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Previous articles have explained how spring mvc passes parameters to the background and returns parameters to the foreground. Today, let's talk about the ModelAttribute annotation of spring mvc. What is the role of this annotation and what scenario is it used in?

1. Application scenario: data update. For example, there is an entity User, which has id, userName, age, email, createTime and other attributes. CreateTime defines the creation time of the user, which is generally automatically obtained and assigned in the background. Other attributes of User except id and createTime can be edited and modified, as shown in the following figure.

At this time, our background generally uses objects to receive parameters, as follows:

@RequestMapping("/testModelAttribute")public String testModelAttribute(User user){ System.out.println(user); return "success";}

Test code, set the default value of user, you can see the background output

User{userName='lxy', password='null', email='test@126.com', age=20, createTime=null, address=null}

, user object only userName, age, emai have values, the rest are null:

userName： age： email：

If we update the database at this time, we need to query the database first, read out the value of the previous user object, and then set the parameters passed from the foreground to user, otherwise some data will be lost.

Another solution is to query the data before editing, then use hidden fields on the page, put other values in hidden fields, and then submit the hidden field data to the background when submitting the form.

2. Use ModelAttribute annotation together. Here, according to parameter userName, simulate reading data from database.

@ModelAttributepublic void getUserById(@RequestParam(value = "userName", required = false) String userName, Map map) { System.out.println("I will be called anytime"); if(userName != null) { //simulate reading data from database User user = new User(); user.setUserName("lxy"); user.setAge(18); user.setPassword("123456"); user.setEmail("lxy@126.com"); user.setCreateTime(new Date()); map.put("user", user); }}

At this point, run the project again, you can see the output of the background is: email and age are the parameters passed from the foreground, and password and createTime are the data read from the database, which can be directly based on the user entity, update the database.

User{userName='lxy', password='123456', email='test@126.com', age=20, createTime=Mon Mar 20 18:03:31 CST 2017, address=null}

Note that accessing any method in controller calls the method annotated with ModelAttribute. Therefore, this annotation should be used with caution. Before applying the annotation, you need to carefully consider whether it is really appropriate.

@RequestMapping("/testRun")public String testRun(){ System.out.println("I'll see if I can call ModelAttribute annotated methods"); return "success";}

Visiting the method above, you can see the following output, the method decorated with ModelAttribute annotation, will execute before the target method:

Anytime I'm called, I'll see if I call ModelAttribute annotated methods.

4. Use ModelAttribute annotation on parameters of target method

Above we see that on methods decorated with ModelAttribute annotations, if you want to get a value, you must set this in the method.

User user = new User();user.setUserName("lxy");user.setAge(18);user.setPassword("123456");user.setEmail("lxy@126.com"); user.setCreateTime(new Date());//If the key is temp, the target method must have a corresponding annotation declaration to have an effect map.put("temp", user);@RequestMapping("/testModelAttribute2")public String testModelAttribute2 (@ModelAttribute("temp")User user){ //temp here is consistent with the stored key of the method decorated with ModelAttribute annotation System.out.println(user); return "success";}

Project source code:

https://git.oschina.net/acesdream/spring-mvc

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