引言

前端传入的 json 数据如何被解析成 Java 对象作为 API入参，API 返回结果又如何将 Java 对象解析成 json 格式数据返回给前端，其实在整个数据流转过程中，HttpMessageConverter 起到了重要作用；另外在转换的过程我们可以加入哪些定制化内容？

源代碼

核心邏輯代碼：

// 对于String类型的，直接 append 返回，不转json
if ("java.lang.String".equals(type.getTypeName())) {

try {
// 1.先解析json對象,如果不是json對象的,走catch邏輯
Object jsonObject = JSON.parse(value.toString());
objectWriter.writeValue(generator, jsonObject);
} catch (Throwable e) {
log.error("OvsHttpMessageConverter writeInternal,JSON.parse(value.toString()) = {}", value, e);
// 2.不是json對象的,就原樣輸出string
objectWriter.writeValue(generator, value);
}

}

自定義的 HttpMessageConverter :

package com.alibaba.ovs.operationcenter.config;

import com.alibaba.fastjson.JSON;
import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.util.DefaultIndenter;
import com.fasterxml.jackson.core.util.DefaultPrettyPrinter;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.ser.FilterProvider;
import lombok.extern.slf4j.Slf4j;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.converter.json.MappingJacksonValue;
import org.springframework.util.TypeUtils;

import java.io.IOException;
import java.lang.reflect.Type;

/**
* @author: Jack
* 2020-08-24 23:10
*/
@Slf4j
public class OvsHttpMessageConverter extends MappingJackson2HttpMessageConverter {

private static final MediaType TEXT_EVENT_STREAM = new MediaType("text", "event-stream");

public OvsHttpMessageConverter() {
}

public OvsHttpMessageConverter(ObjectMapper objectMapper) {
super(objectMapper);
}

@Override
protected void writeInternal(Object object, Type type, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException {

log.info("OvsHttpMessageConverter writeInternal,type={},object={}", type.getTypeName(), JSON.toJSONString(object));

MediaType contentType = outputMessage.getHeaders().getContentType();
JsonEncoding encoding = getJsonEncoding(contentType);
JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);

try {
Object value = object;
Class<?> serializationView = null;
FilterProvider filters = null;
JavaType javaType = null;

if (object instanceof MappingJacksonValue) {
MappingJacksonValue container = (MappingJacksonValue) object;
value = container.getValue();
serializationView = container.getSerializationView();
filters = container.getFilters();
}

ObjectWriter objectWriter = (serializationView != null ? this.objectMapper.writerWithView(serializationView) : this.objectMapper.writer());

// 对于String类型的，直接 append 返回，不转json
if ("java.lang.String".equals(type.getTypeName())) {

try {
// 1.先解析json對象,如果不是json對象的,走catch邏輯
Object jsonObject = JSON.parse(value.toString());
objectWriter.writeValue(generator, jsonObject);
} catch (Throwable e) {
log.error("OvsHttpMessageConverter writeInternal,JSON.parse(value.toString()) = {}", value, e);
// 2.不是json對象的,就原樣輸出string
objectWriter.writeValue(generator, value);
}

} else {

writePrefix(generator, object);

if (type != null && value != null && TypeUtils.isAssignable(type, value.getClass())) {
javaType = getJavaType(type, null);
}

if (filters != null) {
objectWriter = objectWriter.with(filters);
}
if (javaType != null && javaType.isContainerType()) {
objectWriter = objectWriter.forType(javaType);
}
SerializationConfig config = objectWriter.getConfig();
if (contentType != null && contentType.isCompatibleWith(TEXT_EVENT_STREAM) &&
config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {

DefaultPrettyPrinter prettyPrinter = new DefaultPrettyPrinter();
prettyPrinter.indentObjectsWith(new DefaultIndenter("  ", "\ndata:"));
objectWriter = objectWriter.with(prettyPrinter);
}
objectWriter.writeValue(generator, value);
writeSuffix(generator, object);
}

generator.flush();

} catch (JsonProcessingException ex) {
throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
}
}

}

WebMvcConfig代碼：

package com.alibaba.ovs.operationcenter.config;

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.std.ToStringSerializer;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;

