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In this article, the editor introduces in detail the "case analysis of Spring Boot data response problems", with detailed contents, clear steps and proper handling of details. I hope that this article "case analysis of Spring Boot data response problems" can help you solve your doubts.

Preface

The response page refers to how we send a request to jump to the specified page. It will be explained later in the view parsing. Response pages are common in developing monolithic applications. Response data is common in applications where front and back ends are separated from each other. The back-end code is mainly used to receive requests. The front-end page sends us a request and responds to the front-end json data. Or give the front end response to xml, pictures, audio and video data.

In the process of separate front and back end development, the back end generally encapsulates the data set into a JSON object response to the front end, and generally only needs standard ResponseBody to return data to the front end.

1. Response Json data: Jackson.jar+@ResponseBody

Assuming that json data is returned automatically to the front end, relevant dependencies need to be introduced.

Org.springframework.boot spring-boot-starter-web org.springframework.boot spring-boot-starter-json 2.3.4.RELEASE compile

The control layer code is as follows: after introducing the dependency, mark @ ResponseBody on the method, and the JSON data can be returned to the front end automatically.

@ Controllerpublic class ResponseTestController {@ ResponseBody / / the principle is to use the message converter in the return value processor to process @ GetMapping ("/ test/person") public Person getPerson () {Person person = new Person (); person.setAge (28); person.setBirth (new Date ()); person.setUserName ("zhangsan"); return person;}}

Test:

2. Principle analysis

The return value processor determines whether this type of return value supportsReturnType is supported.

The return value processor calls handleReturnValue for processing

RequestResponseBodyMethodProcessor can handle returning values marked with @ ResponseBody annotations.

Content negotiation (by default, the browser tells the server what type of content it can accept in the form of a request header)

The server finally decides what kind of content type data the server can produce according to its own ability.

SpringMVC traverses the HttpMessageConverter at the bottom of all containers one by one to see who can handle it. (that is, convert objects to json data)

Get the MappingJackson2HttpMessageConverter message converter to write the object as json

Use MappingJackson2HttpMessageConverter to convert the object to json and then write it out.

Use MessageConverters for processing and write the data as json

Which return values are supported by SpringMVC

ModelAndView / / contains data and pages

Model

View

ResponseEntity

ResponseBodyEmitter

StreamingResponseBody

HttpEntity

HttpHeaders

Callable / / Asynchronous

DeferredResult

ListenableFuture

CompletionStage

WebAsyncTask

With @ ModelAttribute and is of object type

@ ResponseBody comment-- > RequestResponseBodyMethodProcessor; processor / / that is, whether @ ResponseBody is marked on the method or class

HTTPMessageConverter principle

MessageConverter specification

HttpMessageConverter: see if you can convert objects of type Class to data of type MediaType. Example: CanWrite converts Person objects to JSON. CanRead or JSON to Person

Default MessageConverter

0-only Byte types are supported

1-String

2-String

3-Resource

4-ResourceRegion

5-DOMSource.class\ SAXSource.class)\ StAXSource.class\ StreamSource.class\ Source.class

6-MultiValueMap

7-true / / supports converting any object to a specified object, no matter what it is

8-true

9-support annotated xml processing.

Eventually MappingJackson2HttpMessageConverter converts the object to JSON (using the objectMapper conversion of the underlying jackson)

3. Content negotiation 3.1, Overview

According to the receiving ability of the client, some only receive xml, some only receive json, and return data of different media types. For example, before returning xml data to

Introduce support for XML dependencies:

Com.fasterxml.jackson.dataformat jackson-dataformat-xml

Recompile the project to run and return xml data

3.2. postman test returns json and xml respectively

In the above test, if I send the same request with postman at this time, I get json data. Why the same request, in different ways, and return different data? The reason is the sequence of data responses specified in the request header.

View request header

Content negotiation Accept, what type of data the browser has the ability to receive, you can see that xml data is priority to be received.

You can use Postman software to test and return json and xml respectively: you only need to change the Accept field (application/json, application/xml) in the request header. As specified in the Http protocol, tell the server the types of data that the client can receive.

3.3. Enable the content negotiation function of browser parameter mode.

In order to facilitate content negotiation, the content negotiation function based on request parameters is turned on.

Spring: contentnegotiation: favor-parameter: true # enable request parameter content negotiation mode

Make a request:

Json type: http://localhost:8080/test/person?format=json

Xml type: http://localhost:8080/test/person?format=xml

Determine what type of content the client receives

1. The priority of Parameter policy is to return json data (to obtain the value of format in the request header). Finally, content negotiation can be carried out and returned to the client json.

4. The principle of content negotiation

Determine whether there is a certain media type in the current response header. MediaType

Gets the types of content that clients (PostMan, browsers) support to receive. (get client Accept request header field) [application/xml]

ContentNegotiationManager content negotiation Manager uses request header-based policy by default

HeaderContentNegotiationStrategy determines the types of content that clients can receive

Loop through all the MessageConverter of the current system to see who supports manipulating this object (Person)

Find the converter that supports manipulating Person, and count out the media types supported by converter.

The client needs [application/xml]. Server capabilities [10, json, xml]

The best media type for content negotiation

Use converter that supports converting objects to the best match media type. Call it for conversion.

Import the package of jackson processing xml, and the converter of xml will come in automatically.

WebMvcConfigurationSupportjackson2XmlPresent = ClassUtils.isPresent ("com.fasterxml.jackson.dataformat.xml.XmlMapper", classLoader); if (jackson2XmlPresent) {Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.xml (); if (this.applicationContext! = null) {builder.applicationContext (this.applicationContext);} messageConverters.add (new MappingJackson2XmlHttpMessageConverter (builder.build () } 5. Custom message Converter MessageConverter5.1, Overview

Achieve multi-protocol data compatibility. Json 、 xml 、 x-guigu

@ ResponseBody response data goes out and calls RequestResponseBodyMethodProcessor processing

The Processor processing method returns a value. Through MessageConverter processing

Together, all MessageConverter can support the operation of data of various media types (read and write).

Content negotiation finds the final messageConverter

What function of SpringMVC is to be customized, that is, adding a WebMvcConfigurer to the container through an entry

Suppose you want to negotiate custom content based on custom request parameters. In other words, enter http://localhost:8080/test/person?format=gg in the address bar to return data, which is the same as the custom protocol data returned by http://localhost:8080/test/person and request header parameter `Accept:application/x- guigu`.

Demo

From the above analysis, we only need to implement the WebMvcConfigurer interface and the configureMessageConverters method to achieve the purpose of customizing the message converter. For example, I don't want to use jackson, I want to use fastjson's message converter, we can add fastjson-related MessageConverter.

@ Configurationpublic class WebConfig implements WebMvcConfigurer {@ Override public void configureMessageConverters (List > converters) {}

test

@ Configuration (proxyBeanMethods = false) public class WebConfig {@ Bean public WebMvcConfigurer webMvcConfigurer () {return new WebMvcConfigurer () {@ Override public void extendMessageConverters (List clazz, MediaType mediaType) {return false;} @ Override public boolean canWrite (Class clazz, MediaType mediaType) {return clazz.isAssignableFrom (Person.class) } / * Server needs to count which content types can be written by all MessageConverter * * application/x-guigu * @ return * / @ Override public List getSupportedMediaTypes () {return MediaType.parseMediaTypes ("application/x-guigu");} @ Override public Person read (Class)

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