eugenp · programenth · Nov 17, 2025 · Nov 17, 2025 · Nov 17, 2025 · Nov 17, 2025
diff --git a/apache-poi-3/pom.xml b/apache-poi-3/pom.xml
@@ -14,6 +14,11 @@
     </parent>
 
     <dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-web</artifactId>
+            <version>${spring.web.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.apache.poi</groupId>
             <artifactId>poi-ooxml</artifactId>
@@ -108,6 +113,7 @@
         <logback-core.version>1.5.6</logback-core.version>
         <jmh.version>1.37</jmh.version>
         <log4j.version>2.23.1</log4j.version>
+        <spring.web.version>3.5.7</spring.web.version>
     </properties>
 
-</project>
+</project>
diff --git a/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/DemoApplication.java b/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/DemoApplication.java
@@ -0,0 +1,16 @@
+package com.baeldung.hssfworkbook;
+
+import java.io.IOException;
+
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class DemoApplication {
+
+	public static void main(String[] args) throws IOException {
+		SpringApplication.run(DemoApplication.class, args);
+    }
+
+}
diff --git a/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelController.java b/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelController.java
@@ -0,0 +1,64 @@
+package com.baeldung.hssfworkbook;
+
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+import org.apache.poi.ss.usermodel.Sheet;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.multipart.MultipartFile;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+
+@RestController
+public class ExcelController {
+
+    @GetMapping("/download-excel")
+    public ResponseEntity<byte[]> downloadExcel() {
+        try {
+            HSSFWorkbook workbook = ExcelCreator.createSampleWorkbook();
+
+            try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
+                workbook.write(baos);
+                byte[] bytes = baos.toByteArray();
+
+                return ResponseEntity.ok()
+                  .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=employee_data.xls")
+                  .contentType(MediaType.parseMediaType("application/vnd.ms-excel")) // More specific MIME type
+                  .body(bytes);
+            }
+        } catch (IOException e) {
+            System.err.println("Error generating or writing Excel workbook: " + e.getMessage());
+            return ResponseEntity.internalServerError()
+              .build();
+        }
+    }
+
+    @PostMapping("/upload-excel")
+    public ResponseEntity<String> uploadExcel(@RequestParam("file") MultipartFile file) {
+        if (file.isEmpty()) {
+            return ResponseEntity.badRequest()
+              .body("File is empty. Please upload a file.");
+        }
+
+        try {
+            try (HSSFWorkbook workbook = new HSSFWorkbook(file.getInputStream())) {
+                Sheet sheet = workbook.getSheetAt(0);
+
+                return ResponseEntity.ok("Sheet '" + sheet.getSheetName() + "' uploaded successfully!");
+            }
+        } catch (IOException e) {
+            System.err.println("Error processing uploaded Excel file: " + e.getMessage());
+            return ResponseEntity.internalServerError()
+              .body("Failed to process the Excel file.");
+        } catch (Exception e) {
+            System.err.println("An unexpected error occurred during file upload: " + e.getMessage());
+            return ResponseEntity.internalServerError()
+              .body("An unexpected error occurred.");
+        }
+    }
+}
diff --git a/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelConverter.java b/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelConverter.java
@@ -0,0 +1,23 @@
+package com.baeldung.hssfworkbook;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+
+public class ExcelConverter {
+
+    public static byte[] convertWorkbookToBytes(HSSFWorkbook workbook) throws IOException {
+        try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
+            workbook.write(baos);
+            return baos.toByteArray();
+        }
+    }
+
+    public static HSSFWorkbook convertBytesToWorkbook(byte[] excelBytes) throws IOException {
+        try (ByteArrayInputStream bais = new ByteArrayInputStream(excelBytes)) {
+            return new HSSFWorkbook(bais);
+        }
+    }
+}
diff --git a/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelCreator.java b/apache-poi-3/src/main/java/com/baeldung/hssfworkbook/ExcelCreator.java
@@ -0,0 +1,56 @@
+package com.baeldung.hssfworkbook;
+
+import org.apache.poi.hssf.usermodel.HSSFCellStyle;
+import org.apache.poi.hssf.usermodel.HSSFFont;
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+import org.apache.poi.ss.usermodel.Cell;
+import org.apache.poi.ss.usermodel.Row;
+import org.apache.poi.ss.usermodel.Sheet;
+
+public class ExcelCreator {
