diff --git a/pom.xml b/pom.xml
index 7c3909e..49b7467 100644
--- a/pom.xml
+++ b/pom.xml
@@ -4,7 +4,7 @@
     <groupId>io.prismic</groupId>
     <artifactId>java-kit</artifactId>
     <packaging>jar</packaging>
-    <version>2.0.0</version>
+    <version>2.1.0-SNAPSHOT</version>
     <name>java-kit</name>
     <description>The developer kit to access Prismic.io repositories using the Java language.</description>
     <url>http://maven.apache.org</url>
diff --git a/src/main/java/io/prismic/Api.java b/src/main/java/io/prismic/Api.java
index bdc8d7b..738db19 100644
--- a/src/main/java/io/prismic/Api.java
+++ b/src/main/java/io/prismic/Api.java
@@ -8,6 +8,7 @@
 import java.net.Proxy;
 import java.util.*;
 import java.util.Map.Entry;
+import java.util.concurrent.CompletableFuture;
 
 /**
  * Embodies an API endpoint, from which it is possible to make queries.
@@ -92,7 +93,7 @@ public static Api get(String endpoint, String accessToken, String defaultReferen
     JsonNode json = cache.getOrSet(
       url,
       5000L,
-      () -> HttpClient.fetch(url, logger, null, proxy)
+      CompletableFuture.supplyAsync(() -> HttpClient.fetch(url, logger, null, proxy))
     );
 
     ApiData apiData = ApiData.parse(json);
diff --git a/src/main/java/io/prismic/Cache.java b/src/main/java/io/prismic/Cache.java
index 3a2bb17..249caf0 100644
--- a/src/main/java/io/prismic/Cache.java
+++ b/src/main/java/io/prismic/Cache.java
@@ -3,13 +3,34 @@
 import com.fasterxml.jackson.databind.JsonNode;
 import org.apache.commons.collections4.map.LRUMap;
 
+import java.util.Map;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+
+import static java.util.Collections.synchronizedMap;
+
 public interface Cache {
 
   void set(String key, Long ttl, JsonNode response);
+
   JsonNode get(String key);
-  JsonNode getOrSet(String key, Long ttl, Callback f);
+
+  default JsonNode getOrSet(String key, Long ttl, Future<JsonNode> callback) {
+    JsonNode found = this.get(key);
+    if (found != null) {
+      return found;
+    }
+    try {
+      JsonNode json = callback.get();
+      this.set(key, ttl, json);
+      return json;
+    } catch (InterruptedException | ExecutionException e) {
+      throw new RuntimeException(e);
+    }
+  }
 
   // --
+
   class NoCache implements Cache {
 
     @Override
@@ -21,39 +42,32 @@ public JsonNode get(String key) {
       return null;
     }
 
-    @Override
-    public JsonNode getOrSet(String key, Long ttl, Callback f) {
-      return f.execute();
-    }
-
   }
 
   // --
 
   class DefaultCache {
 
-    private static final Cache defaultCache = new BuiltInCache(999);
+    private static final Cache DEFAULT_CACHE = new BuiltInCache(999);
 
-    private DefaultCache() {}
+    private DefaultCache() {
+    }
 
     public static Cache getInstance() {
-      return defaultCache;
+      return DEFAULT_CACHE;
     }
   }
 
   // --
 
-  interface Callback {
-    JsonNode execute();
-  }
-
   class BuiltInCache implements Cache {
 
-    private final java.util.Map<String, Entry> cache;
+    private final Map<String, Entry> cache;
 
     static class Entry {
       public final Long expiration;
       public final JsonNode value;
+
       public Entry(Long expiration, JsonNode value) {
         this.expiration = expiration;
         this.value = value;
@@ -61,7 +75,7 @@ public Entry(Long expiration, JsonNode value) {
     }
 
     public BuiltInCache(int maxDocuments) {
-      this.cache = java.util.Collections.synchronizedMap(new LRUMap<String, Entry>(maxDocuments));
+      this.cache = synchronizedMap(new LRUMap<String, Entry>(maxDocuments));
     }
 
     @Override
@@ -80,21 +94,9 @@ public void set(String key, Long ttl, JsonNode response) {
       this.cache.put(key, new Entry(expiration, response));
     }
 
-    @Override
-    public JsonNode getOrSet(String key, Long ttl, Callback f) {
-      JsonNode found = this.get(key);
-      if(found == null) {
-        JsonNode json = f.execute();
-        this.set(key, ttl, json);
-        return json;
-      } else {
-        return found;
-      }
-    }
-
     private Boolean isExpired(String key) {
       Entry entry = this.cache.get(key);
-      return entry != null && entry.expiration !=0 && entry.expiration < System.currentTimeMillis();
+      return entry != null && entry.expiration != 0 && entry.expiration < System.currentTimeMillis();
     }
 
   }
diff --git a/src/main/java/io/prismic/Callback.java b/src/main/java/io/prismic/Callback.java
deleted file mode 100644
index 896f5a7..0000000
--- a/src/main/java/io/prismic/Callback.java
+++ /dev/null
@@ -1,5 +0,0 @@
-package io.prismic;
-
-public interface Callback {
-    void execute();
-}
diff --git a/src/test/java/io/prismic/AppTest.java b/src/test/java/io/prismic/AppTest.java
index 3221300..f88dcdb 100644
--- a/src/test/java/io/prismic/AppTest.java
+++ b/src/test/java/io/prismic/AppTest.java
@@ -4,6 +4,7 @@
 import io.netty.handler.codec.http.HttpObject;
 import io.netty.handler.codec.http.HttpRequest;
 import io.netty.handler.codec.http.HttpResponse;
+import org.apache.commons.lang3.exception.ExceptionUtils;
 import org.junit.Assert;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -51,9 +52,17 @@ public void apiIsInitialized() {
   /**
    * Make sure a call to a private repository without a token returns the expected error
    */
-  @Test(expected = Api.Error.class)
+  @Test
   public void invalidToken() {
-    Api api = Api.get("https://private-test.prismic.io/api");
+    Throwable thrown = null;
+    try {
+      Api.get("https://private-test.prismic.io/api");
+    } catch (Throwable t) {
+      thrown = t;
+    }
+    Throwable rootCause = ExceptionUtils.getRootCause(thrown);
+    Assert.assertEquals(Api.Error.class, rootCause.getClass());
+    Assert.assertEquals("Invalid access token", rootCause.getMessage());
   }
 
   /**