/**
* @author: Jack
* 2020-08-14 18:53
*/
@EnableWebMvc
@Configuration
public class WebConfig extends WebMvcConfigurerAdapter {
@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
// MappingJackson2HttpMessageConverter httpMessageConverter = new MappingJackson2HttpMessageConverter();
OvsHttpMessageConverter httpMessageConverter = new OvsHttpMessageConverter();
ObjectMapper objectMapper = new ObjectMapper();
/**
* 序列换成json时,将所有的long变成string, 因为js 中得 Number 数字类型不能包含所有的 java long 值 (js中会被截断)
* 参考文章: https://blog.csdn.net/universsky2015/article/details/108010953
*/
SimpleModule simpleModule = new SimpleModule();
simpleModule.addSerializer(Long.class, ToStringSerializer.instance);
simpleModule.addSerializer(Long.TYPE, ToStringSerializer.instance);
objectMapper.registerModule(simpleModule);
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
httpMessageConverter.setObjectMapper(objectMapper);

// supportedMediaTypes
List<MediaType> supportedMediaTypes = new ArrayList<>(httpMessageConverter.getSupportedMediaTypes());
supportedMediaTypes.add(MediaType.APPLICATION_JSON_UTF8);
supportedMediaTypes.add(MediaType.TEXT_HTML);
supportedMediaTypes.add(MediaType.TEXT_PLAIN);
supportedMediaTypes.add(MediaType.TEXT_XML);
supportedMediaTypes.add(new MediaType("application", "*+json", StandardCharsets.UTF_8));
httpMessageConverter.setSupportedMediaTypes(supportedMediaTypes);

converters.add(httpMessageConverter);
}

@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {

// Spring Boot自动配置本身不会自动把/swagger-ui.html这个路径映射到对应的目录META-INF/resources/下面。
// 这个swagger-ui.html 相关的所有前端静态文件都在springfox-swagger-ui.jar里面。
registry.addResourceHandler("swagger-ui.html")
.addResourceLocations("classpath:/META-INF/resources/");

registry.addResourceHandler("/webjars/**")
.addResourceLocations("classpath:/META-INF/resources/webjars/");
}

@Override
public void addInterceptors(InterceptorRegistry registry) {
// 多个拦截器组成一个拦截器链
// addPathPatterns 用于添加拦截规则
// excludePathPatterns 用户排除拦截

//添加拦截器
registry.addInterceptor(new CheckpreloadInterceptor());

super.addInterceptors(registry);
}

}

HttpMessageConverter是什麼？

其中，

HttpMessageConverter 接口介绍
org.springframework.http.converter.HttpMessageConverter 是一个策略接口，接口说明如下：

/*
* Copyright 2002-2017 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.http.converter;

import java.io.IOException;
import java.util.List;

import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;

/**
* Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.
*
* @author Arjen Poutsma
* @author Juergen Hoeller
* @since 3.0
*/
public interface HttpMessageConverter<T> {

/**
* Indicates whether the given class can be read by this converter.
* @param clazz the class to test for readability
* @param mediaType the media type to read (can be {@code null} if not specified);
* typically the value of a {@code Content-Type} header.
* @return {@code true} if readable; {@code false} otherwise
*/
boolean canRead(Class<?> clazz, MediaType mediaType);

/**
* Indicates whether the given class can be written by this converter.
* @param clazz the class to test for writability
* @param mediaType the media type to write (can be {@code null} if not specified);
* typically the value of an {@code Accept} header.
* @return {@code true} if writable; {@code false} otherwise
*/
boolean canWrite(Class<?> clazz, MediaType mediaType);

/**
* Return the list of {@link MediaType} objects supported by this converter.
* @return the list of supported media types
*/
List<MediaType> getSupportedMediaTypes();

/**
* Read an object of the given type from the given input message, and returns it.
* @param clazz the type of object to return. This type must have previously been passed to the
* {@link #canRead canRead} method of this interface, which must have returned {@code true}.
* @param inputMessage the HTTP input message to read from
* @return the converted object
* @throws IOException in case of I/O errors
* @throws HttpMessageNotReadableException in case of conversion errors
*/
T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException;

/**
* Write an given object to the given output message.
* @param t the object to write to the output message. The type of this object must have previously been
* passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
* @param contentType the content type to use when writing. May be {@code null} to indicate that the
* default content type of the converter must be used. If not {@code null}, this media type must have
* previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
* returned {@code true}.
* @param outputMessage the message to write to
* @throws IOException in case of I/O errors
* @throws HttpMessageNotWritableException in case of conversion errors
*/
void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException;

}

Strategy interface that specifies a converter that can convert from and to HTTP requests and responses. 简单说就是 HTTP request (请求)和response (响应)的转换器。该接口有只有5个方法，简单来说就是获取支持的 MediaType（application/json之类），接收到请求时判断是否能读（canRead），能读则读（read）；返回结果时判断是否能写（canWrite），能写则写（write）。

