Vaadin 上传文件流没有关闭？

Question

Vaadin 7.6.2

从这个文档，得到这个代码示例： https://vaadin.com/docs/-/part/framework/components/components-upload.html

// Show uploaded file in this placeholder
final Embedded image = new Embedded("Uploaded Image");
image.setVisible(false);

// Implement both receiver that saves upload in a file and
// listener for successful upload
class ImageUploader implements Receiver, SucceededListener {
    public File file;

    public OutputStream receiveUpload(String filename,
                                      String mimeType) {
        // Create upload stream
        FileOutputStream fos = null; // Stream to write to
        try {
            // Open the file for writing.
            file = new File("/tmp/uploads/" + filename);
            fos = new FileOutputStream(file);
        } catch (final java.io.FileNotFoundException e) {
            new Notification("Could not open file<br/>",
                             e.getMessage(),
                             Notification.Type.ERROR_MESSAGE)
                .show(Page.getCurrent());
            return null;
        }
        return fos; // Return the output stream to write to
    }

    public void uploadSucceeded(SucceededEvent event) {
        // Show the uploaded file in the image viewer
        image.setVisible(true);
        image.setSource(new FileResource(file));
    }
};
ImageUploader receiver = new ImageUploader();

// Create the upload with a caption and set receiver later
Upload upload = new Upload("Upload Image Here", receiver);
upload.setButtonCaption("Start Upload");
upload.addSucceededListener(receiver);

// Put the components in a panel
Panel panel = new Panel("Cool Image Storage");
Layout panelContent = new VerticalLayout();
panelContent.addComponents(upload, image);
panel.setContent(panelContent);

对此我有几个问题：

1. 为什么没有fos.close() 任何地方（在finally 块中）？
2. 为什么会从方法返回OutputStream？尤其是当receiver 被传递给监听器时？ Listener 是否正在关闭流？
3. 在哪里关闭流？

Answer 1

如果您查看sources，您会看到：

1. 为什么没有fos.close() 任何地方（在finally 块中）？

因为关闭输出流不取决于您。该框架将处理读取它接收到的数据，将其写入文件并关闭流（即使出现问题）。这发生在 streamToReceiver() 方法中的 FileUploadHandler (line 529 in the current version)。

1. 为什么从方法返回OutputStream？特别是当receiver 被传递到Listener 时？ Listener 是否正在关闭流？

因为Receiver 的唯一目的是告诉框架它应该将接收到的数据写入哪里。来自文档：

/**
 * Interface that must be implemented by the upload receivers to provide the
 * Upload component an output stream to write the uploaded data.
 *
 * @author Vaadin Ltd.
 * @since 3.0
 */

为了进一步阐明 receiver 被传递到 Listener 的困惑：事实并非如此。 ImageUploader 实现了两者

• Receiver - 提供应该写入内容的输出流
• SucceededListener - 上传成功时收到通知

因此，相同的ImageUploader 实例在Upload 组件的构造函数中传递，它将用作Receiver，并在addSucceededListener() 方法中传递，它将用作...你猜对了，SucceededListener。大多数人（可能？！）更喜欢在同一个类中实现所有这些接口，但如果您愿意，您可以选择在他们自己的类中实现每个功能，然后将适当的实例传递给每个方法。

1. 在哪里关闭流？

你没有，当上传完成时，框架会按照第 1 点的说明为你关闭它。与此同时，你可以去喝杯啤酒 :-)