Initial implementation of memory-efficient Excel writing for large documents

[gamerover98]

I have implemented an initial version that does not use memory to write a document. This solution is useful in cases where it’s necessary to create large Excel documents sequentially, meaning there’s no need for a second manipulation of the cells already added to the document.

The issue is described here: #480

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Currently, this code only allows text to be written into a cell, and there is no functionality yet for changing its style. I would also like to point out that, due to the rush in which this modification was made, you should see this pull request not as a final change but as a way to demonstrate that even more can be achieved with Fastexcel!

You can compare the memory usage difference using VisualVM with the images from the issue:

• Memory usage:
  

• Memory Profiling:
  

The difference is remarkable, isn’t it? You can now create huge files without wasting memory!

Here’s the code used for the tests:

package org.example;

import lombok.extern.slf4j.Slf4j;
import org.dhatim.fastexcel.Workbook;
import org.dhatim.fastexcel.Worksheet;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

@Slf4j
public class Main {

    private static final String EXCEL_RELATIVE_PATH = "./target/data/example.xlsx";
    private static final int COLUMNS = 100;
    private static final int ROWS = 10000000;

    public static void main(String... args) throws Exception {
        try (var outputStream = new FileOutputStream(getExcelFile());
             var workbook = new Workbook(outputStream, "MyApplication", "1.0");
             var worksheet = workbook.newStreamWorksheet("Sheet 1")) {

           // Start writing on the document.
            worksheet.start(COLUMNS);

            log.debug("Generating data...");

            for (int rowIndex = 0; rowIndex < ROWS; rowIndex++) {
                worksheet.appendRow(column -> "test"); // Append a new row and set "test" at each cell.
            }

           // The autoclosable will close the worksheet and the workbook.
        }
    }

    public static File getExcelFile() throws IOException {
        var file = new File(EXCEL_RELATIVE_PATH);

        if (file.mkdirs()) {
            log.debug("Excel file created");
        }

        if (file.exists()) {
            Files.delete(Path.of(file.toURI()));
            log.debug("Excel file deleted");
        }

        return file;
    }
}

Lastly, I’d like to mention that test cases haven’t been added due to the nature of this pull request. If this approach moves forward, they will be included.

Thank you all <3