Spring Boot 缺省配置

我们写 Demo 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());
}
}

我们看到很熟悉的 MappingJackson2HttpMessageConverter ，如果我们引入 jackson 相关包，Spring 就会为我们添加该 MessageConverter，但是我们通常在搭建框架的时候还是会手动添加配置 MappingJackson2HttpMessageConverter，为什么？ 先思考一下：

当我们配置了自己的 MessageConverter， SpringMVC 启动过程就不会调用 addDefaultHttpMessageConverters 方法，且看下面代码 if 条件，这样做也是为了定制化我们自己的 MessageConverter

protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<HttpMessageConverter<?>>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}

数据流转解析

数据的请求和响应都要经过 DispatcherServlet 类的 doDispatch(HttpServletRequest request, HttpServletResponse response) 方法的处理

请求过程解析

看 doDispatch 方法中的关键代码：

// 这里的 Adapter 实际上是 RequestMappingHandlerAdapter
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 实际处理的handler
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());            mappedHandler.applyPostHandle(processedRequest, response, mv);

从进入handle之后我先将调用栈粘贴在此处，希望小伙伴可以按照调用栈路线动手跟踪尝试：

readWithMessageConverters:192, AbstractMessageConverterMethodArgumentResolver (org.springframework.web.servlet.mvc.method.annotation)
readWithMessageConverters:150, RequestResponseBodyMethodProcessor (org.springframework.web.servlet.mvc.method.annotation)
resolveArgument:128, RequestResponseBodyMethodProcessor (org.springframework.web.servlet.mvc.method.annotation)
resolveArgument:121, HandlerMethodArgumentResolverComposite (org.springframework.web.method.support)
getMethodArgumentValues:158, InvocableHandlerMethod (org.springframework.web.method.support)
invokeForRequest:128, InvocableHandlerMethod (org.springframework.web.method.support)

// 下面的调用栈重点关注，处理请求和返回值的分叉口就在这里

invokeAndHandle:97, ServletInvocableHandlerMethod (org.springframework.web.servlet.mvc.method.annotation)
invokeHandlerMethod:849, RequestMappingHandlerAdapter (org.springframework.web.servlet.mvc.method.annotation)
handleInternal:760, RequestMappingHandlerAdapter (org.springframework.web.servlet.mvc.method.annotation)
handle:85, AbstractHandlerMethodAdapter (org.springframework.web.servlet.mvc.method)
doDispatch:967, DispatcherServlet (org.springframework.web.servlet)

这里重点说明调用栈最顶层 readWithMessageConverters 方法中内容：

// 遍历 messageConverters
for (HttpMessageConverter<?> converter : this.messageConverters) {
Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
// 上文类关系图处要重点记住的地方，主要判断 MappingJackson2HttpMessageConverter 是否是 GenericHttpMessageConverter 类型
if (converter instanceof GenericHttpMessageConverter) {
GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
if (genericConverter.canRead(targetType, contextClass, contentType)) {
if (logger.isDebugEnabled()) {
logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
}
if (inputMessage.getBody() != null) {
inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);
body = genericConverter.read(targetType, contextClass, inputMessage);
body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);
}
else {
body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);
}
break;
}
}
else if (targetClass != null) {
if (converter.canRead(targetClass, contentType)) {
if (logger.isDebugEnabled()) {
logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
}
if (inputMessage.getBody() != null) {
inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);
body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);
}
else {
body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);
}
break;
}
}
}

然后就判断是否canRead，能读就read，最终走到下面代码处将输入的内容反序列化出来：

protected Object _readMapAndClose(JsonParser p0, JavaType valueType)
throws IOException
{
try (JsonParser p = p0) {
Object result;
JsonToken t = _initForReading(p);
if (t == JsonToken.VALUE_NULL) {
// Ask JsonDeserializer what 'null value' to use:
DeserializationContext ctxt = createDeserializationContext(p,
getDeserializationConfig());
result = _findRootDeserializer(ctxt, valueType).getNullValue(ctxt);
} else if (t == JsonToken.END_ARRAY || t == JsonToken.END_OBJECT) {
result = null;
} else {
DeserializationConfig cfg = getDeserializationConfig();
DeserializationContext ctxt = createDeserializationContext(p, cfg);
JsonDeserializer<Object> deser = _findRootDeserializer(ctxt, valueType);
if (cfg.useRootWrapping()) {
result = _unwrapAndDeserialize(p, ctxt, cfg, valueType, deser);
} else {
result = deser.deserialize(p, ctxt);
}
ctxt.checkUnresolvedObjectId();
}
// Need to consume the token too
p.clearCurrentToken();
return result;
}
}

