Ajax提交form表单

var form = new FormData($("#importForm")[0]); 
//这里为什么要有个[0]呢？ 我也不知道 浏览器控制台会报错Unexpected end of JSON input，当时这里卡了好久，大神看到知道的为啥的给说下

$.ajax({
                    url:"",
                    type:"post",
                    data:form,
                    cache: false,
                    processData:false,
                    contentType:false,
                    success:function(data){
                        var boj = JSON.parse(data);
                        console.log(boj);
                    },
                    error:function(e){
                        console.log(e);
                    }
                });

<form id="importForm" action="" enctype="multipart/form-data" class="form-horizontal">
                <div class="control-group">
                <div class="controls">
                <input type="button" class="btn btn-primary" value="批量下单" onclick="importc()" style="margin-right: 25px;">
                <a href="" style="margin-right: 25px;">下载模板</a>
                <input type="file" id="file" name="file">
                </div>
                </div>
                </form>

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
        <property name="defaultEncoding" value="UTF-8"></property>
        <property name="maxInMemorySize" value="10240000"></property>
    </bean>