Possible to download JPA repository in Vaadin as CSV file? - database

Question

Assume that we have defined a entity and it's connected to a database. Now we can access the database by using a repository.
#Autowired
private DataLoggRepository dataLoggRepository;
If I want to get all the rows from the database and download it. Then I can write this code:
List<DataLogg> dataLoggers = dataLoggRepository.findAll();
Now, how can I donwload the object dataLoggers as a CSV file in Vaadin in a proper way?

Answer 1

Here you can see how to create a link to download a file:
Anchor csvLink = new Anchor(new StreamResource("file.csv",
() -> {
String csvString = ...// create the csv
return new ByteArrayInputStream(csvString.getBytes());
}), "Download CSV");
csvLink.getElement().setAttribute("download", true);
To create the CSV you have various options like OpenCSV or directly create the CSV from the SQL query.

Answer 2

Here is a working example
// Download all data
Anchor download = new Anchor(); // Add this to the layout
loggerId.addValueChangeListener(e-> {
String fileName = String.valueOf(loggerId.getValue()) + ".csv";
List<DataLogg> selectedLogger = dataLoggRepository.findByLoggerId(loggerId.getValue());
download.setHref(getStreamResource(fileName, selectedLogger));
});
download.getElement().setAttribute("download",true);
download.add(new Button("Download", new Icon(VaadinIcon.DOWNLOAD_ALT)));
Function
public StreamResource getStreamResource(String filename, List<DataLogg> selectedLogger) {
// Create a large CSV file in a form of StringBuilder and then convert it all to bytes
StringWriter stringWriter = new StringWriter();
stringWriter.write("id, dateTime, DO0, DO1, DO2, DO3, AI0, AI1, AI2, AI3, loggerId, samplingTime\n");
for (int i = 0; i < selectedLogger.size(); ++ i) {
DataLogg dataLogg = selectedLogger.get(i);
String row = dataLogg.getId() + "," +
dataLogg.getDateTime() + "," +
dataLogg.getDO0() + "," +
dataLogg.getDO1() + "," +
dataLogg.getDO2() + "," +
dataLogg.getDO3() + "," +
dataLogg.getAI0() + "," +
dataLogg.getAI1() + "," +
dataLogg.getAI2() + "," +
dataLogg.getAI3() + "," +
dataLogg.getLoggerId() + "," +
dataLogg.getSamplingTime() + "\n";
stringWriter.write(row);
}
// Try to download
try {
byte[] buffer = stringWriter.toString().getBytes("UTF-8");
return new StreamResource(filename, () -> new ByteArrayInputStream(buffer));
} catch (UnsupportedEncodingException e) {
byte[] buffer = new byte[] {0};
return new StreamResource(filename, () -> new ByteArrayInputStream(buffer));
}
}