@Controller
public class RestController {

    @RequestMapping(value = "/book/{bid}", method = RequestMethod.GET)
    public String getBook(@PathVariable("bid") Integer id) {
        System.out.println("获得图书 id " + id);
        return "success";
    }

    @RequestMapping(value = "/book/{bid}", method = RequestMethod.POST)
    public String addBook(@PathVariable("bid") Integer id) {
        System.out.println("添加图书 id " + id);
        return "success";
    }

    @RequestMapping(value = "/book/{bid}", method = RequestMethod.PUT)
    public String modifyBook(@PathVariable("bid") Integer id) {
        System.out.println("修改图书 id " + id);
        return "success";
    }

    @RequestMapping(value = "/book/{bid}", method = RequestMethod.DELETE)
    public String deleteBook(@PathVariable("bid") Integer id) {
        System.out.println("删除图书 id " + id);
        return "success";
    }
}

@RequestMapping("/hello")
// 这个是获得参数 user 给 username。如果不用改名删掉 RequestParam 就行了。
// 也可以使用组合拳 @RequestParam(value = "user", required = false, defaultValue = "qwq")，此时 user 不再必须有，没有的时候有一个默认值 qwq。
public String hello(@RequestParam("user") String username) {
    System.out.println("username 是 " + username);
    return "success";
}

public class Book {
    private String title;
    private Integer price;
    // getter, setter, 构造函数，tostring 略
}

@RequestMapping("/hello")
public String hello(Book book) {
    System.out.println(book);
    return "success";
}

@RequestMapping("/hello")
public String hello(Map<String, Object> map) {
    map.put("msg", "message1");
    return "success";
}

// Model 和 ModelMap 这么写：
@RequestMapping("/hello")
public String hello(Model model) {
    model.addAttribute("msg", "message2");
    return "success";
}

@RequestMapping("/hello")
public ModelAndView hello() {
    ModelAndView mav = new ModelAndView("success");
    mav.addObject("msg", "message3");
    return mav;
}

@Controller
public class HelloController {

    @ModelAttribute//ModelAttribute 写在方法上，比 hello 更先执行。
    public void qwqModelAttribute(Map<String, Object> map) {
        Book book = new Book("title", 12);
        map.put("book", book);//放进去，以便 hello 方法能用。
        System.out.println("model attribute");
    }

    @RequestMapping("/hello")
    public String hello(@ModelAttribute("book") Book book) {
        System.out.println(book);
        return "success";
    }
}

package net.yxchen.component;

import net.yxchen.dao.DepartmentDao;
import net.yxchen.entities.Employee;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.convert.converter.Converter;

public class StringToEmployeeConverter implements Converter<String, Employee> {
    @Autowired
    DepartmentDao departmentDao;

    @Override
    public Employee convert(String s) { // 编写自定义的转换规则
        Employee employee = new Employee();
        // 具体转换方式略
        return employee;
    }
}

@RequestMapping("/simpleAdd")
public String simpleAdd(@RequestParam("empinfo") Employee employee) {
    employeeDao.save(employee);
    return "redirect:/emps";
}

@NotEmpty
@Length(min = 4, max = 18, message = "用户名长度在4-18个字符之间")
private String lastName;

@Email
private String email;
//甚至还能用 @Future 指定 Date 要是未来的 Date

@RequestMapping(value = "/emp", method = RequestMethod.POST)
public String addEmp(@Valid Employee employee, BindingResult result) {// result 紧跟要校验的东西
    if (result.hasErrors()) {
        System.out.println("校验失败");
        return "add";
    }
    employeeDao.save(employee);
    return "redirect:/emps";
}

@ResponseBody
@RequestMapping("ajaxEmployee")
public Collection<Employee> ajaxEmployee() {
    Collection<Employee> all = employeeDao.getAll();
    return all;
}

@RequestMapping("/download")
public ResponseEntity<byte[]> download(HttpServletRequest request) throws Exception {
    ServletContext context = request.getServletContext();
    String realPath = context.getRealPath("/script/jquery-3.4.1.min.js");
    FileInputStream fileInputStream = new FileInputStream(realPath);
    byte[] bytes = new byte[fileInputStream.available()];
    fileInputStream.read(bytes);
    fileInputStream.close();
    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.set("Content-Disposition", "attachment; filename=jquery-3.4.1.min.js");
    return new ResponseEntity<byte[]>(bytes, httpHeaders, HttpStatus.OK);
}

@Controller
public class MainController {
    @RequestMapping("/uploadOne")
    public String uploadOne(@RequestParam(value = "username", required = false) String username,
                            @RequestParam("avatar") MultipartFile file, Model model) {
        System.out.println("username is " + username);
        System.out.println("filename is " + file.getOriginalFilename());
        try {
            file.transferTo(new File("uploads/" + file.getOriginalFilename()));// 别问我存哪儿了，这只是一个例子。可以获得 context 然后存到 real path 底下去。
            model.addAttribute("msg", "upload succeed");
        } catch (Exception e) {
            model.addAttribute("msg", e.getMessage());
        }
        return "forward:/index.jsp";
    }
}