从JSF datatable中的MySQL数据库显示图像

本文介绍了从JSF datatable中的MySQL数据库显示图像的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我有一个MySQL数据库存储图像在一个 blob 列。我想在PrimeFaces < p：dataTable> 中显示它们。如何实现？

解决方案

您可以使用显示存储在字节[ ] ，而不管 byte [] source（DB，磁盘文件系统，网络等）。最简单的例子是：

 < p：graphicImage value =＃{bean.streamedContent} />

它引用了一个 StreamedContent 属性。 p>

然而，这是一个陷阱，特别是在诸如数据表的迭代组件中使用时：getter方法将被调用两次;第一次由JSF自己生成< img src> 的URL，第二次由webbrowser在需要根据< img src> 。为了有效率，您不应该在第一个getter调用中击中DB。另外，要参数化getter方法调用，以便您可以使用通用特定图像ID的通用方法，您应该使用< f：param> （请请注意，传递方法参数的EL 2.2功能将无法正常工作，因为这不会导致< img src> ！的URL）。

总结说，这应该是：

  p：dataTable value =＃{bean.items}var =item> 
< p：column> 
< p：graphicImage value =＃{imageStreamer.image}> 
< f：param name =idvalue =＃{item.imageId}/> 
< / p：graphicImage> 
< / p：column> 
< / p：dataTable>

＃{item.imageId} 返回数据库（主键）中图像的唯一idenfitier，因此不 byte [] 内容。 ＃{imageStreamer} 是一个应用范围的bean，如下所示：

  @ManagedBean 
 @ApplicationScoped 
 public class ImageStreamer {
 
 @EJB 
 private ImageService service; 
 
 public StreamedContent getImage（）throws IOException {
 FacesContext context = FacesContext.getCurrentInstance（）; 
 
 if（context.getCurrentPhaseId（）== PhaseId.RENDER_RESPONSE）{
 //所以，我们渲染HTML。返回一个存根StreamedContent，以便它将生成正确的URL。 
返回新的DefaultStreamedContent（）; 
} else {
 //所以浏览器正在请求图像。用图像字节返回一个真正的StreamedContent。 
 String imageId = context.getExternalContext（）。getRequestParameterMap（）。get（imageId）; 
 Image image = imageService.find（Long.valueOf（imageId））; 
返回新的DefaultStreamedContent（新的ByteArrayInputStream（image.getBytes（）））; 
} 
} 
 
}

code> Image class在这个特殊的例子中只有一个 @Entity 与 @Lob 在字节属性（因为你使用JSF，我的cource假定你正在使用JPA与数据库进行交互）。

  @Entity 
 public class Image {
 
 @Id 
 @ GeneratedValue（strategy = IDENTITY）//根据您的DB，当然。 
 private Long id; 
 
 @Lob 
 private byte [] bytes; 
 
 // ... 
}

ImageService 只是一个标准的 @Stateless EJB，没有什么特别的看法：

  @Stateless 
 public class ImageService {
 
 @PersistenceContext 
 private EntityManager em; 
 
 public Image find（Long id）{
 return em.find（Image.class，id）; 
} 
 
}

另请参见：

I have MySQL database which stores images in a blob column. I would like to show them in a PrimeFaces <p:dataTable>. How can I achieve this?

解决方案

You can use <p:graphicImage> to display images stored in a byte[], regardless of the byte[] source (DB, disk file system, network, etc). Simplest example is:

<p:graphicImage value="#{bean.streamedContent}" />

which refers a StreamedContent property.

This has however a pitfall, particularly when used in an iterating component such as a data table: the getter method will be invoked twice; the first time by JSF itself to generate the URL for <img src> and the second time by webbrowser when it needs to download the image content based on the URL in <img src>. To be efficient, you should not be hitting the DB in the first getter call. Also, to parameterize the getter method call so that you can use a generic method wherein you pass a specific image ID, you should be using a <f:param> (please note that EL 2.2 feature of passing method arguments won't work at all as this doesn't end up in URL of <img src>!).

Summarized, this should do:

<p:dataTable value="#{bean.items}" var="item">
    <p:column>
        <p:graphicImage value="#{imageStreamer.image}">
            <f:param name="id" value="#{item.imageId}" />
        </p:graphicImage>
    </p:column>
</p:dataTable>

The #{item.imageId} obviously returns the unique idenfitier of the image in the DB (the primary key) and thus not the byte[] content. The #{imageStreamer} is an application scoped bean which look like this:

@ManagedBean
@ApplicationScoped
public class ImageStreamer {

    @EJB
    private ImageService service;

    public StreamedContent getImage() throws IOException {
        FacesContext context = FacesContext.getCurrentInstance();

        if (context.getCurrentPhaseId() == PhaseId.RENDER_RESPONSE) {
            // So, we're rendering the HTML. Return a stub StreamedContent so that it will generate right URL.
            return new DefaultStreamedContent();
        } else {
            // So, browser is requesting the image. Return a real StreamedContent with the image bytes.
            String imageId = context.getExternalContext().getRequestParameterMap().get("imageId");
            Image image = imageService.find(Long.valueOf(imageId));
            return new DefaultStreamedContent(new ByteArrayInputStream(image.getBytes()));
        }
    }

}

The Image class is in this particular example just an @Entity with a @Lob on bytes property (as you're using JSF, I of cource assume that you're using JPA to interact with the DB).

@Entity
public class Image {

    @Id
    @GeneratedValue(strategy = IDENTITY) // Depending on your DB, of course.
    private Long id;

    @Lob
    private byte[] bytes;

    // ...
}

The ImageService is just a standard @Stateless EJB, nothing special to see here:

@Stateless
public class ImageService {

    @PersistenceContext
    private EntityManager em;

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

}

mysql数据库