<form id="addForm" action="${pageContext.request.contextPath}/admin/saveAdd" method="post">    
  <input type="text" name="name" placeholder="请输入名字" />
  <input type="password" name="password" placeholder="密码"/>
 </form>
<button type="button" id="submitAdd">确认</button> 

 $("#submitAdd").click(function(){
    
   var targetUrl = $("#addForm").attr("action");    
   var data = $("#addForm").serialize();     
    $.ajax({ 
     type:'post',  
     url:targetUrl, 
     cache: false,
     data:data,  
     dataType:'json', 
     success:function(data){      
       alert('success');
     },
     error:function(){ 
      alert("请求失败")
     }
    })
    
 })

<form id="addForm" action="${pageContext.request.contextPath}/admin/saveAdd" method="post"enctype=" multipart/form-data">    
  <input type="text" name="name" placeholder="请输入名字" />
  <input type="password" name="password" placeholder="密码"/>
  <input type="file" name="avatar" />
 </form>
<button type="button" id="submitAdd">确认</button>

$("#submitAdd").click(function(){
    
   var targetUrl = $("#addForm").attr("action");    
   var data = new FormData($( "#addForm" )[0]);     
    $.ajax({ 
     type:'post',  
     url:targetUrl, 
     cache: false,    //上传文件不需缓存
     processData: false, //需设置为false。因为data值是FormData对象，不需要对数据做处理
     contentType: false, //需设置为false。因为是FormData对象，且已经声明了属性enctype="multipart/form-data"
     data:data,  
     dataType:'json', 
     success:function(data){      
       alert('success');
     },
     error:function(){ 
      alert("请求失败")
     }
    })
    
 })

<div id="uploadFile">
 <input id="file" name="avatar" type="file"/>
 <button id="upload" data-url="/admin/upload" type="button">上传头像</button>
</div>

$("#upload").click(function(){
    
   var targetUrl = $(this).attr("data-url");    
   var data = new FormData();
   //FormData对象加入参数
   data.append('file', $('#file')[0].files[0]); //'file' 为参数名，$('#file')[0].files[0])获取上传的文件，如果需上传多个文件，要在<input>标签加上属性multiple    
    $.ajax({ 
     type:'post',  
     url:targetUrl, 
     cache: false,    
     processData: false, 
     contentType: false, 
     data:data,  
     dataType:'json', 
     success:function(data){      
       alert('success');
     },
     error:function(){ 
      alert("请求失败")
     }
    })
    
 })