Fix #97: Allow custom native library directory via h3.native.dir

src/main/java/com/uber/h3core/H3CoreLoader.java

@@ -93,17 +93,36 @@ public static NativeMethods loadNatives() throws IOException {
     return loadNatives(os, arch);
   }
 
-  private static File createTempLibraryFile(OperatingSystem os) throws IOException {
+  static File createTempLibraryFile(OperatingSystem os) throws IOException {
+    // Check if the user specified a custom directory for native libraries
+    String customDir = System.getProperty("h3.native.dir");
+    File dir = customDir != null ? new File(customDir) : null;
+
+    // Ensure the custom directory exists
+    if (dir != null && !dir.exists()) {
+      dir.mkdirs();
+    }
+
     if (os.isPosix()) {
       // Note this is already done by the implementation of Files.createTempFile that I looked at,
       // but the javadoc does not seem to gaurantee the permissions will be restricted to owner
       // write.
       final FileAttribute<Set<PosixFilePermission>> attr =
           PosixFilePermissions.asFileAttribute(PosixFilePermissions.fromString("rwx------"));
-      return Files.createTempFile("libh3-java", os.getSuffix(), attr).toFile();
+
+      if (dir != null) {
+        return Files.createTempFile(dir.toPath(), "libh3-java", os.getSuffix(), attr).toFile();
+      } else {
+        return Files.createTempFile("libh3-java", os.getSuffix(), attr).toFile();
+      }
     } else {
       // When not a POSIX OS, try to ensure the permissions are secure
-      final File f = Files.createTempFile("libh3-java", os.getSuffix()).toFile();
+      final File f;
+      if (dir != null) {
+        f = Files.createTempFile(dir.toPath(), "libh3-java", os.getSuffix()).toFile();
+      } else {
+        f = Files.createTempFile("libh3-java", os.getSuffix()).toFile();
+      }
       f.setReadable(true, true);
       f.setWritable(true, true);
       f.setExecutable(true, true);

src/test/java/com/uber/h3core/TestH3CoreLoader.java

@@ -68,4 +68,27 @@ void extractNonexistant() throws IOException {
         UnsatisfiedLinkError.class,
         () -> H3CoreLoader.copyResource("/nonexistant-resource", tempFile));
   }
+
+  @Test
+  void testCustomNativeDir() throws Exception {
+    String customDir = System.getProperty("java.io.tmpdir") + "/h3-custom-test-dir";
+    System.setProperty("h3.native.dir", customDir);
+
+    try {
+      H3CoreLoader.OperatingSystem currentOs =
+          H3CoreLoader.detectOs(System.getProperty("java.vendor"), System.getProperty("os.name"));
+
+      // Call the package-private method directly! No reflection needed.
+      File tempFile = H3CoreLoader.createTempLibraryFile(currentOs);
+
+      org.junit.jupiter.api.Assertions.assertTrue(
+          tempFile.getAbsolutePath().startsWith(new File(customDir).getAbsolutePath()),
+          "File should be created inside the custom directory");
+
+      tempFile.delete();
+      new File(customDir).delete();
+    } finally {
+      System.clearProperty("h3.native.dir");
+    }
+  }
 }