到这里从请求中解析参数过程就到此结束了，趁热打铁来看将响应结果返回给前端的过程

返回过程解析

在上面调用栈请求和返回结果分叉口处同样处理返回的内容：

writeWithMessageConverters:224, AbstractMessageConverterMethodProcessor (org.springframework.web.servlet.mvc.method.annotation)
handleReturnValue:174, RequestResponseBodyMethodProcessor (org.springframework.web.servlet.mvc.method.annotation)
handleReturnValue:81, HandlerMethodReturnValueHandlerComposite (org.springframework.web.method.support)
// 分叉口
invokeAndHandle:113, ServletInvocableHandlerMethod (org.springframework.web.servlet.mvc.method.annotation)

重点关注调用栈顶层内容，是不是很熟悉的样子，完全一样的逻辑, 判断是否能写canWrite，能写则write：

for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
if (messageConverter instanceof GenericHttpMessageConverter) {
if (((GenericHttpMessageConverter) messageConverter).canWrite(
declaredType, valueType, selectedMediaType)) {
outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
inputMessage, outputMessage);
if (outputValue != null) {
addContentDispositionHeader(inputMessage, outputMessage);
((GenericHttpMessageConverter) messageConverter).write(
outputValue, declaredType, selectedMediaType, outputMessage);
if (logger.isDebugEnabled()) {
logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
"\" using [" + messageConverter + "]");
}
}
return;
}
}
else if (messageConverter.canWrite(valueType, selectedMediaType)) {
outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
inputMessage, outputMessage);
if (outputValue != null) {
addContentDispositionHeader(inputMessage, outputMessage);
((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
if (logger.isDebugEnabled()) {
logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
"\" using [" + messageConverter + "]");
}
}
return;
}
}

我们看到有这样一行代码：

outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
inputMessage, outputMessage);

我们在设计 RESTful API 接口的时候通常会将返回的数据封装成统一格式，通常我们会实现 ResponseBodyAdvice<T> 接口来处理所有 API 的返回值，在真正 write 之前将数据进行统一的封装

@RestControllerAdvice()
public class CommonResultResponseAdvice implements ResponseBodyAdvice<Object> {

@Override
public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
return true;
}

@Override
public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request,
ServerHttpResponse response) {
if (body instanceof CommonResult) {
return body;
}
return new CommonResult<Object>(body);
}

}

整个处理流程就是这样。

将各种常用 HttpMessageConverter 支持的MediaType 和 JavaType 以及对应关系总结在此处：

參考資料：

https://www.jianshu.com/p/3e1de3d02dd8
https://blog.csdn.net/BryantLmm/article/details/85163590

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大幅减少样板代码量。

2.与Java的100%互操作性

Kotlin可以直接与Java类交互，反之亦然。这个特性使得我们可以直接重用我们的代码库，并将其迁移到 Kotlin中。由于Java的互操作性几乎无处不在。我们可以直接访问平台API以及现有的代码库，同时仍然享受和使用 Kotlin 的所有强大的现代语言功能。

3.扩展函数

Kotlin 类似于 C# 和 Gosu, 它提供了为现有类提供新功能扩展的能力，而不必从该类继承或使用任何类型的设计模式 (如装饰器模式)。

4.函数式编程

Kotlin 语言一等支持函数式编程，就像Scala一样。具备高阶函数、Lambda 表达式等函数式基本特性。

5.默认和命名参数

在Kotlin中，您可以为函数中的参数设置一个默认值，并给每个参数一个名称。这有助于编写易读的代码。

6.强大的开发工具支持

而由于是JetBrains出品，我们拥有很棒的IDE支持。虽然Java到Kotlin的自动转换并不是100% OK 的，但它确实是一个非常好的工具。使用 IDEA 的工具转换Java代码为 Kotlin 代码时，可以轻松地重用60％-70％的结果代码，而且修改成本很小。

Kotlin 除了简洁强大的语法特性外，还有实用性非常强的API以及围绕它构建的生态系统。例如：集合类 API、IO 扩展类、反射API 等。同时 Kotlin 社区也提供了丰富的文档和大量的学习资料，还有在线REPL。

A modern programming language that makes developers happier. Open source forever

图来自《Kotlin从入门到进阶实战》 (陈光剑，清华大学出版社) 图来自《Kotlin从入门到进阶实战》 (陈光剑，清华大学出版社)

https://kotlinlang.org/