java.lang.IllegalArgumentException: No converter found for return value of type: class com.st.bean.User

原因：springmvc默认是没有对象转换成json的转换器的，要添加jackson依赖

在pom.xml中添加

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.5.4</version>
  </dependency>
  <dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
  <version>2.5.4</version>
</dependency>

在springmvc配置文件中进行配置

<!--处理json解析-->
    <mvc:annotation-driven>
        <mvc:message-converters>
            <!--返回普通字符串-->
            <bean class="org.springframework.http.converter.StringHttpMessageConverter"/>
            <bean class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter">
                <property name="supportedMediaTypes">
                    <list>
                        <value>text/html;charset=UTF-8</value>
                        <value>application/json;charset=UTF-8</value>
                    </list>
                </property>
            </bean>
        </mvc:message-converters>
    </mvc:annotation-driven>