perf: stream image outputs directly into zip entries

backend/src/main/java/com/crystalpdf/backend/service/ExtractImagesService.java

@@ -14,6 +14,7 @@
 import java.awt.image.BufferedImage;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.util.Locale;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -24,7 +25,12 @@ public class ExtractImagesService {
      * Extract all images from a PDF and return them as a ZIP file.
      */
     public byte[] extractImages(byte[] pdfBytes, String format) throws IOException {
-        String imgFormat = (format != null && !format.isBlank()) ? format : "png";
+        String imgFormat = (format != null && !format.isBlank())
+                ? format.toLowerCase(Locale.ROOT)
+                : "png";
+        if (!imgFormat.equals("png") && !imgFormat.equals("jpg") && !imgFormat.equals("jpeg")) {
+            throw new IllegalArgumentException("Unsupported image output format: " + imgFormat);
+        }
 
         try (PDDocument doc = Loader.loadPDF(new RandomAccessReadBuffer(pdfBytes));
              ByteArrayOutputStream zipOut = new ByteArrayOutputStream();
@@ -35,18 +41,20 @@ public byte[] extractImages(byte[] pdfBytes, String format) throws IOException {
             for (int pageIdx = 0; pageIdx < doc.getNumberOfPages(); pageIdx++) {
                 PDPage page = doc.getPage(pageIdx);
                 PDResources resources = page.getResources();
-                if (resources == null) continue;
+                if (resources == null) {
+                    continue;
+                }
 
                 for (COSName name : resources.getXObjectNames()) {
                     PDXObject xobj = resources.getXObject(name);
                     if (xobj instanceof PDImageXObject image) {
                         BufferedImage bimg = image.getImage();
-                        ByteArrayOutputStream imgBytes = new ByteArrayOutputStream();
-                        ImageIO.write(bimg, imgFormat, imgBytes);
 
                         String entryName = String.format("page%d_img%d.%s", pageIdx + 1, imgIndex++, imgFormat);
                         zos.putNextEntry(new ZipEntry(entryName));
-                        zos.write(imgBytes.toByteArray());
+                        if (!ImageIO.write(bimg, imgFormat, zos)) {
+                            throw new IllegalArgumentException("Unsupported image output format: " + imgFormat);
+                        }
                         zos.closeEntry();
                     }
                 }

backend/src/main/java/com/crystalpdf/backend/service/PdfToImageService.java

@@ -31,12 +31,11 @@ public byte[] convert(byte[] pdfBytes, String format, int dpi) throws IOExceptio
             for (int i = 0; i < doc.getNumberOfPages(); i++) {
                 BufferedImage image = renderer.renderImageWithDPI(i, dpi, ImageType.RGB);
 
-                ByteArrayOutputStream imgOut = new ByteArrayOutputStream();
-                ImageIO.write(image, imageioFormat, imgOut);
-
                 ZipEntry entry = new ZipEntry(String.format("page_%03d.%s", i + 1, fmt));
                 zos.putNextEntry(entry);
-                zos.write(imgOut.toByteArray());
+                if (!ImageIO.write(image, imageioFormat, zos)) {
+                    throw new IOException("Unsupported image output format: " + imageioFormat);
+                }
                 zos.closeEntry();
             }
 

backend/src/test/java/com/crystalpdf/backend/helper/PdfTestHelper.java

@@ -2,8 +2,14 @@
 
 import org.apache.pdfbox.pdmodel.PDDocument;
 import org.apache.pdfbox.pdmodel.PDPage;
+import org.apache.pdfbox.pdmodel.PDPageContentStream;
 import org.apache.pdfbox.pdmodel.common.PDRectangle;
+import org.apache.pdfbox.pdmodel.graphics.image.LosslessFactory;
+import org.apache.pdfbox.pdmodel.graphics.image.PDImageXObject;
 
