一，先说明下为啥使用multimap这种数据结构

前端需要返回各地区的门店信息，会出现一个key下有多个value,类似：

{
    "data": {
        "上海": [
            {
                "id": 4358,
                "venderId": 200118,
                "cityName": "徐汇区",
                "storeNumbers": 1
            },
            {
                "id": 2668,
                "venderId": 200118,
                "cityName": "虹口区",
                "storeNumbers": 3
            }
        ],
        "北京": [
            {
                "id": 2664,
                "venderId": 200118,
                "cityName": "昌平区",
                "storeNumbers": 1
            },
            {
                "id": 859,
                "venderId": 200118,
                "cityName": "朝阳区",
                "storeNumbers": 3477
            }
        ]
    }
}

二，出现的问题：
如果不加任何处理，在controller里直接返回这种数据结构会出现：

{
 "fields":{
 "empty": false
 }
}

三，原因:
spring不能正确的序列化这种类型的数据结构

四，处理方法：

要手动向jackson的ObjectMapper中注册一个Module

１，添加mvn依赖：

<dependency>
 <groupId>com.fasterxml.jackson.datatype</groupId>
 <artifactId>jackson-datatype-guava</artifactId>
 <version>2.2.0</version>
</dependency>

<dependency>
<groupId>com.fasterxml.jackson.datatype</groupId>
<artifactId>jackson-datatype-guava</artifactId>
<version>2.2.0</version>
</dependency>

<!-- JSON parser configuration-->
<bean id="guavaObjectMapper" class="com.fasterxml.jackson.databind.ObjectMapper"/>

<bean class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
    <property name="targetObject"><ref bean="guavaObjectMapper" /></property>
    <property name="targetMethod"><value>registerModule</value></property>
    <property name="arguments">
        <list>
            <bean id="guavaModule" class="com.fasterxml.jackson.datatype.guava.GuavaModule"/>
        </list>
    </property>
</bean>
<mvc:annotation-driven>
    <mvc:message-converters register-defaults="true">
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <ref bean="guavaObjectMapper"/>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>