springboot项目提示No converter found for return value of type: class java.util.LinkedHashMap,@Request...

先说解决方案，在pom中加入如下依赖，解决。

        <!--json-->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.9.9</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>2.9.9</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.9.9</version>
        </dependency>

说起来，这可真真是一个坑爹的问题，困扰了一天，烦了一天，我真以为我大半年没碰java，连新建个sprintboot的rest接口用完美的intellij idea这么高大上的工具都创建不出来了呢。

接口

看看错误吧.
两个连入门都算不上的接口

package com.marvin.demo.controller;

import com.marvin.demo.vo.DemoEntity;
import lombok.extern.slf4j.Slf4j;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

/**
 * 精诚所至，金石为开。
 * 石の上にも三年;陽気の発する所金石亦透る。
 *
 * @Author mahaiqiang
 * @Create 2020/2/1 11:06 下午
 * @Description TODO
 **/
@Slf4j
@RestController
public class DemoController {

    @GetMapping("/demo1")
    public DemoEntity demo1() {
        log.info("第一个get接口");
        DemoEntity de = new DemoEntity();
        de.setName("李四");
        return de;
    }

    @PostMapping("/demo2")
    public DemoEntity demo2(@RequestBody DemoEntity entity) {
        log.info("第一个post接口, 参数:{}", entity);
        entity.setName("Hello, " + entity.getName());
        return entity;
    }
}

错误

GET是正常的，但是POST的这个接口，因为用了@RequestBody注解，要转换json，所以就死活不行。
错误是这样的：

2020-02-03 22:55:34.382  WARN 40841 --- [nio-8080-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/json;charset=UTF-8' not supported]
2020-02-03 22:55:34.405  WARN 40841 --- [nio-8080-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class java.util.LinkedHashMap]

出这个错我先上网查，有个推荐找不到了说引入jackson看日志我引入的是2.9.6不知道是不是还少引用了啥，反正没成。

让别的同事试了下，人家说啥也不用干，导入进去就能运行，日了狗了，咋回事，肯定就是环境问题了呗。

于是就jdk、maven、intellij idea，重新配置，甚至重装，都搞一遍也没办法了。

第二天继续按照错误提示找，找到个原来就翻出来过的链接，加了依赖后莫名其妙好了，而且删除了依赖立马就能复现问题了。

正常

预期是这样的

com.marvin.demo.DemoApplication

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.4.RELEASE)

2020-02-03 22:51:16.565  INFO 40753 --- [           main] com.marvin.demo.DemoApplication          : Starting DemoApplication on 192.168.0.105 with PID 40753 (/Users/mahaiqiang/git/mygitrepo/secret-demo/target/classes started by mahaiqiang in /Users/mahaiqiang/git/mygitrepo/secret-demo)
2020-02-03 22:51:16.571  INFO 40753 --- [           main] com.marvin.demo.DemoApplication          : No active profile set, falling back to default profiles: default
2020-02-03 22:51:17.751  INFO 40753 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-02-03 22:51:17.763  INFO 40753 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-02-03 22:51:17.764  INFO 40753 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.30]
2020-02-03 22:51:17.838  INFO 40753 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-02-03 22:51:17.838  INFO 40753 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1205 ms
2020-02-03 22:51:18.043  INFO 40753 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-02-03 22:51:18.230  INFO 40753 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-02-03 22:51:18.234  INFO 40753 --- [           main] com.marvin.demo.DemoApplication          : Started DemoApplication in 2.039 seconds (JVM running for 2.681)
2020-02-03 22:51:29.934  INFO 40753 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring DispatcherServlet 'dispatcherServlet'
2020-02-03 22:51:29.934  INFO 40753 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'
2020-02-03 22:51:29.942  INFO 40753 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 8 ms
2020-02-03 22:51:30.045  INFO 40753 --- [nio-8080-exec-1] c.marvin.demo.controller.DemoController  : 第一个post接口, 参数:DemoEntity(name=a)
2020-02-03 22:51:50.866  INFO 40753 --- [nio-8080-exec-3] c.marvin.demo.controller.DemoController  : 第一个post接口, 参数:DemoEntity(name=marvin)

这次是百度救了我，我一般用的必应搜索引擎，没想到按照第二个错误百度出来的第一条直接就是解决方案。

但是按照方案说第一条，我的pom文件里已经有spring-boot-starter-web，

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

但是还是逼着我加了jackson才好使了。
先记录一笔，再慢慢想原因吧。

感谢

SpringBoot2.X 遭遇 No converter found for return value of type: class java.util.LinkedHashMap