将Spring实战第5版中Spring HATEOAS部分代码迁移到Spring HATEOAS 1.0

       最近在阅读Spring实战第五版中文版，书中第6章关于Spring HATEOAS部分代码使用的是Spring HATEOAS 0.25的版本，而最新的Spring HATEOAS 1.0对旧版的API做了升级，导致在使用新版Spring Boot（截至文章发布日最新的Spring Boot版本为2.2.4）加载的Spring HATEOAS 1.0.3无法正常运行书中代码，所以我决定在此对书中的代码进行迁移升级。

在线阅读书中这一部分：https://potoyang.gitbook.io/spring-in-action-v5/di-6-zhang-chuang-jian-rest-fu-wu/6.2-qi-yong-chao-mei-ti

Spring HATEOAS 1.0 版本的变化

封装结构的最大变化是通过引入超媒体类型注册API来实现的，以支持Spring HATEOAS中的其他媒体类型。这导致客户端API和服务器API（分别命名的包）以及包中的媒体类型实现的明确分离 mediatype。

最大的变化就是将原来的资源表示为模型，具体变化如下。

在ResourceSupport/ Resource/ Resources/ PagedResources组类从来没有真正感受到适当命名。毕竟，这些类型实际上并不表示资源，而是表示模型，可以通过超媒体信息和提供的内容来丰富它们。这是新名称映射到旧名称的方式：

• ResourceSupport 就是现在 RepresentationModel

• Resource 就是现在 EntityModel

• Resources 就是现在 CollectionModel

• PagedResources 就是现在 PagedModel

因此，ResourceAssembler已被重命名为RepresentationModelAssembler和及其方法toResource(…)，并分别toResources(…)被重命名为toModel(…)和toCollectionModel(…)。名称更改也反映在中包含的类中TypeReferences。

• RepresentationModel.getLinks()现在公开了一个Links实例（通过List<Link>），该实例公开了其他API，以Links使用各种策略来连接和合并不同的实例。同样，它已经变成了自绑定的泛型类型，以允许向实例添加链接的方法返回实例本身。

• 该LinkDiscovererAPI已移动到client包。

• 在LinkBuilder和EntityLinksAPI已经被移到了server包。

• ControllerLinkBuilder已移入server.mvc，不推荐使用替换WebMvcLinkBuilder。

• RelProvider已重命名为LinkRelationProvider并返回LinkRelation实例，而不是String。

• VndError已移至mediatype.vnderror套件。

另外注意 ResourceProcessor 接口被 RepresentationModelProcessor 取代

更多变化请参考Spring HATEOAS文档：https://spring.io/projects/spring-hateoas

代码迁移升级

书中程序清单6.4　为资源添加超链接

    @GetMapping("/recent")
    public CollectionModel<EntityModel<Taco>> recentTacos() {
        PageRequest page = PageRequest.of(
                0, 12, Sort.by("createdAt").descending());

        List<Taco> tacos = tacoRepo.findAll(page).getContent();
        CollectionModel<EntityModel<Taco>> recentResources = CollectionModel.wrap(tacos);

        recentResources.add(
                new Link("http://localhost:8080/design/recent", "recents"));
        return recentResources;
    }

消除URL硬编码

    @GetMapping("/recent")
    public CollectionModel<EntityModel<Taco>> recentTacos() {
        PageRequest page = PageRequest.of(
                0, 12, Sort.by("createdAt").descending());

        List<Taco> tacos = tacoRepo.findAll(page).getContent();
        CollectionModel<EntityModel<Taco>> recentResources = CollectionModel.wrap(tacos);

        recentResources.add(
                linkTo(methodOn(DesignTacoController.class).recentTacos()).withRel("recents"));
        return recentResources;
    }

程序清单6.5　能够携带领域数据和超链接列表taco资源

public class TacoResource extends RepresentationModel<TacoResource> {

    @Getter
    private String name;

    @Getter
    private Date createdAt;

    @Getter
    private List<Ingredient> ingredients;

    public TacoResource(Taco taco) {
        this.name = taco.getName();
        this.createdAt = taco.getCreatedAt();
        this.ingredients = taco.getIngredients();
    }
}

程序清单6.6　装配taco资源的资源装配器

public class TacoResourceAssembler extends RepresentationModelAssemblerSupport<Taco, TacoResource> {
    /**
     * Creates a new {@link RepresentationModelAssemblerSupport} using the given controller class and resource type.
     *
     * @param controllerClass DesignTacoController {@literal DesignTacoController}.
     * @param resourceType    TacoResource {@literal TacoResource}.
     */
    public TacoResourceAssembler(Class<?> controllerClass, Class<TacoResource> resourceType) {
        super(controllerClass, resourceType);
    }

    @Override
    protected TacoResource instantiateModel(Taco taco) {
        return new TacoResource(taco);
    }


    @Override
    public TacoResource toModel(Taco entity) {
        return createModelWithId(entity.getId(), entity);
    }
}

之后对recentTacos()的调整

@GetMapping("/recentNew")
    public CollectionModel<TacoResource> recentTacos() {
        PageRequest page = PageRequest.of(
                0, 12, Sort.by("createdAt").descending());
        List<Taco> tacos = tacoRepo.findAll(page).getContent();

