LoggentriesAppender: memory efficient concatenation for layout.ignoresThrowable()

src/main/java/com/logentries/log4j/LogentriesAppender.java

@@ -152,22 +152,41 @@ protected void append( LoggingEvent event) {
 		// Append stack trace if present and layout does not handle it
 		if (layout.ignoresThrowable()) {
 			String[] stack = event.getThrowableStrRep();
-			if (stack != null)
-			{
-				int len = stack.length;
-				formattedEvent += ", ";
-				for(int i = 0; i < len; i++)
-				{
-					formattedEvent += stack[i];
-					if(i < len - 1)
-						formattedEvent += "\u2028";
-				}
-			}
+			if (stack != null && stack.length > 0)
+				formattedEvent = appendFormatedEvent(formattedEvent, stack);
 		}
 
 		// Prepare to be queued
 		this.le_async.addLineToQueue(formattedEvent);
 	}
+	private static final String EXCEPTION_SEPARATOR = ", ";
+
+	static String appendFormatedEvent(final String formattedEvent, final String[] stack) {
+		final int len = stack.length;
+
+		// calculate buffer size
+		int buffSize = formattedEvent.length()
+		             + EXCEPTION_SEPARATOR.length()
+		             + len - 1
+		             ;
+		for (int i = 0; i < len; i++) {
+			buffSize += stack[i].length();
+		}
+
+		// concatenate 
+		final StringBuilder sb = new StringBuilder(buffSize);
+		sb.append(formattedEvent);
+		sb.append(EXCEPTION_SEPARATOR);
+
+		for (int i = 0; i < len; i++) {
+			sb.append(stack[i]);
+			if (i < len - 1) {
+				sb.append('\u2028');
+			}
+		}
+		// assert sb.capacity() == sb.length();
+		return sb.toString();
+	}
 
 	/**
 	 * Closes all connections to Logentries

src/test/java/com/logentries/log4j/LogentriesAppenderTest.java

@@ -1,5 +1,6 @@
 package com.logentries.log4j;
 
+import static com.logentries.log4j.LogentriesAppender.*;
 import static org.junit.Assert.*;
 
 import org.junit.Test;
@@ -26,4 +27,8 @@ public void settersTest() {
 		assertEquals(le.le_async.getSsl(),true);
 	}
 
+	@Test
+	public void testAppendFormatedEvent() {
+	    assertEquals("foo, a\u2028b\u2028c", appendFormatedEvent("foo", new String[]{"a","b", "c"}));    
+	}
 }