Vaadin 上传文件流没有关闭？

Question

Vaadin 7.6.2

来自这个文档，来自这个代码示例： 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);

关于这个我有几个问题：

为什么没有 fos.close() 任何地方（在 finally 块中）？
为什么 OutputStream 从方法返回？特别是当 receiver 被传递给一个监听器时？监听器是否正在关闭流？
在哪里关闭流？

Answer 1

如果您查看 sources，您会看到：

Why no fos.close() anywhere (in finally block)?

因为关闭输出流不是由你决定的。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 FileUploadHandler in the streamToReceiver() method (line 529 in the current version).

Why in the world is OutputStream returned from the method? Especially when the receiver is being passed into a Listener? Is the Listener closing the stream?

因为 Receiver 的唯一目的是告诉框架应该将接收到的数据写入何处。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 的混乱：它不是。 ImageUploader 实现了两者

Receiver - 提供应该写入内容的输出流
SucceededListener - 上传成功完成后收到通知

因此，相同的 ImageUploader 实例被传递到 Upload 组件的构造函数中，它将被用作 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 class 中实现所有这些接口，但如果您愿意，您可以选择在它们自己的 class 中实现每个功能，然后传递一个每个方法的适当实例。

Where do I close the stream?

你不需要，正如第 1 点所述，上传完成后框架会为你关闭它。同时，你可以去喝杯啤酒:-)

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