Backport classscanner and fix npe (#63)

* backport class path util from main and fix a potential NPE

* backport class scanner and fix NPE

Author: Piiit

src/main/java/it/aboutbits/springboot/toolbox/jackson/CustomTypeDeserializer.java

@@ -44,6 +44,10 @@ public Class<T> handledType() {
 
     @Override
     public T deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
+        if (jsonParser.getCurrentToken() == null) {
+            return null;
+        }
+
         var value = typeConverter.apply(jsonParser);
 
         try {

src/main/java/it/aboutbits/springboot/toolbox/reflection/util/ClassScannerUtil.java

@@ -6,19 +6,27 @@
 import org.jspecify.annotations.NullMarked;
 
 import java.lang.annotation.Annotation;
+import java.util.Arrays;
+import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 
 @NullMarked
 public final class ClassScannerUtil {
+    private static final Map<String, ClassScanner> CACHE = new ConcurrentHashMap<>();
+
     private ClassScannerUtil() {
     }
 
     public static ClassScanner getScannerForPackages(String... packages) {
-        return new ClassScanner(packages);
+        var cacheKey = Arrays.stream(packages)
+                .sorted()
+                .collect(Collectors.joining("|"));
+        return CACHE.computeIfAbsent(cacheKey, _ -> new ClassScanner(packages));
     }
 
-    public static final class ClassScanner implements AutoCloseable {
+    public static final class ClassScanner {
         private final ScanResult scanResult;
         private final String[] packages;
 
@@ -51,10 +59,5 @@ public Set<Class<?>> getClassesAnnotatedWith(Class<? extends Annotation> clazz)
                     ClassInfo::loadClass
             ).collect(Collectors.toSet());
         }
-
-        @Override
-        public void close() {
-            scanResult.close();
-        }
     }
 }