Fix FlowRuleComparator violating the Comparator contract

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/FlowRuleComparator.java

@@ -37,9 +37,15 @@ public int compare(FlowRule o1, FlowRule o2) {
             return -1;
         }
 
-        if (o1.getLimitApp() == null) {
+        if (o1.getLimitApp() == null && o2.getLimitApp() == null) {
             return 0;
         }
+        if (o1.getLimitApp() == null) {
+            return 1;
+        }
+        if (o2.getLimitApp() == null) {
+            return -1;
+        }
 
         if (o1.getLimitApp().equals(o2.getLimitApp())) {
             return 0;
@@ -50,7 +56,7 @@ public int compare(FlowRule o1, FlowRule o2) {
         } else if (RuleConstant.LIMIT_APP_DEFAULT.equals(o2.getLimitApp())) {
             return -1;
         } else {
-            return 0;
+            return o1.getLimitApp().compareTo(o2.getLimitApp());
         }
     }
 

sentinel-core/src/test/java/com/alibaba/csp/sentinel/slots/block/flow/FlowRuleComparatorTest.java

@@ -42,4 +42,42 @@ private void assertOrderEqual(int size, List<FlowRule> expected, List<FlowRule>
             assertEquals(expected.get(i), actual.get(i));
         }
     }
+
+    @Test
+    public void testNullVsNonNullLimitApp() {
+        FlowRuleComparator comparator = new FlowRuleComparator();
+
+        // o1 has null limitApp, o2 has non-null limitApp
+        FlowRule ruleNull = new FlowRule("abc");
+        ruleNull.setLimitApp(null);
+        FlowRule ruleNonNull = new FlowRule("abc");
+        ruleNonNull.setLimitApp("originA");
+
+        // Comparator should NOT return 0 when one is null and the other is not
+        int result = comparator.compare(ruleNull, ruleNonNull);
+        assertNotEquals("null vs non-null should not be equal", 0, result);
+
+        // Symmetry: compare(o2, o1) should be -compare(o1, o2)
+        int reverseResult = comparator.compare(ruleNonNull, ruleNull);
+        assertEquals("comparator must be antisymmetric", -result, reverseResult);
+    }
+
+    @Test
+    public void testDifferentNonDefaultLimitApps() {
+        FlowRuleComparator comparator = new FlowRuleComparator();
+
+        // Two rules with different non-DEFAULT limitApp values
+        FlowRule ruleA = new FlowRule("abc");
+        ruleA.setLimitApp("originA");
+        FlowRule ruleB = new FlowRule("abc");
+        ruleB.setLimitApp("originB");
+
+        // Different apps should NOT compare as equal (0)
+        int result = comparator.compare(ruleA, ruleB);
+        assertNotEquals("different non-default apps should not be equal", 0, result);
+
+        // Symmetry check
+        int reverseResult = comparator.compare(ruleB, ruleA);
+        assertEquals("comparator must be antisymmetric", -result, reverseResult);
+    }
 }