<p:graphicImage>无法渲染图像

Question

<p:graphicImage>无法渲染图像

3

我有一个JSF页面，想要显示一张图片。这张图片以blob的形式存储在数据库中。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页面如下所示：

<!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>

我所称之为的bean长这样：

@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;
    }
}

调试时，我在PrimeResourceHandler的handleResourceRequest()方法中设置了一个断点。我正在查看的PrimeResourceHandler的代码包含以下内容：

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);
}

当传递线路 StreamedContent content = (StreamedContent) ve.getValue(eLContext); 时，content 看起来是空的。这当然会导致 NullPointerException。然而，在 JSF 页面中，我告诉元素如果值为空，则不要渲染。

- siebz0r

1

可能是重复的问题：如何在ui:repeat中使用DefaultStreamedContent和<p:graphicImage>？ - BalusC

1个回答

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- maple_shaft · Accepted Answer

<p:graphicImage>组件不能拥有指向SessionScoped bean中托管属性的value属性。必须将值设置为RequestScoped bean。

原因是对于图像/ jpeg内容类型HTTP响应的HTTP请求本质上是无状态的。浏览器将首先请求JSF页面内容，然后对于呈现的每个HTML <img>标记，它都会向动态生成的每个<img>标记的URL发送单独的请求来获取它们。在有状态的上下文中获取图像没有多大意义。