Fix singleton initialization

nostr-java-api/src/main/java/nostr/api/NostrSpringWebSocketClient.java

@@ -25,18 +25,34 @@ public class NostrSpringWebSocketClient implements NostrIF {
   @Getter
   private Identity sender;
 
-  private static NostrSpringWebSocketClient INSTANCE;
+  private static volatile NostrSpringWebSocketClient INSTANCE;
 
   public NostrSpringWebSocketClient(String relayName, String relayUri) {
     setRelays(Map.of(relayName, relayUri));
   }
 
   public static NostrIF getInstance() {
-    return (INSTANCE == null) ? new NostrSpringWebSocketClient() : INSTANCE;
+    if (INSTANCE == null) {
+      synchronized (NostrSpringWebSocketClient.class) {
+        if (INSTANCE == null) {
+          INSTANCE = new NostrSpringWebSocketClient();
+        }
+      }
+    }
+    return INSTANCE;
   }
 
   public static NostrIF getInstance(@NonNull Identity sender) {
-    return (INSTANCE == null) ? new NostrSpringWebSocketClient(sender) : INSTANCE;
+    if (INSTANCE == null) {
+      synchronized (NostrSpringWebSocketClient.class) {
+        if (INSTANCE == null) {
+          INSTANCE = new NostrSpringWebSocketClient(sender);
+        } else if (INSTANCE.getSender() == null) {
+          INSTANCE.sender = sender; // Initialize sender if not already set
+        }
+      }
+    }
+    return INSTANCE;
   }
 
   public NostrSpringWebSocketClient(@NonNull Identity sender) {

nostr-java-api/src/test/java/nostr/api/unit/NostrSpringWebSocketClientTest.java

@@ -0,0 +1,36 @@
+package nostr.api.unit;
+
+import nostr.api.NostrIF;
+import nostr.api.NostrSpringWebSocketClient;
+import nostr.id.Identity;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.lang.reflect.Field;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+public class NostrSpringWebSocketClientTest {
+
+    @BeforeEach
+    void resetSingleton() throws Exception {
+        Field instance = NostrSpringWebSocketClient.class.getDeclaredField("INSTANCE");
+        instance.setAccessible(true);
+        instance.set(null, null);
+    }
+
+    @Test
+    void getInstanceShouldReturnSameInstance() {
+        NostrIF first = NostrSpringWebSocketClient.getInstance();
+        NostrIF second = NostrSpringWebSocketClient.getInstance();
+        assertSame(first, second, "Multiple calls should return the same instance");
+    }
+
+    @Test
+    void getInstanceWithIdentityShouldReturnSameInstance() {
+        Identity identity = Identity.generateRandomIdentity();
+        NostrIF first = NostrSpringWebSocketClient.getInstance(identity);
+        NostrIF second = NostrSpringWebSocketClient.getInstance();
+        assertSame(first, second, "Calls with and without identity should return the same instance");
+    }
+}