分离Controller与RequestMapping定义

分离Controller与RequestMapping定义

Spring在解析RequestMapping的注解的时候, 会从父类或者接口上读取RequestMapping注解的信息.

利用这种方法, 可以分离Controller和RequestMapping注解.

例如下面是一个常见的RESTful风格的Controller代码

@RestController
@RequestMapping("/user")
public class UserController {

  private final UserService userService;

  public UserController(UserService userService) {
    this.userService = userService;
  }

  @GetMapping("/{id}")
  public User get(@PathVariable("id") Long id) {
    return userService.findById(id);
  }

  @GetMapping
  public Page<User> page(@QuerydslPredicate Predicate predicate, Pageable pageable) {
    return userService.findByPage(predicate, pageable);
  }

  @PostMapping
  public User create(@RequestBody User user) {
    return userService.create(user);
  }

  @PutMapping("{id}")
  public User update(@PathVariable("id") Long id, @RequestBody User user) {
    return userService.update(id, user);
  }

  @DeleteMapping("{id}")
  public void delete(@PathVariable("id") Long id) {
    userService.deleteById(id);
  }
}

我们可以定义一个UserRequestMappingDefinition的接口, 方法与UserController一致.

public interface UserRequestMappingDefinition {
  @GetMapping("/{id}")
  User get(@PathVariable("id") Long id);

  @GetMapping
  Page<User> page(@QuerydslPredicate Predicate predicate, Pageable pageable);

  @PostMapping
  User create(@RequestBody User user);

  @PutMapping("{id}")
  User update(@PathVariable("id") Long id, @RequestBody User user);

  @DeleteMapping("{id}")
  void delete(@PathVariable("id") Long id);
}

然后让UserController实现该接口, 并去掉UserController上的注解

@RestController
@RequestMapping("/user")
public class UserController implements UserRequestMappingDefinition {

  private final UserService userService;

  public UserController(UserService userService) {
    this.userService = userService;
  }

  public User get(Long id) {
    return userService.findById(id);
  }

  public Page<User> page(Predicate predicate, Pageable pageable) {
    return userService.findByPage(predicate, pageable);
  }

  public User create(User user) {
    return userService.create(user);
  }

  public User update(Long id, User user) {
    return userService.update(id, user);
  }

  public void delete(Long id) {
    userService.deleteById(id);
  }
}

然而, 这又有什么用呢? 如果每个Controller都添加一个RequestMappingDefinition来定义, 无端增加了很多的类.

但是仔细观察, 你会发现, RESTful风格的Controller的注解基本上一样, 每个Controller上的方法中都要同样编写一样的注解来修饰方法和方法参数等, 我们只需利用上面的机制, 就可以省去重复定义注解的过程.

定义一个RequestMappingDefinition

public interface RequestMappingDefinition<T> {
  @GetMapping("/{id}")
  default T get(@PathVariable("id") Long id) {
    return null;
  }

  @GetMapping
  default Page<T> page(@QuerydslPredicate Predicate predicate, Pageable pageable) {
    return null;
  }

  @PostMapping
  default T create(@RequestBody T entity) {
    return null;
  }

  @PutMapping("{id}")
  default T update(@PathVariable("id") Long id, @RequestBody T entity) {
    return null;
  }

  @DeleteMapping("{id}")
  default void delete(@PathVariable("id") Long id) {}
}

UserController使用实现该接口并添加泛型

@RestController
@RequestMapping("user")
public class UserController implements RequestMappingDefinition<User> {
    // ...
}

其它Controller可以使用同样的方式

@RestController
@RequestMapping("role")
public class RoleController implements RequestMappingDefinition<Role>{
	// ...
}

这样就可以在RoleController内编写接口的逻辑, 但不需要重复编写注解, 其它Controller也是同理.

但是这会带来一个问题, 直接实现接口会引入RequestMappingDefinition定义的所有接口, 即RoleController中你没有重写实现的接口方法也会被注解为Handler. 但是很有可能对于某些Controller我们不需要RequestMappingDefinition中的部分接口.

可以用下面的方法解决.

定义一个DefaultController

public class DefaultController<T> {
  public T get(Long id) {
    throw new UnsupportedOperationException();
  }

  public Page<T> page(Predicate predicate, Pageable pageable) {

    throw new UnsupportedOperationException();
  }

  public T create(T entity) {
    throw new UnsupportedOperationException();
  }

  public T update(Long id, T entity) {
    throw new UnsupportedOperationException();
  }

  public void delete(Long id) {
    throw new UnsupportedOperationException();
  }
}

让RoleController继承他.

@RestController
@RequestMapping("role")
public class RoleController extends DefaultController<Role>
    implements RequestMappingDefinition<Role> {
    // ...
}

此时, 没有在RoleController中重写的方法就不会再被注册为Handler.