网上有多种解决办法,发现这种方式最简便:
在springMVC的配置文件中(springMVC-xx.xml),修改<mvc:annotation-driven />为:

<mvc:annotation-driven>
<mvc:message-converters register-defaults="true">
<bean class="org.springframework.http.converter.StringHttpMessageConverter">
<constructor-arg value="UTF-8" />
</bean>
</mvc:message-converters>
</mvc:annotation-driven>

乱码原因:org.springframework.http.converter.StringHttpMessageConverter有个final修饰的Charset DEFAULT_CHARSET=Charset.forName("ISO-8859-1")常量。即当返回类型为String时,返回类型为text/plain,字符被设置为默认字符ISO-8859-1