+import java.awt.Color;
+import java.awt.Graphics2D;
+import java.awt.image.BufferedImage;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 
@@ -34,4 +40,34 @@ public static byte[] createPdf(int pageCount) throws IOException {
     public static byte[] createPdf() throws IOException {
         return createPdf(1);
     }
+
+    /**
+     * Returns a single-page PDF with one embedded image XObject.
+     */
+    public static byte[] createPdfWithImage() throws IOException {
+        try (PDDocument doc = new PDDocument();
+             ByteArrayOutputStream out = new ByteArrayOutputStream()) {
+            PDPage page = new PDPage(PDRectangle.A4);
+            doc.addPage(page);
+
+            BufferedImage image = new BufferedImage(32, 32, BufferedImage.TYPE_INT_RGB);
+            Graphics2D graphics = image.createGraphics();
+            try {
+                graphics.setColor(Color.CYAN);
+                graphics.fillRect(0, 0, 32, 32);
+                graphics.setColor(Color.BLUE);
+                graphics.fillOval(8, 8, 16, 16);
+            } finally {
+                graphics.dispose();
+            }
+
+            PDImageXObject ximage = LosslessFactory.createFromImage(doc, image);
+            try (PDPageContentStream content = new PDPageContentStream(doc, page)) {
+                content.drawImage(ximage, 72, 720, 32, 32);
+            }
+
+            doc.save(out);
+            return out.toByteArray();
+        }
+    }
 }

backend/src/test/java/com/crystalpdf/backend/service/ExtractImagesServiceTest.java

@@ -0,0 +1,49 @@
+package com.crystalpdf.backend.service;
+
+import com.crystalpdf.backend.helper.PdfTestHelper;
+import org.junit.jupiter.api.Test;
+
+import javax.imageio.ImageIO;
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipInputStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class ExtractImagesServiceTest {
+
+    private final ExtractImagesService extractImagesService = new ExtractImagesService();
+
+    @Test
+    void extractImages_returnsZipWithDecodableImages() throws Exception {
+        byte[] pdf = PdfTestHelper.createPdfWithImage();
+        byte[] zipBytes = extractImagesService.extractImages(pdf, "png");
+
+        try (ZipInputStream zip = new ZipInputStream(new ByteArrayInputStream(zipBytes))) {
+            ZipEntry entry = zip.getNextEntry();
+            assertThat(entry).isNotNull();
+            assertThat(entry.getName()).isEqualTo("page1_img1.png");
+            assertThat(ImageIO.read(new ByteArrayInputStream(readEntry(zip)))).isNotNull();
+            zip.closeEntry();
+
+            assertThat(zip.getNextEntry()).isNull();
+        }
+    }
+
+    @Test
+    void extractImages_throwsForUnsupportedFormat() throws Exception {
+        byte[] pdf = PdfTestHelper.createPdfWithImage();
+
+        assertThatThrownBy(() -> extractImagesService.extractImages(pdf, "tiff"))
+                .isInstanceOf(IllegalArgumentException.class)
+                .hasMessageContaining("Unsupported image output format: tiff");
+    }
+
+    private static byte[] readEntry(ZipInputStream zip) throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        zip.transferTo(out);
+        return out.toByteArray();
+    }
+}

backend/src/test/java/com/crystalpdf/backend/service/PdfToImageServiceTest.java

@@ -0,0 +1,45 @@
+package com.crystalpdf.backend.service;
+
+import com.crystalpdf.backend.helper.PdfTestHelper;
+import org.junit.jupiter.api.Test;
+
+import javax.imageio.ImageIO;
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.util.zip.ZipEntry;
+import java.util.zip.ZipInputStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class PdfToImageServiceTest {
+
+    private final PdfToImageService pdfToImageService = new PdfToImageService();
+
+    @Test
+    void convert_returnsZipWithDecodablePageImages() throws Exception {
+        byte[] pdf = PdfTestHelper.createPdf(2);
+        byte[] zipBytes = pdfToImageService.convert(pdf, "png", 72);
+
+        try (ZipInputStream zip = new ZipInputStream(new ByteArrayInputStream(zipBytes))) {
+            ZipEntry first = zip.getNextEntry();
+            assertThat(first).isNotNull();
+            assertThat(first.getName()).isEqualTo("page_001.png");
+            assertThat(ImageIO.read(new ByteArrayInputStream(readEntry(zip)))).isNotNull();
+            zip.closeEntry();
+
+            ZipEntry second = zip.getNextEntry();
+            assertThat(second).isNotNull();
+            assertThat(second.getName()).isEqualTo("page_002.png");
+            assertThat(ImageIO.read(new ByteArrayInputStream(readEntry(zip)))).isNotNull();
+            zip.closeEntry();
+
+            assertThat(zip.getNextEntry()).isNull();
+        }
+    }
+
+    private static byte[] readEntry(ZipInputStream zip) throws Exception {
+        ByteArrayOutputStream out = new ByteArrayOutputStream();
+        zip.transferTo(out);
+        return out.toByteArray();
+    }
+}