+
+    public static HSSFWorkbook createSampleWorkbook() {
+        HSSFWorkbook workbook = new HSSFWorkbook();
+
+        final String SHEET_NAME = "Employees";
+        final String[] COLUMN_HEADERS = { "ID", "Name", "Department" };
+        Object[][] data = { { 101, "John Doe", "Finance" }, { 102, "Jane Smith", "HR" }, { 103, "Michael Clark", "IT" } };
+
+        Sheet sheet = workbook.createSheet(SHEET_NAME);
+
+        HSSFFont font = workbook.createFont();
+        font.setBold(true);
+        HSSFCellStyle headerStyle = workbook.createCellStyle();
+        headerStyle.setFont(font);
+
+        Row header = sheet.createRow(0);
+        for (int i = 0; i < COLUMN_HEADERS.length; i++) {
+            Cell cell = header.createCell(i);
+            cell.setCellValue(COLUMN_HEADERS[i]);
+            cell.setCellStyle(headerStyle);
+        }
+
+        int rowNum = 1;
+        for (Object[] rowData : data) {
+            Row row = sheet.createRow(rowNum++);
+            for (int i = 0; i < rowData.length; i++) {
+                Cell cell = row.createCell(i);
+                Object value = rowData[i];
+
+                if (value instanceof Integer) {
+                    cell.setCellValue(((Integer) value).doubleValue());
+                } else if (value instanceof Double) {
+                    cell.setCellValue((Double) value);
+                } else if (value != null) {
+                    cell.setCellValue(value.toString());
+                }
+            }
+        }
+
+        for (int i = 0; i < COLUMN_HEADERS.length; i++) {
+            sheet.autoSizeColumn(i);
+        }
+
+        return workbook;
+    }
+}
diff --git a/apache-poi-3/src/test/java/com/baeldung/hssfworkbook/ExcelConverterUnitTest.java b/apache-poi-3/src/test/java/com/baeldung/hssfworkbook/ExcelConverterUnitTest.java
@@ -0,0 +1,86 @@
+package com.baeldung.hssfworkbook;
+
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+import org.apache.poi.ss.usermodel.Sheet;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.UncheckedIOException;
+
+import com.baeldung.hssfworkbook.ExcelConverter;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class ExcelConverterTest {
+
+    private HSSFWorkbook createMinimalWorkbook() {
+        HSSFWorkbook workbook = new HSSFWorkbook();
+        Sheet sheet = workbook.createSheet("TestSheet");
+        sheet.createRow(0)
+          .createCell(0)
+          .setCellValue("Test Data");
+        return workbook;
+    }
+
+    private byte[] convertWorkbookToBytesSafely(HSSFWorkbook workbook) {
+        try (ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
+            workbook.write(baos);
+            return baos.toByteArray();
+        } catch (IOException e) {
+            throw new UncheckedIOException("Failed to create test bytes.", e);
+        }
+    }
+
+    @Test
+    @DisplayName("GivenAValidHSSFWorkbook_whenConvertingToBytes_thenByteArrayIsNonEmptyAndValid")
+    void givenAValidHSSFWorkbook_whenConvertingToBytes_thenByteArrayIsNonEmptyAndValid() throws IOException {
+        HSSFWorkbook workbook = createMinimalWorkbook();
+
+        byte[] resultBytes = ExcelConverter.convertWorkbookToBytes(workbook);
+
+        assertNotNull(resultBytes, "The byte array should not be null.");
+        assertTrue(resultBytes.length > 0, "The byte array should not be empty.");
+
+        HSSFWorkbook convertedWorkbook = ExcelConverter.convertBytesToWorkbook(resultBytes);
+        assertEquals("TestSheet", convertedWorkbook.getSheetName(0));
+    }
+
+    @Test
+    @DisplayName("GivenAValidExcelByteArray_whenConvertingToWorkbook_thenHSSFWorkbookIsReturned")
+    void givenAValidExcelByteArray_whenConvertingToWorkbook_thenHSSFWorkbookIsReturned() throws IOException {
+        HSSFWorkbook originalWorkbook = createMinimalWorkbook();
+        byte[] validExcelBytes = convertWorkbookToBytesSafely(originalWorkbook);
+
+        HSSFWorkbook resultWorkbook = ExcelConverter.convertBytesToWorkbook(validExcelBytes);
+
+        assertNotNull(resultWorkbook, "The resulting workbook should not be null.");
+        assertEquals(1, resultWorkbook.getNumberOfSheets(), "The resulting workbook should have 1 sheet.");
+        assertEquals("TestSheet", resultWorkbook.getSheetName(0), "Sheet name should match the original.");
+        assertEquals("Test Data", resultWorkbook.getSheetAt(0)
+          .getRow(0)
+          .getCell(0)
+          .getStringCellValue());
+    }
+
+    @Test
+    @DisplayName("GivenAnEmptyByteArray_whenConvertingToWorkbook_thenIOExceptionIsThrown")
+    void givenAnEmptyByteArray_whenConvertingToWorkbook_thenIOExceptionIsThrown() {
+        byte[] emptyBytes = new byte[0];
+
+        assertThrows(IOException.class, () -> ExcelConverter.convertBytesToWorkbook(emptyBytes), "Expected IOException for empty byte array.");
+    }
+
+    @Test
