Error creating bean with name 'requestMappingHandlerAdapter'

for me removing following from pom.xml fixed the issue

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>4.0.0.RELEASE</version>
</dependency>

as i was also using

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

Your cause is

nested exception is java.lang.ClassCastException: org.springframework.web.accept.ContentNegotiationManagerFactoryBean$$EnhancerByCGLIB$$88a811cb cannot be cast to org.springframework.web.accept.ContentNegotiationManager

This is probably due to both <mvc:annotation-driven /> and @EnableWebMvc are used together. As they are not complementary, either use @EnableWebMvc with Java based config or <mvc:annotation-driven /> with xml.

    <!-- https://mvnrepository.com/artifact/javax.validation/validation-api -->
    <dependency>
        <groupId>javax.validation</groupId>
        <artifactId>validation-api</artifactId>
        <version>1.0.0.GA</version>
    </dependency>

I received the same error. I removed the version.

    <!-- https://mvnrepository.com/artifact/javax.validation/validation-api -->
    <dependency>
        <groupId>javax.validation</groupId>
        <artifactId>validation-api</artifactId>
    </dependency>

This effectively downloaded: <javax-validation.version>2.0.1.Final</javax-validation.version> which works without the error.

In your User.java class add @javax.persistence.Id above getUserID() method

just like this...

@javax.persistence.Id
public int getUserID() {
    return userID;
}

It worked for me.