문제 발생 원인: 아마도 _csrf
<input type="hidden" name="${_csrf.parameterName }" value="${_csrf.token }"/>
해결
[web.xml]
<filter>
<filter-name>encodingFilter</filter-name>
<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
<init-param>
<param-name>encoding</param-name>
<param-value>UTF-8</param-value>
</init-param>
<init-param>
<param-name>forceEncoding</param-name>
<param-value>true</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>encodingFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
GET방식은 _csrf값 넘기지 않아도 된다.
POST방식일 땐 넘겨야 한다.