Consider the StringBuilder's capacity instead of content length when being trimmed

log4j-api/src/main/java/org/apache/logging/log4j/util/StringBuilders.java

@@ -153,7 +153,7 @@ public static boolean equalsIgnoreCase(final CharSequence left, final int leftOf
      * @since 2.9
      */
     public static void trimToMaxSize(final StringBuilder stringBuilder, final int maxSize) {
-        if (stringBuilder != null && stringBuilder.length() > maxSize) {
+        if (stringBuilder != null && stringBuilder.capacity() > maxSize) {
             stringBuilder.setLength(maxSize);
             stringBuilder.trimToSize();
         }

log4j-api/src/test/java/org/apache/logging/log4j/util/StringBuildersTest.java

@@ -34,4 +34,18 @@ public void trimToMaxSize() throws Exception {
         StringBuilders.trimToMaxSize(sb, Constants.MAX_REUSABLE_MESSAGE_SIZE);
         assertTrue("trimmed OK", sb.length() <= Constants.MAX_REUSABLE_MESSAGE_SIZE);
     }
+
+    @Test
+    public void trimToMaxSizeWithLargeCapacity() throws Exception {
+        final StringBuilder sb = new StringBuilder();
+        final char[] value = new char[4 * 1024];
+        sb.append(value);
+        sb.setLength(0);
+
+        assertTrue("needs trimming", sb.capacity() > Constants.MAX_REUSABLE_MESSAGE_SIZE);
+        StringBuilders.trimToMaxSize(sb, Constants.MAX_REUSABLE_MESSAGE_SIZE);
+        assertTrue("trimmed OK", sb.capacity() <= Constants.MAX_REUSABLE_MESSAGE_SIZE);
+    }
+
+
 }

log4j-core/src/test/java/org/apache/logging/log4j/core/layout/AbstractStringLayoutTest.java

@@ -47,26 +47,31 @@ public String toSerializable(final LogEvent event) {
     @Test
     public void testGetStringBuilderCapacityRestrictedToMax() throws Exception {
         final StringBuilder sb = ConcreteStringLayout.getStringBuilder();
+        int initialCapacity = sb.capacity();
         assertEquals("initial capacity", ConcreteStringLayout.DEFAULT_STRING_BUILDER_SIZE, sb.capacity());
 
-        final int MEDIUM = ConcreteStringLayout.DEFAULT_STRING_BUILDER_SIZE + 100;
-        final String mediumMessage = new String(new char[MEDIUM]);
+        final int SMALL = 100;
+        final String mediumMessage = new String(new char[SMALL]);
         sb.append(mediumMessage);
-        final int GROWN = sb.capacity();
-        assertTrue("capacity has grown", GROWN >= MEDIUM);
-        assertEquals("length=msg length", MEDIUM, sb.length());
+        assertTrue("capacity not grown", sb.capacity() == initialCapacity);
+        assertEquals("length=msg length", SMALL, sb.length());
 
-        final int LARGE = 4096;
-        final String largeMessage = new String(new char[LARGE]);
         final StringBuilder sb2 = ConcreteStringLayout.getStringBuilder();
-        assertEquals("resized capacity", GROWN, sb2.capacity());
+        assertEquals("capacity unchanged", sb2.capacity(), initialCapacity);
         assertEquals("empty, ready for use", 0, sb2.length());
+
+        final int LARGE = ConcreteStringLayout.MAX_STRING_BUILDER_SIZE * 2;
+        final String largeMessage = new String(new char[LARGE]);
         sb2.append(largeMessage);
         assertTrue("capacity grown to fit msg length", sb2.capacity() >= LARGE);
+        assertTrue("capacity is now greater than max length", sb2.capacity() >= ConcreteStringLayout.MAX_STRING_BUILDER_SIZE);
         assertEquals("length=msg length", LARGE, sb2.length());
+        sb2.setLength(0); // set 0 before next getStringBuilder
+        assertEquals("empty, cleared", 0, sb2.length());
+        assertTrue("capacity remains very large", sb2.capacity() >= ConcreteStringLayout.MAX_STRING_BUILDER_SIZE);
 
         final StringBuilder next = ConcreteStringLayout.getStringBuilder();
-        assertEquals("max capacity", ConcreteStringLayout.MAX_STRING_BUILDER_SIZE, next.capacity());
+        assertEquals("capacity, trimmed to MAX_STRING_BUILDER_SIZE", ConcreteStringLayout.MAX_STRING_BUILDER_SIZE, next.capacity());
         assertEquals("empty, ready for use", 0, next.length());
     }
 }