Why did REST forget about JSON?

Question

Java

I actually know the depths of the fiber, and took up this manual: https://spring.io/guides/gs/rest-service/

Actually the problem:

o.s.web.servlet.DispatcherServlet        : Completed initialization in 2 ms
 o.s.web.servlet.DispatcherServlet        : GET "/message", parameters={}
 s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped to by.alex.webrest.RESTApp#GetMessage(String)
 .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class by.alex.webrest.RESTMessage]
 o.s.web.servlet.DispatcherServlet        : Completed 500 INTERNAL_SERVER_ERROR
 o.s.web.servlet.DispatcherServlet        : "ERROR" dispatch for GET "/error", parameters={}
 s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped to org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController#errorHtml(HttpServletRequest, HttpServletResponse)
 o.s.w.s.v.ContentNegotiatingViewResolver : Selected 'text/html' given [text/html, text/html;q=0.8]
 o.s.web.servlet.DispatcherServlet        : Exiting from "ERROR" dispatch, status 500

pom.xml:

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.4.2</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>
  <groupId>by.alex</groupId>
  <artifactId>webrest</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>webrest</name>
  <description>Some just REST service</description>

  <properties>
    <java.version>1.8</java.version>
  </properties>

  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
    </plugins>
  </build>

</project>

RestApp.java:

RESTApp.java

package by.alex.webrest;

import java.util.concurrent.atomic.AtomicLong; 
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

@RestController
public class RESTApp {
  private static final String template = "Hello, %s";
  private final AtomicLong requestCounter = new AtomicLong();
  
  @GetMapping("/message")
  @ResponseBody
  RESTMessage GetMessage(@RequestParam(name = "name", defaultValue = "my friend!") String target) {
    return new RESTMessage(requestCounter.incrementAndGet(), String.format(template, target));
  }
}

RESTMessage.java:

RESTMessage.java

package by.alex.webrest;

public class RESTMessage {
  private final long id;
  private final String content;
  
  RESTMessage(long id, String content) {
    this.id = id;
    this.content = content; 
  }
  
  long getId() {
    return this.id;
  }
  
  String getContent() {
    return this.content;
  }
  
  
}

WebRestApplication.java:

WebRestApplication.java

package by.alex.webrest;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.xml.MarshallingHttpMessageConverter;

@SpringBootApplication
public class WebrestApplication {
    
  public static void main(String[] args) {
    SpringApplication.run(WebrestApplication.class, args);
  }

}

Tried tips from internet:
1. Remove .m2 repository from PC
2. add jackson.fastxml to pom.xml DID

NOT HELP

so sad when their guides don't work....

Answer 1

Despite the fact that the example of the spring itself works without additional. annotations to the class/class fields, the solution turned out to be very simple:

@JsonSerialize
public class Tag {
  @JsonProperty("id")
  private long Id;
  
  @JsonProperty("name")
  private String Name;
  
  @JsonProperty("createdAt")
  private ZonedDateTime CreatedAt;
  
  @JsonProperty("updatedAt")
  private ZonedDateTime UpdatedAt;
....

With this usage:

@RestController
public class RESTAPI {
  private static final AtomicLong counter = new AtomicLong();
  @GetMapping("/tags")
  Tag tags(@RequestParam(name = "name", defaultValue = "anytag") String tagName) {
    return new Tag(this.counter.incrementAndGet(), tagName);
  }
}