From 0824f0d53b7931bf7a11391f7a47152e6d5e49a3 Mon Sep 17 00:00:00 2001
From: Elliotte Rusty Harold <elharo@ibiblio.org>
Date: Sun, 5 Jul 2026 11:49:28 +0000
Subject: [PATCH] fix #88: use CopyOnWriteArrayList for thread-safe message
 list

- Change the messages list from ArrayList to CopyOnWriteArrayList
- Add concurrency test for adding messages from multiple threads
---
 .../io/logging/DefaultMessageHolder.java      |  4 ++--
 .../io/logging/DefaultMessageHolderTest.java  | 22 +++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/src/main/java/org/apache/maven/shared/io/logging/DefaultMessageHolder.java b/src/main/java/org/apache/maven/shared/io/logging/DefaultMessageHolder.java
index 765e0c1..e97fd7f 100644
--- a/src/main/java/org/apache/maven/shared/io/logging/DefaultMessageHolder.java
+++ b/src/main/java/org/apache/maven/shared/io/logging/DefaultMessageHolder.java
@@ -20,9 +20,9 @@
 
 import java.io.PrintWriter;
 import java.io.StringWriter;
-import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * Default Message Holder.
@@ -30,7 +30,7 @@
  */
 public class DefaultMessageHolder implements MessageHolder {
 
-    private List<Message> messages = new ArrayList<>();
+    private List<Message> messages = new CopyOnWriteArrayList<>();
 
     private Message currentMessage;
 
diff --git a/src/test/java/org/apache/maven/shared/io/logging/DefaultMessageHolderTest.java b/src/test/java/org/apache/maven/shared/io/logging/DefaultMessageHolderTest.java
index 9186148..4827c3b 100644
--- a/src/test/java/org/apache/maven/shared/io/logging/DefaultMessageHolderTest.java
+++ b/src/test/java/org/apache/maven/shared/io/logging/DefaultMessageHolderTest.java
@@ -20,6 +20,11 @@
 
 import java.io.PrintWriter;
 import java.io.StringWriter;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 
 import org.junit.jupiter.api.Test;
 
@@ -302,4 +307,21 @@ void appendCharSequence() {
 
         assertTrue(mh.render().contains("This is a test"));
     }
+
+    @Test
+    void shouldAddMessagesConcurrently() throws Exception {
+        DefaultMessageHolder mh = new DefaultMessageHolder();
+        ExecutorService executor = Executors.newFixedThreadPool(4);
+        List<Future<?>> futures = new ArrayList<>();
+        for (int i = 0; i < 10; i++) {
+            futures.add(executor.submit(() -> {
+                mh.addMessage("concurrent message");
+            }));
+        }
+        for (Future<?> future : futures) {
+            future.get();
+        }
+        assertEquals(10, mh.size());
+        executor.shutdown();
+    }
 }