springmvc:配置
<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
        <property name="messageConverters">
            <list>
                <ref bean="jsonHttpMessageConverter" />
            </list>
        </property>
    </bean>

    <bean id="jsonHttpMessageConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
        <property name="supportedMediaTypes">
            <list>
                <value>application/json;charset=UTF-8</value>
            </list>
        </property>
    </bean>

Controller：返回一个自定义的json：只有message和success属性
@RequestMapping("saveClass")
    @ResponseBody
    public Object saveClass(Classes classes){
        System.out.println(classes.getClassName());
        classesDao.save(classes);
        MyJson mj=new MyJson();
        mj.setMessage("添加成功");
        mj.setSuccess(true);
        return mj;
    }

function save(){
        $("#fm").form({
            url:"classes/saveClass",
            success:function(result){
                alert(result);//{‘message’:'成功','success':true}
                alert(res.message);//undefined
                                alert(res['message'])//undefined
                if(result.success){
                    $.messager.show({  
                        title:'Info',  
                        msg:result.msg,  
                        showType:'fade',  
                        style:{  
                            right:'',  
                            bottom:''  
                            }  
                        });  
                        $('#dlg').dialog('close');      // close the dialog  
                        $('#dg').datagrid('reload');  
                }else {  
                    $.messager.show({  
                        title: 'Error',  
                        msg: result.msg  
                    });  
                }  
            }
        });
        $("#fm").submit();
    }