diff --git a/src/main/java/com/featureprobe/sdk/example/FeatureProbeDemo.java b/src/main/java/com/featureprobe/sdk/example/FeatureProbeDemo.java
index 1e43a2c..61ceeda 100644
--- a/src/main/java/com/featureprobe/sdk/example/FeatureProbeDemo.java
+++ b/src/main/java/com/featureprobe/sdk/example/FeatureProbeDemo.java
@@ -32,8 +32,9 @@ public static void main(String[] args) throws MalformedURLException, Interrupted
         final FeatureProbe fpClient = new FeatureProbe(FEATURE_PROBE_SERVER_SDK_KEY, config);
 
         // Create one user.
-        FPUser user = new FPUser("00001")   // key is for percentage rollout, normally use userId as key
-            .with("userId", "00001");            // "userId" is used in rules, should be filled in.
+        FPUser user = new FPUser()
+            .stableRollout("00001")          // key is for percentage rollout, normally use userId as key
+            .with("userId", "00001");        // "userId" is used in rules, should be filled in.
 
         // Get toggle result for this user.
         final String YOUR_TOGGLE_KEY = "campaign_allow_list";
diff --git a/src/main/java/com/featureprobe/sdk/server/FPUser.java b/src/main/java/com/featureprobe/sdk/server/FPUser.java
index 18c77a2..8990c48 100644
--- a/src/main/java/com/featureprobe/sdk/server/FPUser.java
+++ b/src/main/java/com/featureprobe/sdk/server/FPUser.java
@@ -1,5 +1,4 @@
 package com.featureprobe.sdk.server;
-
 import java.util.HashMap;
 import java.util.Map;
 
@@ -13,14 +12,29 @@ public class FPUser {
 
     private Map<String, String> attrs = new HashMap<>();
 
+    /**
+     * Creates a new FPUser
+     */
+    public FPUser() {}
+
     /**
      * Creates a new FPUser
      * @param key user unique id for percentage rollout
      */
+    @Deprecated
     public FPUser(String key) {
         this.key = key;
     }
 
+    /**
+     * Set user unique id for percentage rollout
+     * @param key user unique id for percentage rollout
+     */
+    public FPUser stableRollout(String key) {
+        this.key = key;
+        return this;
+    }
+
     /**
      * Add an attribute to the user
      * @param name attribute name
diff --git a/src/main/java/com/featureprobe/sdk/server/model/Split.java b/src/main/java/com/featureprobe/sdk/server/model/Split.java
index ec22028..163fbcb 100644
--- a/src/main/java/com/featureprobe/sdk/server/model/Split.java
+++ b/src/main/java/com/featureprobe/sdk/server/model/Split.java
@@ -35,7 +35,7 @@ public Split(List<List<List<Integer>>> distribution) {
     }
 
     public HitResult findIndex(FPUser user, String toggleKey) {
-        String hashKey = user.getKey();
+        String hashKey = user.getKey() != null ? user.getKey() : String.valueOf(System.nanoTime());
         if (StringUtils.isNotBlank(bucketBy)) {
             if (user.containAttr(bucketBy)) {
                 hashKey = user.getAttr(bucketBy);
diff --git a/src/test/groovy/com/featureprobe/sdk/server/AccessRecorderSpec.groovy b/src/test/groovy/com/featureprobe/sdk/server/AccessRecorderSpec.groovy
index 253666d..e99c235 100644
--- a/src/test/groovy/com/featureprobe/sdk/server/AccessRecorderSpec.groovy
+++ b/src/test/groovy/com/featureprobe/sdk/server/AccessRecorderSpec.groovy
@@ -9,7 +9,7 @@ class AccessRecorderSpec extends Specification {
 
     def setup() {
         accessRecorder = new AccessRecorder()
-        FPUser user = new FPUser("test_user")
+        FPUser user = new FPUser().stableRollout("test_user")
         event = new AccessEvent(System.currentTimeMillis(), user, "test_toggle", "true", 1, 0)
     }
 
diff --git a/src/test/groovy/com/featureprobe/sdk/server/ConditionSpec.groovy b/src/test/groovy/com/featureprobe/sdk/server/ConditionSpec.groovy
index 8bb38f8..bbedcb5 100644
--- a/src/test/groovy/com/featureprobe/sdk/server/ConditionSpec.groovy
+++ b/src/test/groovy/com/featureprobe/sdk/server/ConditionSpec.groovy
@@ -16,7 +16,7 @@ class ConditionSpec extends Specification {
         condition = new Condition()
         condition.setType(ConditionType.STRING)
         condition.setSubject("userId")
-        user = new FPUser("test_user")
+        user = new FPUser().stableRollout("test_user")
         segments = ["test_project\$test_segment": new Segment(uniqueId: "test_project\$test_segment", version: 1,
                 rules: [new SegmentRule(conditions: [new Condition(type: ConditionType.STRING, subject: "userId",
                         predicate: PredicateType.IS_ONE_OF, objects: ["1", "2"])])])]
diff --git a/src/test/groovy/com/featureprobe/sdk/server/FeatureProbeSpec.groovy b/src/test/groovy/com/featureprobe/sdk/server/FeatureProbeSpec.groovy
index 649d948..65ec0f9 100644
--- a/src/test/groovy/com/featureprobe/sdk/server/FeatureProbeSpec.groovy
+++ b/src/test/groovy/com/featureprobe/sdk/server/FeatureProbeSpec.groovy
@@ -63,7 +63,7 @@ class FeatureProbeSpec extends Specification {
                 def caseName = testCase.get("name").asText()
                 println("starting execute scenario : " + name + ",case : " + caseName)
                 def userCase = testCase.get("user")
-                FPUser user = new FPUser(userCase.get("key").asText())
+                FPUser user = new FPUser().stableRollout(userCase.get("key").asText())
                 def customValues = userCase.get("customValues").asList()
                 for (int x = 0; x < customValues.size(); x++) {
                     def customValue = customValues.get(x)
diff --git a/src/test/groovy/com/featureprobe/sdk/server/SplitSpec.groovy b/src/test/groovy/com/featureprobe/sdk/server/SplitSpec.groovy
index 20f4878..9b49e4b 100644
--- a/src/test/groovy/com/featureprobe/sdk/server/SplitSpec.groovy
+++ b/src/test/groovy/com/featureprobe/sdk/server/SplitSpec.groovy
@@ -10,7 +10,7 @@ class SplitSpec extends Specification {
 
     def setup() {
         split = new Split([[[0, 5000]], [[5000, 10000]]])
-        user = new FPUser("test_user_key")
+        user = new FPUser().stableRollout("test_user_key")
     }
 
     def "Get user group"() {