Improve writing XML elements to socket

Instead of allocating one big continuous memory block before "writing"
the XMPP stream element to the socket, write the single CharSequences of
LazyStringBuilder/XmlStringBuilder.

Also change Obserable writer to only notify the listeners after a
flush *or* if a certain limit has been reached since the last
notification. Otherwise the debugger would "print" every single XML part
as result of this change.

smack-core/src/main/java/org/jivesoftware/smack/util/LazyStringBuilder.java

@@ -17,6 +17,7 @@
 package org.jivesoftware.smack.util;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 public class LazyStringBuilder implements Appendable, CharSequence {
@@ -105,4 +106,17 @@ public class LazyStringBuilder implements Appendable, CharSequence {
         }
         return cache;
     }
+
+    /**
+     * Get the List of CharSequences representation of this instance. The list is unmodifiable. If
+     * the resulting String was already cached, a list with a single String entry will be returned.
+     *
+     * @return a List of CharSequences representing this instance.
+     */
+    public List<CharSequence> getAsList() {
+        if (cache != null) {
+            return Collections.singletonList((CharSequence) cache);
+        }
+        return Collections.unmodifiableList(list);
+    }
 }

smack-core/src/main/java/org/jivesoftware/smack/util/ObservableWriter.java

@@ -28,9 +28,11 @@ import java.util.List;
  * @author Gaston Dombiak
  */
 public class ObservableWriter extends Writer {
+    private static final int MAX_STRING_BUILDER_SIZE = 4096;
 
     Writer wrappedWriter = null;
     List<WriterListener> listeners = new ArrayList<WriterListener>();
+    private final StringBuilder stringBuilder = new StringBuilder(MAX_STRING_BUILDER_SIZE);
 
     public ObservableWriter(Writer wrappedWriter) {
         this.wrappedWriter = wrappedWriter;
@@ -39,10 +41,11 @@ public class ObservableWriter extends Writer {
     public void write(char[] cbuf, int off, int len) throws IOException {
         wrappedWriter.write(cbuf, off, len);
         String str = new String(cbuf, off, len);
-        notifyListeners(str);
+        maybeNotifyListeners(str);
     }
 
     public void flush() throws IOException {
+        notifyListeners();
         wrappedWriter.flush();
     }
 
@@ -57,18 +60,25 @@ public class ObservableWriter extends Writer {
     public void write(char[] cbuf) throws IOException {
         wrappedWriter.write(cbuf);
         String str = new String(cbuf);
-        notifyListeners(str);
+        maybeNotifyListeners(str);
     }
 
     public void write(String str) throws IOException {
         wrappedWriter.write(str);
-        notifyListeners(str);
+        maybeNotifyListeners(str);
     }
 
     public void write(String str, int off, int len) throws IOException {
         wrappedWriter.write(str, off, len);
         str = str.substring(off, off + len);
-        notifyListeners(str);
+        maybeNotifyListeners(str);
+    }
+
+    private void maybeNotifyListeners(String s) {
+        stringBuilder.append(s);
+        if (stringBuilder.length() > MAX_STRING_BUILDER_SIZE) {
+            notifyListeners();
+        }
     }
 
     /**
@@ -76,12 +86,14 @@ public class ObservableWriter extends Writer {
      * 
      * @param str the written String to notify 
      */
-    private void notifyListeners(String str) {
+    private void notifyListeners() {
         WriterListener[] writerListeners = null;
         synchronized (listeners) {
             writerListeners = new WriterListener[listeners.size()];
             listeners.toArray(writerListeners);
         }
+        String str = stringBuilder.toString();
+        stringBuilder.setLength(0);
         for (int i = 0; i < writerListeners.length; i++) {
             writerListeners[i].write(str);
         }

smack-core/src/main/java/org/jivesoftware/smack/util/XmlStringBuilder.java

@@ -16,6 +16,8 @@
  */
 package org.jivesoftware.smack.util;
 
+import java.io.IOException;
+import java.io.Writer;
 import java.util.Collection;
 import java.util.Date;
 
@@ -459,4 +461,23 @@ public class XmlStringBuilder implements Appendable, CharSequence {
     public int hashCode() {
         return toString().hashCode();
     }
+
+    /**
+     * Write the contents of this <code>XmlStringBuilder</code> to a {@link Writer}. This will write
+     * the single parts one-by-one, avoiding allocation of a big continuous memory block holding the
+     * XmlStringBuilder contents.
+     *
+     * @param writer
+     * @throws IOException
+     */
+    public void write(Writer writer) throws IOException {
+        for (CharSequence csq : sb.getAsList()) {
+            if (csq instanceof XmlStringBuilder) {
+                ((XmlStringBuilder) csq).write(writer);
+            }
+            else {
+                writer.write(csq.toString());
+            }
+        }
+    }
 }

smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java

@@ -73,6 +73,7 @@ import org.jivesoftware.smack.util.Async;
 import org.jivesoftware.smack.util.PacketParserUtils;
 import org.jivesoftware.smack.util.StringUtils;
 import org.jivesoftware.smack.util.TLSUtils;
+import org.jivesoftware.smack.util.XmlStringBuilder;
 import org.jivesoftware.smack.util.dns.HostAddress;
 import org.jxmpp.jid.impl.JidCreate;
 import org.jxmpp.jid.parts.Resourcepart;
@@ -1316,7 +1317,15 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
                             throw new IllegalStateException(e);
                         }
                     }
-                    writer.write(element.toXML().toString());
+
+                    CharSequence elementXml = element.toXML();
+                    if (elementXml instanceof XmlStringBuilder) {
+                        ((XmlStringBuilder) elementXml).write(writer);
+                    }
+                    else {
+                        writer.write(elementXml.toString());
+                    }
+
                     if (queue.isEmpty()) {
                         writer.flush();
                     }