參考資料:spring-framework-reference的Using @ModelAttribute on a method和Using @ModelAttribute on a method argument
先看一個沒有使用@ModelAttribute的Controller方法.
- @RequestMapping(“/save”)
- public String save(User user) {
- user.setUsername(”U love me”);
- userService.save(user);
- return “result”;
- }
@RequestMapping("/save")
public String save(User user) {
user.setUsername("U love me");
userService.save(user);
return "result";
}
其中User包含id和username兩個私有屬性,含有公共setter和getter方法.執行此方法時會將key爲”user”(注意:這裏即使參數名稱是user1,key一樣還是”user”),value爲user的對象加入到model.在jsp頁面可以通過
- @RequestMapping(“/save”)
- public String save(Model model,int id,String username) {
- User user=new User();
- //這裏是通過反射從request裏面拿值再set到user
- user.setId(id);
- user.setUsername(username);
- model.addAttribute(”user”,user);
- user.setUsername(”U love me”);
- userService.save(user);
- return “result”;
- }
@RequestMapping("/save")
public String save(Model model,int id,String username) {
User user=new User();
//這裏是通過反射從request裏面拿值再set到user
user.setId(id);
user.setUsername(username);
model.addAttribute("user",user);
user.setUsername("U love me");
userService.save(user);
return "result";
}
一.下面再來看看@ModelAttribute的基本用法.1.在方法上使用@ModelAttribute
@ModelAttribute("user1")
public User addUser(User user) {
return new User(520,"I love U");
}
假設此方法是寫在UserController內,那麼執行UserController內帶有@RequestMapping的方法之前,都會先執行此addUser方法.並且執行addUser過程中會添加兩個對象到model,先將key爲”user”的對象(由addUser方法的User user引起的),再添加key爲”user1”的對象(由註解@ModelAttribute(“user1”)引起的).2.在方法參數上使用@ModelAttribute.
- @RequestMapping(“/save”)
- public String save(@ModelAttribute User user) {
- user.setUsername(”U love me”);
- userService.save(user);
- return “result”;
- }
@RequestMapping("/save")
public String save(@ModelAttribute User user) {
user.setUsername("U love me");
userService.save(user);
return "result";
}
此方法會先從model去獲取key爲”user”的對象,如果獲取不到會通過反射實例化一個User對象,再從request裏面拿值set到這個對象,然後把這個User對象添加到model(其中key爲”user”).使用了@ModelAttribute可修改這個key,不一定是”user”,此情況下,用與不用@ModelAttribute沒有區別.
3.再來看看在方法和方法參數上結合使用@ModelAttribute,即上面兩步的兩個方法都添加UserController,如下:
- @ModelAttribute(“user1”)
- public User addUser(User user) {
- return new User(520,“I love U”);
- }
- @RequestMapping(“/save”)
- public String save(@ModelAttribute User user) {
- user.setUsername(”U love me”);
- userService.save(user);
- return “result”;
- }
@ModelAttribute("user1")
public User addUser(User user) {
return new User(520,"I love U");
}
@RequestMapping("/save")
public String save(@ModelAttribute User user) {
user.setUsername("U love me");
userService.save(user);
return "result";
}
假設要執行保存用戶操作,根據一分析可知,先執行完會產生兩個User類型的對象(一個key是”user”,另一個key是”user1”)添加到model,再執行save方法,此時會先從model去找key爲”user”的對象,能找到再從request取值set到這個User對象.最後返回到jsp頁面,model裏也只有兩個User類型對象.再來個小小假設,將上面@ModelAttribute(“user1”)的user1改爲user,其它不變.雖然執行了addUser方法,那麼執行到save方法內,user對象的字段值還是來源於請求,最後返回到jsp頁面,model裏也只有一個User類型對象.
4.另類:
- @ModelAttribute
- @RequestMapping(“/save”)
- public String save(@ModelAttribute User user) {
- user.setUsername(”U love me”);
- userService.save(user);
- return “result”;
- }
@ModelAttribute
@RequestMapping("/save")
public String save(@ModelAttribute User user) {
user.setUsername("U love me");
userService.save(user);
return "result";
}
此種情況,會添加一個key爲”user”的User對象到model,還會添加一個key爲”string”,value爲”result”的對象到model,而視圖名稱則變爲了”save”而不是”result”.相信一般開發者都不會這樣用.
二.下面再看看結合@SessionAttributes的用法.
1.先看看這段代碼
- @Controller
- @RequestMapping(“user”)
- @SessionAttributes(“test”)
- public class UserController {
- @RequestMapping(value = “test1”)
- public String test1(Map<String, Object> model, SessionStatus sessionStatus) {
- model.put(”test”,“something”);
- //sessionStatus.setComplete();
- return “user/list”;
- }
- @RequestMapping(value = “test2”)
- public String test2(Map<String,?> model) {
- Object test = model.get(”test”);
- System.out.println(test);
- return “user/list”;
- }
- }
@Controller
@RequestMapping("user")
@SessionAttributes("test")
public class UserController {
@RequestMapping(value = "test1")
public String test1(Map<String, Object> model, SessionStatus sessionStatus) {
model.put("test","something");
//sessionStatus.setComplete();
return "user/list";
}
@RequestMapping(value = "test2")
public String test2(Map<String,?> model) {
Object test = model.get("test");
System.out.println(test);
return "user/list";
}
}
在類級別上使用@SessionAttributes(“test”),它的作用是在controller共享 model 屬性,直到調用org.springframework.web.bind.support.SessionStatus#setComplete會清除此session值.否則長期保留(session過期,這個值也不再保留).因此先調用/user/test1,再調用/user/test2.在test2方法能得到test1方法put進去的值.當移除上面的註解,因爲屬性爲test的session也被清除了,所以test2方法會得到是null.
2.再看結合@ModelAttribute的用法.
- @Controller
- @RequestMapping(“user”)
- @SessionAttributes(“test”)
- public class UserController {
- @RequestMapping(value = “test1”)
- public String test1(HttpServletRequest request) {
- User user = new User(“xiejx618”);
- request.getSession().setAttribute(”test”,user);
- return “user/list”;
- }
- @RequestMapping(value = “test2”)
- public String test2(@ModelAttribute(“test”) User user,SessionStatus sessionStatus) {
- System.out.println(user.getUsername());
- sessionStatus.setComplete();
- return “user/list”;
- }
- }
@Controller
@RequestMapping("user")
@SessionAttributes("test")
public class UserController {
@RequestMapping(value = "test1")
public String test1(HttpServletRequest request) {
User user = new User("xiejx618");
request.getSession().setAttribute("test",user);
return "user/list";
}
@RequestMapping(value = "test2")
public String test2(@ModelAttribute("test") User user,SessionStatus sessionStatus) {
System.out.println(user.getUsername());
sessionStatus.setComplete();
return "user/list";
}
}
一樣先調用/user/test1,再調用/user/test2.在test2方法能得到test1方法set進去的值.使用了@SessionAttributes,如果在controller的方法參數上有@ModelAttribute,那麼此方法會確保屬性test的session會存在,否則會拋org.springframework.web.HttpSessionRequiredException異常,即上面代碼先不調/user/test1,直接調/user/test2就會拋此異常
我個人很少這樣用,用原始的寫法就清晰很多了,但要看明白別人寫的代碼。