        CollectionModel<TacoResource> tacoResources =
                new TacoResourceAssembler(DesignTacoController.class, TacoResource.class).toCollectionModel(tacos);

        tacoResources.add(linkTo(methodOn(DesignTacoController.class)
                .recentTacos())
                        .withRel("recents"));
        return tacoResources;
    }

创建 IngredientResource 对象

@Data
public class IngredientResource extends RepresentationModel<IngredientResource> {
    public IngredientResource(Ingredient ingredient) {
        this.name = ingredient.getName();
        this.type = ingredient.getType();
    }

    private final String name;
    private final Ingredient.Type type;
}

IngredientResourceAssembler 对象

public class IngredientResourceAssembler extends RepresentationModelAssemblerSupport<Ingredient, IngredientResource> {
    /**
     * Creates a new {@link RepresentationModelAssemblerSupport} using the given controller class and resource type.
     *
     * @param controllerClass IngredientController {@literal IngredientController}.
     * @param resourceType    IngredientResource {@literal IngredientResource}.
     */
    public IngredientResourceAssembler(Class<?> controllerClass, Class<IngredientResource> resourceType) {
        super(controllerClass, resourceType);
    }

    @Override
    protected IngredientResource instantiateModel(Ingredient entity) {
        return new IngredientResource(entity);
    }

    @Override
    public IngredientResource toModel(Ingredient entity) {
        return createModelWithId(entity.getId(), entity);
    }
}

对 TacoResource 对象的修改

public class TacoResource extends RepresentationModel<TacoResource> {
    private static final IngredientResourceAssembler
            ingredientAssembler = new IngredientResourceAssembler(IngredientController.class, IngredientResource.class);

    @Getter
    private String name;

    @Getter
    private Date createdAt;

    @Getter
    private CollectionModel<IngredientResource> ingredients;

    public TacoResource(Taco taco) {
        this.name = taco.getName();
        this.createdAt = taco.getCreatedAt();
        this.ingredients = ingredientAssembler.toCollectionModel(taco.getIngredients());

    }
}

 程序清单6.7

@RepositoryRestController
public class RecentTacosController {
    private TacoRepository tacoRepo;

    public RecentTacosController(TacoRepository tacoRepo) {
        this.tacoRepo = tacoRepo;
    }

    /**
     * 虽然@GetMapping映射到了“/tacos/recent”路径，但是类级别的@Repository RestController注解会确保这个路径添加
     * Spring Data REST的基础路径作为前缀。按照我们的配置，recentTacos()方法将会处理针对“/api/tacos/recent”的GET请求。
     * */
    @GetMapping(path="/tacos/recent", produces="application/hal+json")
    public ResponseEntity<CollectionModel<TacoResource>> recentTacos() {
        PageRequest page = PageRequest.of(
                0, 12, Sort.by("createdAt").descending());
        List<Taco> tacos = tacoRepo.findAll(page).getContent();

        CollectionModel<TacoResource> tacoResources =
                new TacoResourceAssembler(DesignTacoController.class, TacoResource.class).toCollectionModel(tacos);

        tacoResources.add(
                linkTo(methodOn(RecentTacosController.class).recentTacos())
                        .withRel("recents"));
        return new ResponseEntity<>(tacoResources, HttpStatus.OK);
    }

}

程序清单6.8　为Spring Data REST端点添加自定义的链接

    @Bean
    public RepresentationModelProcessor<PagedModel<EntityModel<Taco>>> tacoProcessor(EntityLinks links) {

        return new RepresentationModelProcessor<PagedModel<EntityModel<Taco>>>() {
            @Override
            public PagedModel<EntityModel<Taco>> process(PagedModel<EntityModel<Taco>> resource) {
                resource.add(
                        links.linkFor(Taco.class)
                                .slash("recent")
                                .withRel("recents"));
                return resource;
            }
        };
    }

另一种写法

如果你觉得写使用资源装配器有点麻烦，那么你还可以采用这种方法。

    @GetMapping("/employees")
    public ResponseEntity<CollectionModel<EntityModel<Taco>>> findAll() {
        PageRequest page = PageRequest.of(
                0, 12, Sort.by("createdAt").descending());
        List<EntityModel<Taco>> employees = StreamSupport.stream(tacoRepo.findAll(page).spliterator(), false)
                .map(employee -> new EntityModel<>(employee,
                        linkTo(methodOn(DesignTacoController.class).findOne(employee.getId())).withSelfRel(),
                        linkTo(methodOn(DesignTacoController.class).findAll()).withRel("employees")))
                .collect(Collectors.toList());

        return ResponseEntity.ok(
                new CollectionModel<>(employees,
                        linkTo(methodOn(DesignTacoController.class).findAll()).withSelfRel()));
    }

@GetMapping("/employees/{id}")
    public ResponseEntity<EntityModel<Taco>> findOne(@PathVariable long id) {

        return tacoRepo.findById(id)
                .map(employee -> new EntityModel<>(employee,
                        linkTo(methodOn(DesignTacoController.class).findOne(employee.getId())).withSelfRel(), //
                        linkTo(methodOn(DesignTacoController.class).findAll()).withRel("employees"))) //
                .map(ResponseEntity::ok)
                .orElse(ResponseEntity.notFound().build());
    }

参考来源：https://github.com/spring-projects/spring-hateoas-examples/tree/master/simplified

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