Vaadin Web应用开发教程(22):UI组件-Upload组件
程序员文章站
2022-03-30 08:33:39
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Upload 组件用于向服务器上传文件。它显示一个文件名输入框,一个文件选择按钮和一个上传确认按钮。
可以通过setButtonCaption 修改”Upload” 按钮的文字。对于”Browser” 按钮由于浏览器安全方面的考虑,难以修改它的外观,”Brower” 显示语言取决于浏览器本身。 因此如果你想保持”Upload” 语言显示的一致,你必须使用和“Browser”一样的语言。
通常情况上传的文件可以存放在文件系统,数据库或是临时存放在内存中,Upload 组件将上传的文件数据写到一个java.io.OutputStream 对象中,因此你可以使用你喜欢的方法来处理上传到服务器的文件。
使用Upload 组件,需要实现Upload.Receiver 接口来处理文件数据,它将在用户点击”upload”按钮后调用。
当文件上传结束后,成功与否Upload组件将触发Upload.FinishedEvent 事件。可以通过Upload.FinishedListener 接口来处理这个事件,事件参数包括文件名,MIME类型和文件长度。
下面的例子将上传的图像文件存放在/tmp/uploads 目录下 并显示最后上传的图像。
import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import com.vaadin.terminal.FileResource;
import com.vaadin.ui.*;
public class MyUploader extends CustomComponent
implements Upload.SucceededListener,
Upload.FailedListener,
Upload.Receiver {
Panel root; // Root element for contained components.
Panel imagePanel; // Panel that contains the uploaded image.
File file; // File to write to.
MyUploader() {
root = new Panel("My Upload Component");
setCompositionRoot(root);
// Create the Upload component.
final Upload upload =
new Upload("Upload the file here", this);
// Use a custom button caption instead of plain "Upload".
upload.setButtonCaption("Upload Now");
// Listen for events regarding the success of upload.
upload.addListener((Upload.SucceededListener) this);
upload.addListener((Upload.FailedListener) this);
root.addComponent(upload);
root.addComponent(new Label("Click 'Browse' to "+
"select a file and then click 'Upload'."));
// Create a panel for displaying the uploaded image.
imagePanel = new Panel("Uploaded image");
imagePanel.addComponent(
new Label("No image uploaded yet"));
root.addComponent(imagePanel);
}
// Callback method to begin receiving the upload.
public OutputStream receiveUpload(String filename,
String MIMEType) {
FileOutputStream fos = null; // Output stream to write to
file = new File("/tmp/uploads/" + filename);
try {
// Open the file for writing.
fos = new FileOutputStream(file);
} catch (final java.io.FileNotFoundException e) {
// Error while opening the file. Not reported here.
e.printStackTrace();
return null;
}
return fos; // Return the output stream to write to
}
// This is called if the upload is finished.
public void uploadSucceeded(Upload.SucceededEvent event) {
// Log the upload on screen.
root.addComponent(new Label("File " + event.getFilename()
+ " of type '" + event.getMIMEType()
+ "' uploaded."));
// Display the uploaded file in the image panel.
final FileResource imageResource =
new FileResource(file, getApplication());
imagePanel.removeAllComponents();
imagePanel.addComponent(new Embedded("", imageResource));
}
// This is called if the upload fails.
public void uploadFailed(Upload.FailedEvent event) {
// Log the failure on screen.
root.addComponent(new Label("Uploading "
+ event.getFilename() + " of type '"
+ event.getMIMEType() + "' failed."));
}
}
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