fix #88: use CopyOnWriteArrayList for thread-safe message list

src/main/java/org/apache/maven/shared/io/logging/DefaultMessageHolder.java

@@ -20,17 +20,17 @@
 
 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.
  *
  */
 public class DefaultMessageHolder implements MessageHolder {
 
-    private List<Message> messages = new ArrayList<>();
+    private List<Message> messages = new CopyOnWriteArrayList<>();
 
     private Message currentMessage;
 

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();
+    }
 }