详细解析ajax中设置contentType: “application/json”的作用

最近在做项目交互的时候，刚开始向后台传递数据返回 415 ，后来百度添加了 contentType：“application/json“ 之后返回400，然后把传输的数据格式改为json字符串就传输成功了，现在我们来看看 contentType：“application/json“的作用：

添加 contentType：“application/json“之后，向后台发送数据的格式必须为json字符串

$.ajax({
  type: "post",
  url: "mobile/notice/addMessageInfo.jspx",
  contentType: "application/json",
  data:"{'name':'zhangsan','age':'15'}",
  dataType: "json",
  success: function(data) {
    console.log(data);
  },
  error: function(msg) {
    console.log(msg)
  }
})

不添加 contentType：“application/json“的时候可以向后天发送json对象形式

$.ajax({
  type: "post",
  url: "mobile/notice/addMessageInfo.jspx",
  data:{name:'zhangsan',age:'15'},
  dataType: "json",
  success: function(data) {
    console.log(data);
  },
  error: function(msg) {
    console.log(msg)
  }
})

另外，当向后台传递复杂json的时候，同样需要添加 contentType：“application/json“，然后将数据转化为字符串

var data = {
  uploadarray: uploadarray,
  messageInfo: {
    messageTitle: messageTitle,
    messageContent: messageContent,
    publisher: publisher
  },
  userId: userId
}

$.ajax({ 
  type: 'post',
  url: "mobile/notice/addMessageInfo.jspx",
  contentType: 'application/json',
  data: JSON.stringify(data),
  dataType: "json",
  success: function(data) {
    console.log(data);
  },
  error: function(msg) {
    console.log(msg)
  }
})

补充：下面看下$.ajax中contentType: “application/json” 的用法

不使用contentType: “application/json”则data可以是对象

$.ajax({
url: actionurl,
type: "POST",
datType: "JSON",
data: { id: nodeId },
async: false,
success: function () {}
});

使用contentType: “application/json”则data只能是json字符串

$.ajax({
url: actionurl,
type: "POST",
datType: "JSON",
contentType: "application/json"
data: "{'id': " + nodeId +"}",
async: false,
success: function () {}
});