[improve][broker] Improve performance of StatsOutputStream by using ArrayDeque instead of Stack (apache#20808)

Author: JooHyukKim

pulsar-broker/src/main/java/org/apache/pulsar/utils/StatsOutputStream.java

@@ -19,52 +19,53 @@
 package org.apache.pulsar.utils;
 
 import io.netty.buffer.ByteBuf;
-import java.util.Stack;
+import java.util.ArrayDeque;
+import java.util.Deque;
 import org.apache.pulsar.common.util.SimpleTextOutputStream;
 
 public class StatsOutputStream extends SimpleTextOutputStream {
-    private final Stack<Boolean> separators = new Stack<>();
+    private final Deque<Boolean> separators = new ArrayDeque<>();
 
     public StatsOutputStream(ByteBuf buffer) {
         super(buffer);
     }
 
     public StatsOutputStream startObject() {
         checkSeparator();
-        separators.push(Boolean.FALSE);
+        separators.addLast(Boolean.FALSE);
         write('{');
         return this;
     }
 
     public StatsOutputStream startObject(String key) {
         checkSeparator();
         write('"').writeEncoded(key).write("\":{");
-        separators.push(Boolean.FALSE);
+        separators.addLast(Boolean.FALSE);
         return this;
     }
 
     public StatsOutputStream endObject() {
-        separators.pop();
+        separators.removeLast();
         write('}');
         return this;
     }
 
     public StatsOutputStream startList() {
         checkSeparator();
-        separators.push(Boolean.FALSE);
+        separators.addLast(Boolean.FALSE);
         write('[');
         return this;
     }
 
     public StatsOutputStream startList(String key) {
         checkSeparator();
         write('"').writeEncoded(key).write("\":[");
-        separators.push(Boolean.FALSE);
+        separators.addLast(Boolean.FALSE);
         return this;
     }
 
     public StatsOutputStream endList() {
-        separators.pop();
+        separators.removeLast();
         write(']');
         return this;
     }
@@ -121,10 +122,11 @@ StatsOutputStream writeItem(String s) {
     private void checkSeparator() {
         if (separators.isEmpty()) {
             return;
-        } else if (separators.peek() == Boolean.TRUE) {
+        } else if (separators.peekLast() == Boolean.TRUE) {
             write(",");
         } else {
-            separators.set(separators.size() - 1, Boolean.TRUE);
+            separators.pollLast();
+            separators.addLast(Boolean.TRUE);
         }
     }
 }

pulsar-broker/src/test/java/org/apache/pulsar/utils/StatsOutputStreamTest.java

@@ -150,4 +150,26 @@ public String str() {
         reset();
         return s;
     }
+
+    @Test
+    public void testBehaviorOfStatsOutputStreamWithDeque() {
+        // Create a byte buffer for collecting output
+        ByteBuf buffer = Unpooled.buffer();
+
+        // Create an instance of StatsOutputStream using Deque
+        StatsOutputStream output = new StatsOutputStream(buffer);
+        output.startObject()
+                .writePair("name", "test")
+                .startList("items")
+                .writeItem(true)
+                .writeItem(123L)
+                .writeItem("sample")
+                .endList()
+                .endObject();
+        
+        // Assert
+        assertEquals(buffer.toString(java.nio.charset.StandardCharsets.UTF_8), 
+                "{\"name\":\"test\",\"items\":[true,123,\"sample\"]}");
+    }
+
 }