remove unnecessary sync

Author: mredolatti

client/src/main/java/io/split/client/impressions/ImpressionObserver.java

@@ -3,6 +3,7 @@
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
 import io.split.client.dtos.KeyImpression;
+import org.apache.http.annotation.NotThreadSafe;
 
 /*
 According to guava's docs (https://guava.dev/releases/18.0/api/docs/com/google/common/annotations/Beta.html),
@@ -12,27 +13,26 @@
  */
 
 @SuppressWarnings("UnstableApiUsage")
+@NotThreadSafe
 public class ImpressionObserver {
 
     private final Cache<Long, Long> _cache;
 
     public ImpressionObserver(long size) {
         _cache = CacheBuilder.newBuilder()
                 .maximumSize(size)
+                .concurrencyLevel(4)  // Just setting the default value explicitly
                 .build();
     }
-
+    
     public Long testAndSet(KeyImpression impression) {
         if (null == impression) {
             return null;
         }
 
         Long hash = ImpressionHasher.process(impression);
-
-        synchronized(this) {
-            Long previous = _cache.getIfPresent(hash);
-            _cache.put(hash, impression.time);
-            return previous;
-        }
+        Long previous = _cache.getIfPresent(hash);
+        _cache.put(hash, impression.time);
+        return previous;
     }
 }

client/src/test/java/io/split/client/impressions/ImpressionHasherTest.java

@@ -67,7 +67,7 @@ public void doesNotCrash() {
 
         imp1.treatment = null;
         assertNotNull(ImpressionHasher.process(imp1));
-        
+
         assertNull(ImpressionHasher.process(null));
     }
 }