+    @DisplayName("GivenAnInvalidByteArray_whenConvertingToWorkbook_thenIOExceptionIsThrown")
+    void givenAnInvalidByteArray_whenConvertingToWorkbook_thenIOExceptionIsThrown() {
+        byte[] invalidBytes = { 0x01, 0x02, 0x03, 0x04, 0x05 };
+
+        assertThrows(IOException.class, () -> ExcelConverter.convertBytesToWorkbook(invalidBytes), "Expected IOException for invalid byte array format.");
+    }
+}
diff --git a/apache-poi-3/src/test/java/com/baeldung/hssfworkbook/ExcelCreatorUnitTest.java b/apache-poi-3/src/test/java/com/baeldung/hssfworkbook/ExcelCreatorUnitTest.java
@@ -0,0 +1,94 @@
+package com.baeldung.hssfworkbook;
+
+import org.apache.poi.hssf.usermodel.HSSFWorkbook;
+import org.apache.poi.ss.usermodel.Cell;
+import org.apache.poi.ss.usermodel.CellStyle;
+import org.apache.poi.ss.usermodel.CellType;
+import org.apache.poi.ss.usermodel.Font;
+import org.apache.poi.ss.usermodel.Row;
+import org.apache.poi.ss.usermodel.Sheet;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import com.baeldung.hssfworkbook.ExcelCreator;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class ExcelCreatorTest {
+
+    private static final String SHEET_NAME = "Employees";
+    private static final String[] COLUMN_HEADERS = { "ID", "Name", "Department" };
+    private static final Object[][] DATA = { { 101, "John Doe", "Finance" }, { 102, "Jane Smith", "HR" }, { 103, "Michael Clark", "IT" } };
+
+    @Test
+    @DisplayName("Workbook should be created and contain one sheet named 'Employees'")
+    void givenExcelCreator_whenCreateSampleWorkbookCalled_thenWorkbookContainsOneSheetNamedEmployees() {
+        HSSFWorkbook workbook = ExcelCreator.createSampleWorkbook();
+
+        assertNotNull(workbook, "The workbook should not be null.");
+        assertEquals(1, workbook.getNumberOfSheets(), "The workbook should contain exactly one sheet.");
+
+        Sheet sheet = workbook.getSheet(SHEET_NAME);
+        assertNotNull(sheet, "The sheet named '" + SHEET_NAME + "' must exist.");
+    }
+
+    @Test
+    @DisplayName("Header row should have correct content and bold style")
+    void givenExcelCreator_whenCreateSampleWorkbookCalled_thenHeaderRowContainsCorrectTextAndBoldStyle() {
+        HSSFWorkbook workbook = ExcelCreator.createSampleWorkbook();
+        Sheet sheet = workbook.getSheet(SHEET_NAME);
+
+        Row headerRow = sheet.getRow(0);
+        assertNotNull(headerRow, "Header row (row 0) should exist.");
+
+        Font expectedFont = workbook.getFontAt(headerRow.getCell(0)
+          .getCellStyle()
+          .getFontIndex());
+        assertTrue(expectedFont.getBold(), "Header font should be bold.");
+
+        for (int i = 0; i < COLUMN_HEADERS.length; i++) {
+            Cell cell = headerRow.getCell(i);
+            assertNotNull(cell, "Header cell at index " + i + " should exist.");
+            assertEquals(COLUMN_HEADERS[i], cell.getStringCellValue(), "Header text mismatch at column " + i + ".");
+
+            CellStyle cellStyle = cell.getCellStyle();
+            assertNotNull(cellStyle, "Header cell should have a style applied.");
+
+            Font actualFont = workbook.getFontAt(cellStyle.getFontIndex());
+            assertTrue(actualFont.getBold(), "Header cell at index " + i + " style must be bold.");
+        }
+    }
+
+    @Test
+    @DisplayName("Data rows should have correct content and data type handling")
+    void givenExcelCreator_whenCreateSampleWorkbookCalled_thenDataRowsContainCorrectContentAndTypes() {
+        HSSFWorkbook workbook = ExcelCreator.createSampleWorkbook();
+        Sheet sheet = workbook.getSheet(SHEET_NAME);
+
+        assertEquals(DATA.length + 1, sheet.getLastRowNum() + 1, "The sheet should contain header row + data rows.");
+
+        for (int i = 0; i < DATA.length; i++) {
+            Row dataRow = sheet.getRow(i + 1);
+            Object[] expectedRowData = DATA[i];
+
+            assertNotNull(dataRow, "Data row at index " + (i + 1) + " should exist.");
+
+            for (int j = 0; j < expectedRowData.length; j++) {
+                Cell cell = dataRow.getCell(j);
+                Object expectedValue = expectedRowData[j];
+
+                assertNotNull(cell, "Cell [" + (i + 1) + ", " + j + "] should exist.");
+
+                if (expectedValue instanceof Integer) {
+                    assertEquals(CellType.NUMERIC, cell.getCellType(), "Cell " + j + " type should be NUMERIC for Integer.");
+                    assertEquals(((Integer) expectedValue).doubleValue(), cell.getNumericCellValue(), 0.001, "Numeric value mismatch.");
+                } else {
+                    assertEquals(CellType.STRING, cell.getCellType(), "Cell " + j + " type should be STRING.");
+                    assertEquals(expectedValue.toString(), cell.getStringCellValue(), "String value mismatch.");
+                }
+            }
+        }
+    }
+}