>>) converter.getclass();generichttpmessageconverter<?> genericconverter =(converter instanceof generichttpmessageconverter ? (generichttpmessageconverter<?>) converter : null);if (genericconverter != null ? genericconverter.canread(targettype, contextclass, contenttype) :(targetclass != null && converter.canread(targetclass, contenttype))) {if (message.hasbody()) {httpinputmessage msgtouse =getadvice().beforebodyread(message, parameter, targettype, convertertype);body = (genericconverter != null ? genericconverter.read(targettype, contextclass, msgtouse) :((httpmessageconverter<t>) converter).read(targetclass, msgtouse));body = getadvice().afterbodyread(body, msgtouse, parameter, targettype, convertertype);}else {body = getadvice().handleemptybody(null, message, parameter, targettype, convertertype);}break;}}}catch (ioexception ex) {throw new httpmessagenotreadableexception("i/o error while reading input message", ex, inputmessage);}if (body == no_value) {if (httpmethod == null || !supported_methods.contains(httpmethod) ||(nocontenttype && !message.hasbody())) {return null;}throw new httpmediatypenotsupportedexception(contenttype, this.allsupportedmediatypes);}mediatype selectedcontenttype = contenttype;object thebody = body;logformatutils.tracedebug(logger, traceon -> {string formatted = logformatutils.formatvalue(thebody, !traceon);return "read \"" selectedcontenttype "\" to [" formatted "]";});return body;}

@responsebody 底层调用的abstractmessageconvertermethodprocessor

protected <t> void writewithmessageconverters(@nullable t value, methodparameter returntype,servletserverhttprequest inputmessage, servletserverhttpresponse outputmessage)throws ioexception, httpmediatypenotacceptableexception, httpmessagenotwritableexception {object body;class<?> valuetype;type targettype;if (value instanceof charsequence) {body = value.tostring();valuetype = string.class;targettype = string.class;}else {body = value;valuetype = getreturnvaluetype(body, returntype);targettype = generictyperesolver.resolvetype(getgenerictype(returntype), returntype.getcontainingclass());}if (isresourcetype(value, returntype)) {outputmessage.getheaders().set(httpheaders.accept_ranges, "bytes");if (value != null && inputmessage.getheaders().getfirst(httpheaders.range) != null &&outputmessage.getservletresponse().getstatus() == 200) {resource resource = (resource) value;try {list<httprange> httpranges = inputmessage.getheaders().getrange();outputmessage.getservletresponse().setstatus(httpstatus.partial_content.value());body = httprange.toresourceregions(httpranges, resource);valuetype = body.getclass();targettype = resource_region_list_type;}catch (illegalargumentexception ex) {outputmessage.getheaders().set(httpheaders.content_range, "bytes */" resource.contentlength());outputmessage.getservletresponse().setstatus(httpstatus.requested_range_not_satisfiable.value());}}}mediatype selectedmediatype = null;mediatype contenttype = outputmessage.getheaders().getcontenttype();if (contenttype != null && contenttype.isconcrete()) {if (logger.isdebugenabled()) {logger.debug("found 'content-type:" contenttype "' in response");}selectedmediatype = contenttype;}else {httpservletrequest request = inputmessage.getservletrequest();list<mediatype> acceptabletypes = getacceptablemediatypes(request);list<mediatype> producibletypes = getproduciblemediatypes(request, valuetype, targettype);if (body != null && producibletypes.isempty()) {throw new httpmessagenotwritableexception("no converter found for return value of type: " valuetype);}list<mediatype> mediatypestouse = new arraylist<>();for (mediatype requestedtype : acceptabletypes) {for (mediatype producibletype : producibletypes) {if (requestedtype.iscompatiblewith(producibletype)) {mediatypestouse.add(getmostspecificmediatype(requestedtype, producibletype));}}}if (mediatypestouse.isempty()) {if (body != null) {throw new httpmediatypenotacceptableexception(producibletypes);}if (logger.isdebugenabled()) {logger.debug("no match for " acceptabletypes ", supported: " producibletypes);}return;}mediatype.sortbyspecificityandquality(mediatypestouse);for (mediatype mediatype : mediatypestouse) {if (mediatype.isconcrete()) {selectedmediatype = mediatype;break;}else if (mediatype.ispresentin(all_application_media_types)) {selectedmediatype = mediatype.application_octet_stream;break;}}if (logger.isdebugenabled()) {logger.debug("using '" selectedmediatype "', given " acceptabletypes " and supported " producibletypes);}}if (selectedmediatype != null) {selectedmediatype = selectedmediatype.removequalityvalue();for (httpmessageconverter<?> converter : this.messageconverters) {generichttpmessageconverter genericconverter = (converter instanceof generichttpmessageconverter ?(generichttpmessageconverter<?>) converter : null);if (genericconverter != null ?((generichttpmessageconverter) converter).canwrite(targettype, valuetype, selectedmediatype) :converter.canwrite(valuetype, selectedmediatype)) {body = getadvice().beforebodywrite(body, returntype, selectedmediatype,(class<? extends httpmessageconverter<?>>) converter.getclass(),inputmessage, outputmessage);if (body != null) {object thebody = body;logformatutils.tracedebug(logger, traceon ->"writing [" logformatutils.formatvalue(thebody, !traceon) "]");addcontentdispositionheader(inputmessage, outputmessage);if (genericconverter != null) {genericconverter.write(body, targettype, selectedmediatype, outputmessage);}else {((httpmessageconverter) converter).write(body, selectedmediatype, outputmessage);}}else {if (logger.isdebugenabled()) {logger.debug("nothing to write: null body");}}return;}}}if (body != null) {throw new httpmediatypenotacceptableexception(this.allsupportedmediatypes);}}

