Spring Hateoas ControllerLinkBuilder 添加空字段 [英] Spring Hateoas ControllerLinkBuilder adds null fields

问题描述

我正在学习有关 Spring REST 的教程，并尝试将 HATEOAS 链接添加到我的控制器结果中.

我有一个简单的 User 类和一个 CRUD 控制器.

class 用户 {私有整数 ID；私人字符串名称；私人 LocalDate 生日；//和 getter/setter}

服务:

@Component类用户服务{私有静态列表<用户>用户 = 新的 ArrayList();列出<用户>查找全部(){返回 Collections.unmodifiableList(users);}公共可选<用户>findById(int id) {返回 users.stream().filter(u -> u.getId() == id).findFirst();}//当然还有添加和删除方法，但这里不重要}

除了在我的控制器中，一切正常，我想将所有用户列表中的链接添加到单个用户:

import static org.springframework.hateoas.mvc.ControllerLinkBuilder.linkTo;导入静态 org.springframework.hateoas.mvc.ControllerLinkBuilder.methodOn;@RestController公共类用户控制器{@自动连线私人用户服务用户服务；@GetMapping("/用户")公共列表<资源<用户>>getAllUsers() {列表<资源<用户>>userResources = userService.findAll().stream().map(u -> new Resource<>(u, linkToSingleUser(u))).collect(Collectors.toList());返回用户资源；}链接 linkToSingleUser(用户用户){返回链接(methodOn(UserController.class).getById(user.getId())).withSelfRel();}

这样对于结果列表中的每个用户，都会添加一个指向用户本身的链接.

链接本身创建得很好，但生成的 JSON 中有多余的条目:

<预><代码>[{身份证":1，"name": "亚当","生日": "2018-04-02",链接":[{"rel": "自我","href": "http://localhost:8080/users/1",hreflang":空，媒体":空，标题":空，类型":空，弃用":空}]}]

具有空值的字段(hreflang、media 等)来自哪里，为什么要添加它们?有没有办法摆脱它们?

在建立指向所有用户列表的链接时它们不会出现:

@GetMapping("/users/{id}")公共资源<用户>getById(@PathVariable("id") int id) {最终用户用户 = userService.findById(id).orElseThrow(() -> new UserNotFoundException(id));链接 linkToAll = linkTo(methodOn(UserController.class).getAllUsers()).withRel("所有用户");返回新资源<用户>(用户，linkToAll)；}

解决方案

为了进一步参考，以防其他人偶然发现这一点，我想通了:我在 application.properties 中添加了一个条目，即

spring.jackson.default-property-inclusion=NON_NULL

为什么这对于 Link 对象是必要的，但对于 User 我不知道(也没有深入研究).

I'm following a tutorial on Spring REST and am trying to add HATEOAS links to my Controller results.

I have a simple User class and a CRUD controller for it.

class User {
    private int id;
    private String name;
    private LocalDate birthdate;
    // and getters/setters
}

Service:

@Component
class UserService {
    private static List<User> users = new ArrayList<>();
    List<User> findAll() {
        return Collections.unmodifiableList(users);
    }
    public Optional<User> findById(int id) {
        return users.stream().filter(u -> u.getId() == id).findFirst();
    }
    // and add and delete methods of course, but not important here
}

Everything works fine except in my Controller, I want to add links from the all user list to the single users:

import static org.springframework.hateoas.mvc.ControllerLinkBuilder.linkTo;
import static org.springframework.hateoas.mvc.ControllerLinkBuilder.methodOn;

@RestController
public class UserController {
    @Autowired
    private UserService userService;
    @GetMapping("/users")
    public List<Resource<User>> getAllUsers() {
        List<Resource<User>> userResources = userService.findAll().stream()
            .map(u -> new Resource<>(u, linkToSingleUser(u)))
            .collect(Collectors.toList());
        return userResources;
    }
    Link linkToSingleUser(User user) {
        return linkTo(methodOn(UserController.class)
                      .getById(user.getId()))
                      .withSelfRel();
    }

so that for every User in the result list, a link to the user itself is added.

The link itself is created fine, but there are superfluous entries in the resulting JSON:

[
    {
        "id": 1,
        "name": "Adam",
        "birthdate": "2018-04-02",
        "links": [
            {
                "rel": "self",
                "href": "http://localhost:8080/users/1",
                "hreflang": null,
                "media": null,
                "title": null,
                "type": null,
                "deprecation": null
            }
        ]
    }
]

Where do the fields with null value (hreflang, media etc) come from and why are they added? Is there a way to get rid of them?

They do not appear when building a link to the all users list:

@GetMapping("/users/{id}")
public Resource<User> getById(@PathVariable("id") int id) {
    final User user = userService.findById(id)
                                 .orElseThrow(() -> new UserNotFoundException(id));
    Link linkToAll = linkTo(methodOn(UserController.class)
                            .getAllUsers())
                            .withRel("all-users");
    return new Resource<User>(user, linkToAll);
}

解决方案

For further reference in case anyone else stumbles upon this and I figured it out: I added an entry to application.properties, namely

spring.jackson.default-property-inclusion=NON_NULL

Why this was necessary for the Link objects, but not for the User I don't know (and haven't diven in deeper).

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