ListAppender: Synchronize on list while iterating

* Prevent ConcurrentModificationException while making snapshots
* Returned snapshots no longer require an unmodifiable view

Author: bjmi

log4j-core-test/src/main/java/org/apache/logging/log4j/core/test/appender/ListAppender.java

@@ -195,14 +195,18 @@ public ListAppender clear() {
         return this;
     }
 
-    /** Returns an immutable snapshot of captured log events */
+    /** Returns a snapshot of captured log events */
     public List<LogEvent> getEvents() {
-        return Collections.<LogEvent>unmodifiableList(new ArrayList<>(events));
+        synchronized (events) {
+            return new ArrayList<>(events);
+        }
     }
 
-    /** Returns an immutable snapshot of captured messages */
+    /** Returns a snapshot of captured messages */
     public List<String> getMessages() {
-        return Collections.<String>unmodifiableList(new ArrayList<>(messages));
+        synchronized (messages) {
+            return new ArrayList<>(messages);
+        }
     }
 
     /**
@@ -215,9 +219,11 @@ public List<String> getMessages(final int minSize, final long timeout, final Tim
         return getMessages();
     }
 
-    /** Returns an immutable snapshot of captured data */
+    /** Returns a snapshot of captured data */
     public List<byte[]> getData() {
-        return Collections.<byte[]>unmodifiableList(new ArrayList<>(data));
+        synchronized (data) {
+            return new ArrayList<>(data);
+        }
     }
 
     public static ListAppender createAppender(
@@ -297,9 +303,9 @@ public static ListAppender getListAppender(final String name) {
 
     @Override
     public String toString() {
-        return "ListAppender [events=" + events + ", messages=" + messages + ", data=" + data + ", newLine=" + newLine
-                + ", raw=" + raw + ", countDownLatch=" + countDownLatch + ", getHandler()=" + getHandler()
-                + ", getLayout()=" + getLayout() + ", getName()=" + getName() + ", ignoreExceptions()="
+        return "ListAppender [events=" + getEvents() + ", messages=" + getMessages() + ", data=" + getData()
+                + ", newLine=" + newLine + ", raw=" + raw + ", countDownLatch=" + countDownLatch + ", getHandler()="
+                + getHandler() + ", getLayout()=" + getLayout() + ", getName()=" + getName() + ", ignoreExceptions()="
                 + ignoreExceptions() + ", getFilter()=" + getFilter() + ", getState()=" + getState() + "]";
     }
 }