为实体创建主从页面，如何链接它们以及选择哪个 bean 范围 [英] Creating master-detail pages for entities, how to link them and which bean scope to choose

问题描述

我已经开始学习 JSF，但遗憾的是，大多数教程只提供登录或注册部分.

I have started learning JSF, but sadly most tutorials out there present only a log in or a register section.

你能指出一些更深入的例子吗?我感兴趣的一件事是显示产品列表的页面.我在页面 home 上并按下页面 products 以便我可以看到最新添加的 products.每次我访问该页面时，都会根据数据库中的最新条目创建产品列表.我该如何处理?

Can you point me to some more in depth examples? One thing I'm interested in is a page presenting a list of products. I'm on page home and I press on page products so that I can see the latest products added. And every time I visit the page, the product list will be created from the latest entries in the database. How can I handle this?

解决这个问题的一种方法是创建一个会话范围的托管 bean，我将在其中放置通过其他托管 bean 更新的不同实体.我在一些教程中发现了这种方法，但看起来相当困难和笨拙.

One way to solve this would be to create a session scoped managed bean in which I would place different entities updated through other managed beans. I found this kind of approach in some tutorials, but it seems quite difficult and clumsy.

哪种方法是解决此类问题的最佳方法?两页主从用户界面中会话范围的正确用法是什么?

Which would be the best approach to solve a thing like this? What is the correct usage of session scope in two-page master-detail user interface?

推荐答案

会话作用域的正确用法是什么

仅将其用于会话范围的数据，仅此而已.例如，登录用户、其设置、选择的语言等.

Use it for session scoped data only, nothing else. For example, the logged-in user, its settings, the chosen language, etcetera.

• 如何选择合适的 bean 范围?

每次我访问该页面时，都会根据数据库中的最新条目创建产品列表.我该如何处理?

通常您使用请求或查看范围.列表的加载应该在 @PostConstruct 方法中发生.如果页面不包含任何 ，那么请求范围就可以了.无论如何，当没有 时，视图作用域 bean 的行为就像一个作用域请求.

Typically you use the request or view scope for it. Loading of the list should happen in a @PostConstruct method. If the page doesn't contain any <h:form>, then the request scope is fine. A view scoped bean would behave like a request scoped when there's no <h:form> anyway.

所有查看产品"和编辑产品"仅检索信息(即幂等)的链接/按钮应该只是简单的 GET /其中，您通过 将实体标识符作为请求参数传递.

All "view product" and "edit product" links/buttons which just retrieve information (i.e. idempotent) whould be just plain GET <h:link> / <h:button> wherein you pass the entity identifier as a request parameter by <f:param>.

所有删除产品"和保存产品"将操作信息(即非幂等)的链接/按钮应该通过 /(您不希望它们可添加书签/搜索机器人可索引！).这反过来又需要 .为了保留验证和 ajax 请求的数据(这样你就不需要在每个请求上重新加载/预初始化实体)，bean 最好是视图范围的.

All "delete product" and "save product" links/buttons which will manipulate information (i.e. non-idempotent) should perform POST by <h:commandLink>/<h:commandButton> (you don't want them to be bookmarkable/searchbot-indexable!). This in turn requires a <h:form>. In order to preserve the data for validations and ajax requests (so that you don't need to reload/preinitialize the entity on every request), the bean should preferably be view scoped.

请注意，基本上每个视图都应该有一个单独的 bean，并且还请注意，这些 bean 不一定需要相互引用.

Note that you should basically have a separate bean for each view and also note that those beans doesn't necessarily need to reference each other.

因此，鉴于此产品"实体:

So, given this "product" entity:

@Entity
public class Product {

    @Id
    private Long id;
    private String name;
    private String description;

    // ...
}

而这个产品服务"EJB:

And this "product service" EJB:

@Stateless
public class ProductService {

    @PersistenceContext
    private EntityManager em;

    public Product find(Long id) {
        return em.find(Product.class, id);
    }

    public List<Product> list() {
        return em.createQuery("SELECT p FROM Product p", Product.class).getResultList();
    }

    public void create(Product product) {
        em.persist(product);
    }

    public void update(Product product) {
        em.merge(product);
    }

    public void delete(Product product) {
        em.remove(em.contains(product) ? product : em.merge(product));
    }

    // ...
}

您可以拥有此查看产品"；在 /products.xhtml 上:

You can have this "view products" on /products.xhtml:

<h:dataTable value="#{viewProducts.products}" var="product">
    <h:column>#{product.id}</h:column>
    <h:column>#{product.name}</h:column>
    <h:column>#{product.description}</h:column>
    <h:column>
        <h:link value="Edit" outcome="/products/edit">
            <f:param name="id" value="#{product.id}" />
        </h:link>
    </h:column>
</h:dataTable>

@Named
@RequestScoped
public class ViewProducts {

    private List<Product> products; // +getter

    @EJB
    private ProductService productService;

    @PostConstruct
    public void init() {
        products = productService.list();
    }

    // ...
}

你可以拥有这个编辑产品"在 /products/edit.xhtml 上:

And you can have this "edit product" on /products/edit.xhtml:

<f:metadata>
    <f:viewParam name="id" value="#{editProduct.product}" 
        converter="#{productConverter}" converterMessage="Unknown product, please use a link from within the system."
        required="true" requiredMessage="Bad request, please use a link from within the system."
    />
</f:metadata>

<h:messages />

<h:form rendered="#{not empty editProduct.product}>
    <h:inputText value="#{editProduct.product.name}" />
    <h:inputTextarea value="#{editProduct.product.description}" />
    ...
    <h:commandButton value="save" action="#{editProduct.save}" />
</h:form>

@Named
@ViewScoped
public class EditProduct {

    private Product product; // +getter +setter

    @EJB
    private ProductService productService;

    public String save() {
        productService.update(product);
        return "/products?faces-redirect=true";
    }

    // ...
}

以及用于编辑产品"的 的转换器:

And this converter for <f:viewParam> of "edit product":

@Named
@RequestScoped
public class ProductConverter implements Converter {

    @EJB
    private ProductService productService;

    @Override
    public Object getAsObject(FacesContext context, UIComponent component, String value) {
        if (value == null || value.isEmpty()) {
            return null;
        }

        try {
            Long id = Long.valueOf(value);
            return productService.find(id);
        } catch (NumberFormatException e) {
            throw new ConverterException("The value is not a valid Product ID: " + value, e);
        }
    }

    @Override    
    public String getAsString(FacesContext context, UIComponent component, Object value) {        
        if (value == null) {
            return "";
        }

        if (value instanceof Product) {
            Long id = ((Product) value).getId();
            return (id != null) ? String.valueOf(id) : null;
        } else {
            throw new ConverterException("The value is not a valid Product instance: " + value);
        }
    }

}

您甚至可以使用通用转换器，这在实现转换器中进行了解释带有 Java 泛型的实体.

You can even use a generic converter, this is explained in Implement converters for entities with Java Generics.

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