fix: try-with-resources for Streams interacting with Files to ensure proper resource management (#4404)

# Description of Changes

The Javadoc recommends wrapping Files.list(), Files.walk(),
Files.find(), and Files.lines() in try-with-resources so the stream’s
close() is called as soon as the terminal operation completes.

This is because when Stream interact with files, Java can ONLY close the
Stream during garbage-collection finalization, which is not guaranteed
to run promptly or at all before the JVM exits, creating a memory leak.

Direct quote:

> Streams have a
[BaseStream.close()](https://docs.oracle.com/javase/8/docs/api/java/util/stream/BaseStream.html#close--)
method and implement
[AutoCloseable](https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html),
but nearly all stream instances do not actually need to be closed after
use. Generally, only streams whose source is an IO channel (such as
those returned by [Files.lines(Path,
Charset)](https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html#lines-java.nio.file.Path-java.nio.charset.Charset-))
will require closing. Most streams are backed by collections, arrays, or
generating functions, which require no special resource management. (If
a stream does require closing, it can be declared as a resource in a
try-with-resources statement.)

> A DirectoryStream is opened upon creation and is closed by invoking
the close method. Closing a directory stream releases any resources
associated with the stream. Failure to close the stream may result in a
resource leak. The try-with-resources statement provides a useful
construct to ensure that the stream is closed:

Sources:
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html

https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/nio/file/DirectoryStream.html

https://stackoverflow.com/questions/79078272/using-try-with-resources-for-a-java-files-walk-stream-created-in-a-separate-meth

https://stackoverflow.com/questions/36990053/resource-leak-in-files-listpath-dir-when-stream-is-not-explicitly-closed

<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
-->

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [x] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Signed-off-by: Balázs Szücs <bszucs1209@gmail.com>

app/common/src/main/java/stirling/software/common/util/FileToPdf.java

@@ -124,8 +124,8 @@ public class FileToPdf {
     private static void zipDirectory(Path sourceDir, Path zipFilePath) throws IOException {
         try (ZipOutputStream zos =
                 new ZipOutputStream(new FileOutputStream(zipFilePath.toFile()))) {
-            Files.walk(sourceDir)
-                    .filter(path -> !Files.isDirectory(path))
+            try (Stream<Path> walk = Files.walk(sourceDir)) {
+                walk.filter(path -> !Files.isDirectory(path))
                         .forEach(
                                 path -> {
                                     ZipEntry zipEntry =
@@ -140,6 +140,7 @@ public class FileToPdf {
                                 });
             }
         }
+    }
 
     private static void deleteDirectory(Path dir) throws IOException {
         Files.walkFileTree(

app/core/src/main/java/stirling/software/SPDF/controller/api/converters/ConvertImgPDFController.java

@@ -9,6 +9,7 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.stream.Stream;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -150,10 +151,11 @@ public class ConvertImgPDFController {
                                 .runCommandWithOutputHandling(command);
 
                 // Find all WebP files in the output directory
-                List<Path> webpFiles =
-                        Files.walk(tempOutputDir)
-                                .filter(path -> path.toString().endsWith(".webp"))
-                                .toList();
+                List<Path> webpFiles;
+                try (Stream<Path> walkStream = Files.walk(tempOutputDir)) {
+                    webpFiles =
+                            walkStream.filter(path -> path.toString().endsWith(".webp")).toList();
+                }
 
                 if (webpFiles.isEmpty()) {
                     log.error("No WebP files were created in: {}", tempOutputDir.toString());

app/core/src/main/java/stirling/software/SPDF/controller/api/filters/FilterController.java

@@ -48,10 +48,12 @@ public class FilterController {
         String text = request.getText();
         String pageNumber = request.getPageNumbers();
 
-        PDDocument pdfDocument = pdfDocumentFactory.load(inputFile);
-        if (PdfUtils.hasText(pdfDocument, pageNumber, text))
+        try (PDDocument pdfDocument = pdfDocumentFactory.load(inputFile)) {
+            if (PdfUtils.hasText(pdfDocument, pageNumber, text)) {
                 return WebResponseUtils.pdfDocToWebResponse(
                         pdfDocument, Filenames.toSimpleFileName(inputFile.getOriginalFilename()));
+            }
+        }
         return null;
     }
 

app/core/src/main/java/stirling/software/SPDF/controller/api/misc/ExtractImageScansController.java

@@ -8,6 +8,7 @@ import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.stream.Stream;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
@@ -142,7 +143,10 @@ public class ExtractImageScansController {
                                 .runCommandWithOutputHandling(command);
 
                 // Read the output photos in temp directory
-                List<Path> tempOutputFiles = Files.list(tempDir).sorted().toList();
+                List<Path> tempOutputFiles;
+                try (Stream<Path> listStream = Files.list(tempDir)) {
+                    tempOutputFiles = listStream.sorted().toList();
+                }
                 for (Path tempOutputFile : tempOutputFiles) {
                     byte[] imageBytes = Files.readAllBytes(tempOutputFile);
                     processedImageBytes.add(imageBytes);

app/core/src/main/java/stirling/software/SPDF/service/SignatureService.java

@@ -7,6 +7,7 @@ import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.stream.Stream;
 
 import org.springframework.stereotype.Service;
 import org.thymeleaf.util.StringUtils;
@@ -66,11 +67,12 @@ public class SignatureService {
 
     private List<SignatureFile> getSignaturesFromFolder(Path folder, String category)
             throws IOException {
-        return Files.list(folder)
-                .filter(path -> isImageFile(path))
+        try (Stream<Path> stream = Files.list(folder)) {
+            return stream.filter(this::isImageFile)
                     .map(path -> new SignatureFile(path.getFileName().toString(), category))
                     .toList();
         }
+    }
 
     public byte[] getSignatureBytes(String username, String fileName) throws IOException {
         validateFileName(fileName);