SpringMVC implementation method of front-end and back-end data interaction through ajax

During the front-end development process, the front-end data interaction is often performed on the html page through ajax, and the SpringMVC controller receives data, but sometimes the back-end data cannot be received. This is because our parameters and the contentType parameter type of the front-end ajax, or what kind of relationship does the contentType and the background controller method parameters exist?

When we pass ordinary parameters, there are often two situations like this:

contentType: "application/x-www-form-urlencoded",
contentType: "application/json",

What are the differences between these two methods?

The first method: the application/x-www-form-urlencoded parameter will be parsed into a parameter table, such as:

name=John+Doe&age=30&city=New+York

Passing through ajax, the ajax is written as follows:

           	let params={
            	    "username":"Zhang San",
            	    "password":"123456",
            	}
            	$.ajax({
            		url: "dologin4",
            		contentType: "application/x-www-form-urlencoded",
            		headers: {  'Authorization': "****",'Access-Control-Allow-Origin':'*'},
            		type: 'post',
            		data: params,
            		success: function(result) {
            			(result)
            		},
            		error: function(data) {
            			('The interface is not connected');
            		}
            	});

When accepting this form in the background, use:()or@RequestParam, for example:

@RequestMapping("/dologin")
	public ModelAndView dologin(@RequestParam String username,@RequestParam String password) throws Exception {
		(username);
		(password);
		ModelAndView mav = new ModelAndView();
		("info", "Welcome");
		("success");
		return mav;
	}
	@ResponseBody
	@RequestMapping("/dologin2")
	public Map&lt;String,Object&gt; dologin2(HttpServletRequest request,HttpServletResponse response) throws Exception {
		String username=("username");
		(username);
		Map&lt;String,Object&gt; map=new HashMap&lt;&gt;();
		("aa", "1111");
		return map;
	}

Suitablex-www-form-urlencodedSituation:

Traditional HTML form submission
Simple key-value pair data
Need backward compatibility with old systems
File upload (combinedmultipart/form-data）

The second method: contentType: "application/json", the entire body is processed as a single data stream, such as:

{
  "name": "John Doe",
  "age": 30,
  "city": "New York",
  "hobbies": ["reading", "swimming"]
}

Passing through ajax, the ajax is written as follows:

           function dologin(){
            	let params={
            	    "username":"Zhang San",
            	    "password":"123456",
            	}
            	$.ajax({
            		url: "dologin4",
            		contentType: "application/json",
            		headers: {  'Authorization': "****",'Access-Control-Allow-Origin':'*'},
            		type: 'post',
            		data: (params),
            		success: function(result) {
            			(result)
            		},
            		error: function(data) {
            			('The interface is not connected');
            		}
            	});
            }

The code for Java backend accepting parameters passed in foreground is as follows: @RequestBody

@ResponseBody
	@RequestMapping("/dologin4")
	public Map<String,Object> dologin4(HttpServletRequest request,HttpServletResponse response) throws Exception {
		String uu=("username");
		(uu);
	    StringBuilder jsonBuilder = new StringBuilder();
	    try (BufferedReader reader = ()) {
	        String line;
	        while ((line = ()) != null) {
	            (line);
	        }
	    }
	    String jsonString = ();
	    ObjectMapper mapper = new ObjectMapper();
	    Map<String, Object> jsonMap = (jsonString, );  
	     String username = (String) ("username");
		(username);
		Map<String,Object> map=new HashMap<>();
		("aa", "1111");
		return map;   
	}
	@ResponseBody
	@RequestMapping("/dologin3")
	public Map<String,Object> dologin3(@RequestBody Map<String,Object> reqMap) throws Exception {
		String username=("username").toString();
		(username);
		Map<String,Object> map=new HashMap<>();
		("aa", "1111");
		return map;
	}

Suitableapplication/jsonSituation:

RESTful API Communication
Complex data structures
Need to clarify the data type
Front-end separation architecture
Mobile application communication with server

The two scenarios are different. If the front-end is passing in json format, then you use the back-end:

   String uu=("username");
    (uu);

This way, data cannot be received, and the received parameters are always null, because json is transmitted to the background as a stream.