Your first Excel report in 5 minutes - a TBEG quick start (Kotlin/Java)

In Part 1, we explored what TBEG is. In this post, we'll write actual code and experience TBEG's core features hands-on.

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Step 1. Add Dependency

// build.gradle.kts
dependencies {
    implementation("io.github.jogakdal:tbeg:1.2.5")
}

Groovy / Maven:

// build.gradle
dependencies {
    implementation 'io.github.jogakdal:tbeg:1.2.5'
}

<!-- pom.xml -->
<dependency>
    <groupId>io.github.jogakdal</groupId>
    <artifactId>tbeg</artifactId>
    <version>1.2.5</version>
</dependency>

The version above is as of this writing. Check the latest version on Maven Central.

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Step 2. Simplest Example - Variable Substitution

Create a Template

Open Excel and enter the following:

${...} is TBEG's variable marker. The values passed from code will fill these placeholders.

Write the Code

import io.github.jogakdal.tbeg.ExcelGenerator
import java.io.File
import java.time.LocalDate

fun main() {
    val data = mapOf(
        "title" to "Monthly Report",
        "date" to LocalDate.now().toString(),
        "author" to "Yongho Hwang"
    )

    ExcelGenerator().use { generator ->
        val template = File("template.xlsx").inputStream()
        val bytes = generator.generate(template, data)
        File("output.xlsx").writeBytes(bytes)
    }
}

Result

That's it. Open output.xlsx and you'll find "Monthly Report" where ${title} was, and today's date where ${date} was.

Core pattern: ExcelGenerator().use { ... } → read template → pass data → get byte array. This pattern is the same in every example that follows.

Java version:

import io.github.jogakdal.tbeg.ExcelGenerator;
import java.io.*;
import java.time.LocalDate;
import java.util.*;

public class QuickStart {
    public static void main(String[] args) throws Exception {
        Map<String, Object> data = new HashMap<>();
        data.put("title", "Monthly Report");
        data.put("date", LocalDate.now().toString());
        data.put("author", "Yongho Hwang");

        try (ExcelGenerator generator = new ExcelGenerator();
             InputStream template = new FileInputStream("template.xlsx")) {
            byte[] bytes = generator.generate(template, data);
            try (FileOutputStream output = new FileOutputStream("output.xlsx")) {
                output.write(bytes);
            }
        }
    }
}

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Step 3. Repeating List Data - repeat

This is the most commonly used feature in practice. Use it whenever you need to expand a list into rows - employee lists, order histories, sales data, etc.

Template

• Row 1: ${repeat(employees, A3:C3, emp)} - "For each item in the employees collection, name it emp and repeat the A3:C3 range"
• Row 2: Header (outside the repeat range, stays as-is)
• Row 3: Repeat range - emp's fields are filled into each row

Code

data class Employee(val name: String, val position: String, val salary: Int)

fun main() {
    val data = mapOf(
        "employees" to listOf(
            Employee("Yongho Hwang", "Director", 8000),
            Employee("Yongho Han", "Manager", 6500),
            Employee("Yongho Hong", "Assistant Manager", 4500),
            Employee("Yongho Kim", "Staff", 3500)
        )
    )

    ExcelGenerator().use { generator ->
        val bytes = generator.generate(File("template.xlsx").inputStream(), data)
        File("output.xlsx").writeBytes(bytes)
    }
}

Result

4 rows of data expanded into 4 rows. If there were formulas (like =SUM()), their ranges would be automatically adjusted.

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Step 4. DataProvider - Structured Data Delivery

You can simply use a Map, but for advanced features like images or lazy loading, it's recommended to use DataProvider.

Kotlin DSL

import io.github.jogakdal.tbeg.simpleDataProvider

val provider = simpleDataProvider {
    // Simple variables
    value("title", "Employee Status")
    value("date", LocalDate.now().toString())

    // Collection (eager loading)
    items("employees", employeeList)

    // Collection (lazy loading) - called when data is needed
    items("bigData") {
        repository.streamAll().iterator()
    }

    // Images
    image("logo", logoBytes)
    imageUrl("banner", "https://example.com/banner.png")
}

ExcelGenerator().use { generator ->
    val bytes = generator.generate(template, provider)
}

Java Builder

import io.github.jogakdal.tbeg.SimpleDataProvider;

SimpleDataProvider provider = SimpleDataProvider.builder()
    .value("title", "Employee Status")
    .value("date", LocalDate.now().toString())
    .items("employees", employeeList)
    .itemsFromSupplier("bigData", () -> repository.streamAll().iterator())
    .image("logo", logoBytes)
    .build();

Approach	Advantages	Best For
Map	Simple, less code	Small data, simple reports
DataProvider	Lazy loading, images, metadata	Large data, images, production

In production, you'll almost always use DataProvider.

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Step 5. Image Insertion

Place a ${image(logo)} marker in the template and an image will be inserted at that cell position.

val provider = simpleDataProvider {
    value("company", "TBEG Inc.")

    // From file
    image("logo", File("logo.png").readBytes())

    // From URL (automatically downloaded during rendering)
    imageUrl("banner", "https://example.com/banner.png")
}

Images can be provided as byte arrays or URLs. The URL approach requires no separate download code - it's handled automatically during rendering.

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Step 6. File Saving Options

Instead of getting a byte array, you can save directly to a file.

ExcelGenerator().use { generator ->
    val path = generator.generateToFile(
        template = template,
        dataProvider = SimpleDataProvider.of(data),
        outputDir = Path.of("./output"),
        baseFileName = "report"
    )
    // Result: ./output/report_20260115_143052.xlsx
}

A timestamp is automatically appended to prevent filename collisions.

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Putting It All Together

1. Design a template in Excel (freely use formatting, charts, formulas, etc.)
2. Place ${variableName} markers where data should go
3. Use ${repeat(collection, range, variable)} for list data
4. Pass only the data from code
5. TBEG combines them → final Excel generated

Formatting, charts, conditional formatting, and formulas are all managed in the template.

Your code focuses solely on data binding.

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Next Up

In the next post, we'll master TBEG's template syntax. We'll cover the various options of repeat (DOWN/RIGHT directions, nested repeats), automatic cell merge (merge), bundle (bundle), empty collection handling, and more.

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Links

• GitHub: jogakdal/data-processors-with-excel
• Maven Central: TBEG

📖 Documentation

• User Guide
• Basic Examples
• Template Syntax Reference

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⭐ Found TBEG useful? Star it on GitHub - it really helps the project grow.