Make UnitImageFactory synchronized.

It's already used from multiple threads, per #12623, so making the methods that access internal collections synchronized to fix ConcurrentModificationException.

This is using the lombok synchronized annotation which is better than the Java keyword since it keeps an internal lock object.

game-app/game-core/src/main/java/games/strategy/triplea/image/UnitImageFactory.java

@@ -34,10 +34,11 @@
 import javax.swing.ImageIcon;
 import lombok.Builder;
 import lombok.Getter;
+import lombok.Synchronized;
 import lombok.Value;
 import lombok.extern.slf4j.Slf4j;
 
-/** A factory with an image cache for creating unit images. */
+/** A factory with a thread-safe image cache for creating unit images. */
 @Slf4j
 public class UnitImageFactory {
   public static final int DEFAULT_UNIT_ICON_SIZE = 48;
@@ -83,13 +84,15 @@ public UnitImageFactory(
     this.mapData = mapData;
   }
 
+  @Synchronized
   public void clearCache() {
     images.clear();
     icons.clear();
     deleteTempFiles();
     colorizedTempFiles.clear();
   }
 
+  @Synchronized
   public void deleteTempFiles() {
     tempFiles.forEach(File::delete);
     tempFiles.clear();
@@ -222,6 +225,7 @@ public int getUnitCounterOffsetHeight() {
     return (int) (scaleFactor * unitCounterOffsetHeight);
   }
 
+  @Synchronized
   public boolean hasImage(final ImageKey imageKey) {
     return images.containsKey(imageKey) || getBaseImageUrl(imageKey).isPresent();
   }
@@ -230,32 +234,40 @@ public boolean hasImage(final ImageKey imageKey) {
    * Return the appropriate scaled unit image. If an image cannot be found, a placeholder 'no-image'
    * image is returned.
    */
+  @Synchronized
   public Image getImage(final ImageKey imageKey) {
     return Optional.ofNullable(images.get(imageKey))
-        .or(
-            () ->
-                getTransformedImage(imageKey)
-                    .map(
-                        baseImage -> {
-                          // We want to scale units according to the given scale factor.
-                          // We use smooth scaling since the images are cached to allow to take our
-                          // time in doing the scaling.
-                          // Image observer is null, since the image should have been guaranteed to
-                          // be loaded.
-                          final int baseWidth = baseImage.getWidth(null);
-                          final int baseHeight = baseImage.getHeight(null);
-                          final int width = Math.max(1, (int) (baseWidth * scaleFactor));
-                          final int height = Math.max(1, (int) (baseHeight * scaleFactor));
-                          final Image scaledImage =
-                              baseImage.getScaledInstance(width, height, Image.SCALE_SMOOTH);
-                          // Ensure the scaling is completed.
-                          Util.ensureImageLoaded(scaledImage);
-                          images.put(imageKey, scaledImage);
-                          return scaledImage;
-                        }))
+        .or(() -> getTransformedScaledImage(imageKey))
         .orElseGet(() -> createNoImageImage(imageKey));
   }
 
+  private Optional<Image> getTransformedScaledImage(final ImageKey imageKey) {
+    return getTransformedImage(imageKey)
+        .map(
+            baseImage -> {
+              // We want to scale units according to the given scale factor.
+              // We use smooth scaling since the images are cached to allow to take our
+              // time in doing the scaling.
+              // Image observer is null, since the image should have been guaranteed to
+              // be loaded.
+              final int baseWidth = baseImage.getWidth(null);
+              final int baseHeight = baseImage.getHeight(null);
+              final int width = Math.max(1, (int) (baseWidth * scaleFactor));
+              final int height = Math.max(1, (int) (baseHeight * scaleFactor));
+              final Image scaledImage =
+                  baseImage.getScaledInstance(width, height, Image.SCALE_SMOOTH);
+              // Ensure the scaling is completed.
+              Util.ensureImageLoaded(scaledImage);
+              images.put(imageKey, scaledImage);
+              return scaledImage;
+            });
+  }
+
+  private Optional<Image> getTransformedImage(final ImageKey imageKey) {
+    return getBaseImageUrl(imageKey)
+        .map(imageLocation -> loadImageAndTransform(imageLocation, imageKey));
+  }
+
   private Image createNoImageImage(ImageKey imageKey) {
     BufferedImage image = resourceLoader.getImageOrThrow(FILE_NAME_BASE + "missing_unit_image.png");
     Color playerColor = mapData.getPlayerColor(imageKey.getPlayer().getName());
@@ -270,7 +282,7 @@ private Image createNoImageImage(ImageKey imageKey) {
     return image;
   }
 
-  public Optional<URL> getBaseImageUrl(final ImageKey imageKey) {
+  private Optional<URL> getBaseImageUrl(final ImageKey imageKey) {
     final String baseImageName = imageKey.getBaseImageName();
     final GamePlayer gamePlayer = imageKey.getPlayer();
     // URL uses '/' not '\'
@@ -280,6 +292,7 @@ public Optional<URL> getBaseImageUrl(final ImageKey imageKey) {
     return Optional.ofNullable(url);
   }
 
+  @Synchronized
   public Optional<URL> getPossiblyTransformedImageUrl(final ImageKey imageKey) {
     final Optional<URL> url = getBaseImageUrl(imageKey);
     if (url.isEmpty() || !shouldTransformImage(imageKey)) {
@@ -309,11 +322,6 @@ public Optional<URL> getPossiblyTransformedImageUrl(final ImageKey imageKey) {
             }));
   }
 
-  private Optional<Image> getTransformedImage(final ImageKey imageKey) {
-    return getBaseImageUrl(imageKey)
-        .map(imageLocation -> loadImageAndTransform(imageLocation, imageKey));
-  }
-
   private Image loadImageAndTransform(URL imageLocation, ImageKey imageKey) {
     Image image = Toolkit.getDefaultToolkit().getImage(imageLocation);
     Util.ensureImageLoaded(image);
@@ -376,6 +384,7 @@ public Image getHighlightImage(final ImageKey imageKey) {
   }
 
   /** Return an _unscaled_ icon image for a unit. */
+  @Synchronized
   public ImageIcon getIcon(final ImageKey imageKey) {
     final String fullName = imageKey.getFullName();
     return icons.computeIfAbsent(