二、自定义httpmessageconverter

一般springboot会有默认的消息管理器，stringhttpmessageconverter(字符串转换器),fastjsonhttpmessageconverter ,mappingjackson2httpmessageconverter等等。

今天我要说的是如何自定义消息管理器-fastjsonhttpmessageconverter
1.引用fastjson依赖

<dependency><groupid>com.alibaba</groupid><artifactid>fastjson</artifactid><version>1.2.58</version> </dependency>

2. 创建配置文件jsonhttpmessageconfiguration.java

@configuration public class myjsonhttpmessageconfiguration {@bean@conditionalonmissingbean(fastjsonhttpmessageconverter.class)public fastjsonhttpmessageconverter fastjsonhttpmessageconverter() {//1.需要定义一个convert转换消息的对象fastjsonhttpmessageconverter fastjsonhttpmessageconverter = new fastjsonhttpmessageconverter();//2.添加fastjson的配置信息;fastjsonconfig config = new fastjsonconfig();//添加配置-config.setserializerfeatures() 方法中可以添加多个配置（以，分开）config.setserializerfeatures(serializerfeature.writemapnullvalue,serializerfeature.disablecircularreferencedetect);//添加配置-config.setserializerfeatures() 方法中可以添加多个配置（以，分开）config.setserializerfeatures(serializerfeature.writemapnullvalue,serializerfeature.disablecircularreferencedetect);fastjsonhttpmessageconverter.setfastjsonconfig(config);fastjsonhttpmessageconverter.setsupportedmediatypes(getsupportedmediatypes());fastjsonhttpmessageconverter.setdefaultcharset(charset.forname("utf-8"));return fastjsonhttpmessageconverter;}/*** 配置全局支持的媒体类型*/private list<mediatype> getsupportedmediatypes() {list<mediatype> supportedmediatypes = new arraylist<>();supportedmediatypes.add(mediatype.application_json);supportedmediatypes.add(mediatype.application_atom_xml);supportedmediatypes.add(mediatype.application_form_urlencoded);supportedmediatypes.add(mediatype.application_octet_stream);supportedmediatypes.add(mediatype.application_pdf);supportedmediatypes.add(mediatype.application_rss_xml);supportedmediatypes.add(mediatype.application_xhtml_xml);supportedmediatypes.add(mediatype.application_xml);supportedmediatypes.add(mediatype.image_gif);supportedmediatypes.add(mediatype.image_jpeg);supportedmediatypes.add(mediatype.image_png);supportedmediatypes.add(mediatype.text_event_stream);supportedmediatypes.add(mediatype.text_html);supportedmediatypes.add(mediatype.text_markdown);supportedmediatypes.add(mediatype.text_plain);supportedmediatypes.add(mediatype.text_xml);return supportedmediatypes;} }

serializerfeature配置的属性解释

属性名称含义

quotefieldnames	输出key时是否使用双引号,默认为true
usesinglequotes	使用单引号而不是双引号,默认为false
writemapnullvalue	是否输出值为null的字段,默认为false应用场景：前端必须需要所有字段
useiso8601dateformat	date使用iso8601格式输出，默认为false
writenulllistasempty	list字段如果为null,输出为[],而不是null
writenullstringasempty	字符类型字段如果为null,输出为"",而不是null
writenullnumberaszero	数值字段如果为null,输出为0,而非null
writenullbooleanasfalse	boolean字段如果为null,输出为false,而非null
skiptransientfield	如果是true，类中的get方法对应的field是transient，序列化时将会被忽略。默认为true
sortfield	按字段名称排序后输出。默认为false

到此springmvc中的httpmessageconverter介绍完毕。如果还有不明白的可以留言。
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由于本人水平有限，难免有不足，恳请各位大佬不吝赐教！

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