如何在静态上下文中使用泛型类和特定对象？

问题描述

我会尽力解释。

我使用Play Framework 2，我会做很多CRUD操作。其中一些将是identitcal，所以我想KISS和DRY所以起初我在考虑一个包含列表的抽象类，详细信息，创建，更新和删除方法，使用通用对象，并通过指定要使用的对象（Model& Form）扩展此类：

I use Play Framework 2, and I will do a lot of CRUD actions. Some of them will be identitcal, so I'd like to KISS and DRY so at first I was thinking about an abstract class containing the list, details, create, update and delete methods, with generic object, and extend this class by specifying which object to use (Model & Form) :

public abstract class CrudController extends Controller {
    protected static Model.Finder<Long, Model> finder = null;
    protected static Form<Model> form = null;

    public static Result list() {
        // some code here
    }

    public static Result details(Long id) {
        // some code here
    }

    public static Result create() {
        // some code here
    }

    public static Result update(Long id) {
        // some code here
    }

    public static Result delete(Long id) {
        // some code here
    }
}

一个将使用CRUD的类：

And a class that will use CRUD :

public class Cities extends CrudController {
    protected static Model.Finder<Long, City> finder = City.find;
    protected static Form<City> form = form(City.class);

    // I can override a method in order to change it's behavior :
    public static Result list() {
        // some different code here, like adding some where condition
    }
}

如果我不在静态环境中，这将有效。

This would work if I wasn't in a static context.

但是既然如此，我该怎么办？

But since it's the case, how can I do ?

推荐答案

这可以使用委托来实现：定义一个包含CRUD动作逻辑的常规Java类：

This can be achieved using delegation: define a regular Java class containing the CRUD actions logic:

public class Crud<T extends Model> {

    private final Model.Finder<Long, T> find;
    private final Form<T> form;

    public Crud(Model.Finder<Long, T> find, Form<T> form) {
        this.find = find;
        this.form = form;
    }

    public Result list() {
        return ok(Json.toJson(find.all()));
    }

    public Result create() {
        Form<T> createForm = form.bindFromRequest();
        if (createForm.hasErrors()) {
            return badRequest();
        } else {
            createForm.get().save();
            return ok();
        }
    }

    public Result read(Long id) {
        T t = find.byId(id);
        if (t == null) {
            return notFound();
        }
        return ok(Json.toJson(t));
    }

    // … same for update and delete
}

然后你可以定义一个Play控制器，其静态字段包含 Crud< City> 的实例：

Then you can define a Play controller having a static field containing an instance of Crud<City>:

public class Cities extends Controller {
    public final static Crud<City> crud = new Crud<City>(City.find, form(City.class));
}

你差不多完成了：你只需要定义路线Crud行动：

And you’re almost done: you just need to define the routes for the Crud actions:

GET     /                     controllers.Cities.crud.list()
POST    /                     controllers.Cities.crud.create()
GET     /:id                  controllers.Cities.crud.read(id: Long)

注意：此示例为brevety生成JSON响应，但可以呈现HTML模板。但是，由于Play 2模板是静态类型的，因此您需要将所有这些模板作为 Crud 类的参数传递。

Note: this example produces JSON responses for brevety but it’s possible to render HTML templates. However, since Play 2 templates are statically typed you’ll need to pass all of them as parameters of the Crud class.

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