我正在使用Sping Boot 和Web依赖项创建一个简单的rest控制器。我尝试将JSON主体反序列化为只有3个字段的测试POJO，但当我尝试发出POST请求时，服务器响应500错误，我在控制台中得到的错误是：

.w.s.m.s.DefaultHandlerExceptionResolver : Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class java.util.LinkedHashMap

我写的所有代码如下：
EmailApplication.java:

package com.test.email.app;

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan(basePackages = { "com.test.email" })
public class EmailApplication {

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

    @Bean
    public CommandLineRunner commandLineRunner(ApplicationContext ctx) {
        return args -> {
            System.out.println("----- You're up and running with test-email-app! -----");
        };
    }
}

EmailController.java:

package com.test.email.controller;

import com.test.email.entity.TestEntity;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.RequestMapping;

import java.net.URI;

@RestController
public class EmailController {
    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String index() {
        TestEntity entity = new TestEntity();
        return "test-email-app index";
    }

    @RequestMapping(value = "/poster", method = RequestMethod.POST)
    public ResponseEntity<String> poster(@RequestBody TestEntity testEntity) {
        URI location = URI.create("/poster");
        return ResponseEntity.created(location).body(testEntity.getUrl());
    }
}

TestEntity.java

package com.test.email.entity;

/**
 * An entity for serialization/deserialization testing
 */
public class TestEntity {
    public String url;
    public int count;
    public double height;

    public TestEntity() {}

    public TestEntity(String url, int count, double height) {
        this.url = url;
        this.count = count;
        this.height = height;
    }

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public int getCount() {
        return count;
    }

    public void setCount(int count) {
        this.count = count;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }
}

我用Postman只使用头部Content-Type: application/json和主体来发出POST请求：

{ "url": "http://google.com", "count": 3, "height": 2.4 }

我不知道为什么Spring不能从LinkedHashMap转换到我的POJO。任何帮助将不胜感激。
编辑：
我的pom.xml如下：

<?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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.test.email</groupId>
    <artifactId>test-email-app</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>test-email-app</name>
    <description>Demo project for Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.3.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>

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

        <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>