SpringMVC(十二)：SpringMVC 处理输出模型数据之@ModelAttribute

Spring MVC提供了以下几种途径输出模型数据：
1）ModelAndView：处理方法返回值类型为ModelAndView时，方法体即可通过该对象添加模型数据；
2）Map及Model：处理方法入参为org.springframework.ui.Model、org.springframework.ui.ModelMap或java.util.Map时，处理方法返回时，Map中的数据会自动被添加到模型中；
3）@SessionAttributes：将模型中的某个属性暂存到HttpSeession中，以便多个请求之间可以共享这个属性；
4）@ModelAttribute：方法入参标注该注解后，入参的对象就会放到数据模型中。

在SpringMVC的hanlder类中用@ModelAttribute标注的方法，会在该handler内所有目标（action）方法执行之前被SpringMVC调用。

用法示例：

Account.java

 package com.dx.springlearn.entities;

 public class Account {
     public Integer id;
     private String username;
     private String password;
     private String registerDate;
     private String registerIP;

     public Account() {
     }

     public Account(Integer id, String username, String password, String registerDate, String registerIP) {
         super();
         this.id = id;
         this.username = username;
         this.password = password;
         this.registerDate = registerDate;
         this.registerIP = registerIP;
     }

     public Account(String username, String password, String registerDate, String registerIP) {
         super();
         this.username = username;
         this.password = password;
         this.registerDate = registerDate;
         this.registerIP = registerIP;
     }

     public Integer getId() {
         return id;
     }

     public void setId(Integer id) {
         this.id = id;
     }

     public String getUsername() {
         return username;
     }

     public void setUsername(String username) {
         this.username = username;
     }

     public String getPassword() {
         return password;
     }

     public void setPassword(String password) {
         this.password = password;
     }

     public String getRegisterDate() {
         return registerDate;
     }

     public void setRegisterDate(String registerDate) {
         this.registerDate = registerDate;
     }

     public String getRegisterIP() {
         return registerIP;
     }

     public void setRegisterIP(String registerIP) {
         this.registerIP = registerIP;
     }

     @Override
     public String toString() {
         return "Account [id=" + id + ", username=" + username + ", password=" + password + ", registerDate="
                 + registerDate + ", registerIP=" + registerIP + "]";
     }

 }

Handler类TestModelData.java

 package com.dx.springlearn.hanlders;

 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.Map;

 import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.ModelAttribute;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.servlet.ModelAndView;

 import com.dx.springlearn.entities.Account;

 @Controller
 public class TestModelData {
     private final String SUCCESS = "success";

     @ModelAttribute
     public void getAccount(@RequestParam(name = "id", required = false) Integer id, Map<String, Object> map) {
         if (id != null) {
             System.out.println("read account(id=" + id + ") from db");
             Account account = new Account(1, "tommy", "123456", "2018-01-20 21:56:09", "127.0.0.1");
             map.put("account", account);
         } else {
             System.out.println("the acount id is null");
         }
     }

     @RequestMapping("/testModelAttribute")
     public ModelAndView testModelAttribute(Account account) {
         System.out.println("accept account:" + account);
         String viewName = SUCCESS;
         ModelAndView modelAndView = new ModelAndView(viewName);
         modelAndView.addObject("currentTime", new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()));

         return modelAndView;
     }
 }

测试表单页面：

    <form id="form_testModelAttribute" action="testModelAttribute"
        method="POST">
        <input name="id" type="hidden" value="1"/>
        username:<input name="username" type="text" value="tommy" /><br>
        <!-- password:<input name="password" type="password" /><br> -->
        register date:<input name="registerDate" type="text" value="2018-01-20 21:56:09" /><br>
        register ip:<input name="registerIP" type="text" value="127.0.0.1"/><br>
        <input type="submit" value="Submit"/>
    </form>

除了handler目标方法参数第一个参数名称与存放到map中的对象名称一致外，也可以使用@ModelAttribute标注handler目标方法参数：

package com.dx.springlearn.hanlders;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Map;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.servlet.ModelAndView;

import com.dx.springlearn.entities.Account;

@Controller
public class TestModelData {
    private final String SUCCESS = "success";

    @ModelAttribute
    public void getAccount(@RequestParam(name = "id", required = false) Integer id, Map<String, Object> map) {
        if (id != null) {
            System.out.println("read account(id=" + id + ") from db");
            Account account = new Account(1, "tommy", "123456", "2018-01-20 21:56:09", "127.0.0.1");
            map.put("testabc", account);
        } else {
            System.out.println("the acount id is null");
        }
    }

    @RequestMapping("/testModelAttribute")
    public ModelAndView testModelAttribute(@ModelAttribute("testabc") Account account) {
        System.out.println("accept account:" + account);
        String viewName = SUCCESS;
        ModelAndView modelAndView = new ModelAndView(viewName);
        modelAndView.addObject("currentTime", new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()));

        return modelAndView;
    }
}

运行流程：

1）运行有@ModelAttribute注解标注的方法：从数据库中取出对象，把对象放入到Map中，键值为：user；

2）SpringMVC从Map中取出User对象，并把表单的请求参数赋值给该User对象的对应属性；

3）SpringMVC把上述对象传入目标方法的参数（参数名称必须与Map中的键值一致）。

注意：在@ModelAttribute修饰的方法中，放入Map是的键类型要与目标方法入参类型一直，而且Map是的键名称要与目标方法入参的参数名一致。