java Primefaces文件下载仅在使用默认构造函数时有效

Question

我使用HttpURLConnection下载一个文件，然后我想让用户选择下载该文件。我使用这个例子http://www.primefaces.org/showcase/ui/file/download.xhtml

我的问题是，如果我通过默认构造函数下载硬编码文件（如示例中所述），一切都正常。但是，如果我将文件名传递给接受param的构造函数，就会得到一个空指针

下面是包含两个构造函数的代码（只有包含硬编码文件的默认构造函数有效）

@ManagedBean

public class FileDownloadView {

    private StreamedContent file;
    private InputStream stream;
    private String fileName;

    public String getFileName() {
        return fileName;
    }


    public FileDownloadView() {
        InputStream stream = FacesContext.getCurrentInstance().getExternalContext().getResourceAsStream("/tmp/1kb.txt");
        file = new DefaultStreamedContent(stream, "text/plain", "text.txt");
        System.out.println("fileName......." + "test.txt");

    }

    public  FileDownloadView(String fileName) {
        this.fileName = fileName;

        InputStream stream = FacesContext.getCurrentInstance().getExternalContext().getResourceAsStream("/tmp/" + fileName);
        this.fileName = fileName;
        file = new DefaultStreamedContent(stream, "text/plain", fileName);
        System.out.println("fileName......." + file.getName());

    }

    public StreamedContent getFile() {
        System.out.println("file  "+file.getName());

        return file;
    }
}

这是我如何提取文件的

     <p:commandButton value="Download" ajax="false" onclick="PrimeFaces.monitorDownload(start, stop);" icon="ui-icon-arrowthick-1-s">
                <p:fileDownload value="#{fileDownloadView.file}" />
            </p:commandButton>

            <p:outputLabel value="#{fileDownloadView.fileName}"/>
        </h:form>
        <script type="text/javascript">
                                       function start() {
                                           PF('statusDialog').show();
                                       }

                                       function stop() {
                                           PF('statusDialog').hide();
                                       }
        </script>

Answer 1

你可以试试这个

@ManagedBean
public class FileDownloadView {

private StreamedContent file;
private InputStream stream;
private String fileName = "error";

public String getFileName() {
    return fileName;
}
public void setFileName(String _filename) {
    this.fileName = _fileName;
}
public FileDownloadView() {

}

prepareToDownload方法允许管理文件下载。通过这种方式，我们可以恢复文件名

public void prepareToDownload(ActionEvent actionEvent){
    setFileName((String)actionEvent.getComponent().getAttributes().get("fileName"));
    InputStream stream = FacesContext.getCurrentInstance().getExternalContext().getResourceAsStream("/tmp/" + fileName);
    setFile( new DefaultStreamedContent(stream, "text/plain", fileName));
}
public StreamedContent getFile() {
    System.out.println("file  "+file.getName());

    return file;
}
public void setFile(StreamedContent _file) {
    this.file  = _file;

}
}
}

然后在xhtml文件中：

<h:commandLink id="downloadLink"
title="Download"
actionListener="#{fileDownloadView.prepareToDownload}">
<p:graphicImage value="/resources/common/images/download.gif"
    alt="Download" />
    <f:attribute name="fileName" value="#{fileDownloadView.fileName}" />
    <p:fileDownload value="#{fileDownloadView.file}" />
</h:commandLink>
<h:outputText value="#{fileDownloadView.fileName}"/> 

Answer 2

您可以使用f:attribute将文件名从UI传递到控制器，并调用下载

根据标签的定义，它提供了一个选项，可以通过动作监听器将属性值传递给组件，或将参数传递给组件

因此，在您的情况下，您希望将文件名传递给控制器，并相应地下载文件

    <p:commandButton value="Download" ajax="true" actionListener="#{fileDownloadView.prepareToDownload}" icon="ui-icon-arrowthick-1-s">
                <p:fileDownload value="#{fileDownloadView.file}" />
                <f:attribute name="fileName" value="#{fileDownloadView.fileName}" />
   </p:commandButton>

或者

<h:commandLink id="downloadLink"
    title="Download"
    actionListener="#{fileDownloadView.prepareToDownload}">
    <p:graphicImage value="/resources/common/images/download.gif"
        alt="Download" />
        <f:attribute name="fileName" value="#{fileDownloadView.fileName}" />
        <p:fileDownload value="#{fileDownloadView.file}" />
</h:commandLink>

在控制器中编写actionEvent并管理下载

public void prepareToDownload(ActionEvent actionEvent){
        String fileName = (String)actionEvent.getComponent().getAttributes().get("fileName");
        InputStream stream = FacesContext.getCurrentInstance().getExternalContext().getResourceAsStream("/tmp/" + fileName);
        file = new DefaultStreamedContent(stream, "text/plain", fileName);
    }

Answer 3

不能在带有注释@ManagedBean的类中创建第二个构造函数来初始化filename变量