Question

我有一个JSF页面，我想要显示图像。图像作为blob存储在数据库中。该实体看起来像这样：

@Entity
public class Player
{
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Long            id;
    @Lob
    private byte[]          pictureData;
    @Transient
    private StreamedContent streamedPicture;

    public StreamedContent getStreamedPicture()
    {
        if (streamedPicture == null && pictureData != null)
        {
            try
            {
                ByteArrayOutputStream os = new ByteArrayOutputStream();
                os.write(pictureData);
                streamedPicture = new DefaultStreamedContent(
                                                    new ByteArrayInputStream(
                                                                                os.toByteArray()),
                                                    "image/png");
            }
            catch (FileNotFoundException e)
            {}
            catch (IOException e)
            {}
        }
        return streamedPicture;
    }
}

JSF页面是这样的：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"
    xmlns:h="http://java.sun.com/jsf/html"
    xmlns:f="http://java.sun.com/jsf/core"
    xmlns:ui="http://java.sun.com/jsf/facelets"
    xmlns:p="http://primefaces.org/ui">

<h:head></h:head>
<body>
    <ui:repeat var="player" value="#{playerbean.cachedPlayers}">
        <h:outputText value="#{player.id}" />
        <p:graphicImage value="#{player.streamedPicture}" rendered="#{player.streamedPicture != null}"/>
    </ui:repeat>
</body>
</html>

我调用的bean看起来像这样：

@ManagedBean(name = "playerbean")
@SessionScoped
public class PlayerBean
        implements Serializable
{
    @EJB
    private PlayerManager   playerManager;
    private List<Player>    cachedPlayers;

    public List<Player> getCachedPlayers()
    {
        if (cachedPlayers == null)
        {
            cachedPlayers = playerManager.getAll();
        }
        return cachedPlayers;
    }
}

在调试时，我在方法PrimeResourceHandler中的handleResourceRequest()中设置了一个断点。我正在查看的PrimeResourceHandler代码包含：

try {
    String dynamicContentEL = (String) session.get(dynamicContentId);
    ELContext eLContext = context.getELContext();
    ValueExpression ve = context.getApplication().getExpressionFactory().createValueExpression(context.getELContext(), dynamicContentEL, StreamedContent.class);
    StreamedContent content = (StreamedContent) ve.getValue(eLContext);
    HttpServletResponse response = (HttpServletResponse) context.getExternalContext().getResponse();

    response.setContentType(content.getContentType());

    byte[] buffer = new byte[2048];

    int length;
    InputStream inputStream = content.getStream();
    while ((length = (inputStream.read(buffer))) >= 0) {
        response.getOutputStream().write(buffer, 0, length);
    }

    response.setStatus(200);
    response.getOutputStream().flush();
    context.responseComplete();

} catch(Exception e) {
    logger.log(Level.SEVERE, "Error in streaming dynamic resource.");
} finally {
    session.remove(dynamicContentId);
}

传递行StreamedContent content = (StreamedContent) ve.getValue(eLContext); content时似乎为空。这当然会导致NullPointerException。但是在JSF页面中，我告诉元素如果值为null则不呈现。

Answer 1

<p:graphicImage>组件不能具有指向SessionScoped bean中的托管属性的value属性。该值必须设置为RequestScoped bean。

这是因为image/jpeg内容类型HTTP响应的HTTP请求本质上是无状态的。浏览器将对JSF页面内容进行初始请求，然后对于呈现的每个HTML <img>标记，它将对每个<img>标记的动态生成的URL进行单独请求以获取这些内容。在有状态的环境中获取图像并没有多大意义。

<p：graphicimage>不渲染图像</p：graphicimage>

1 个答案: