Relates to change proposed by QFJ-421.

No option to attach patch, so will try in-lining:

Index: core/src/main/java/quickfix/Session.java
===================================================================
— core/src/main/java/quickfix/Session.java (revision 926)
+++ core/src/main/java/quickfix/Session.java (working copy)
@@ -27,6 +27,8 @@
import java.util.HashMap;
import java.util.List;
import java.util.Map;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;

import quickfix.Message.Header;
import quickfix.field.BeginSeqNo;
@@ -231,7 +233,7 @@

private boolean enabled;

• private final String responderSync = "SessionResponderSync";
  + private final Lock responderSync = new ReentrantLock(true);
  // @GuardedBy(responderSync)
  private Responder responder;

@@ -330,19 +332,25 @@

• @param responder a responder implementation
  */
  public void setResponder(Responder responder) {

• synchronized (responderSync) {
  + responderSync.lock();
  + try
  Unknown macro: { this.responder = responder; if (responder != null) { stateListener.onConnect(); } else { stateListener.onDisconnect(); }+ }

  finally

  { + responderSync.unlock(); }
  }
  
  public Responder getResponder() {
  - synchronized (responderSync) {
  + responderSync.lock();
  + try { // FIXME why this???? return responder; + } finally {+ responderSync.unlock(); }

  }

@@ -1436,13 +1444,16 @@

• @throws IOException IO error
  */
  public void disconnect() throws IOException {

• synchronized (responderSync) {
  + responderSync.lock();
  + try
  Unknown macro: { if (!hasResponder()) { return; } getLog().onEvent("Disconnecting"); responder.disconnect(); setResponder(null);+ }

  finally

  { + responderSync.unlock(); }
  
  boolean logonReceived = state.isLogonReceived();
  @@ -1778,13 +1789,16 @@
  
  private boolean send(String messageString) {
  getLog().onOutgoing(messageString);
  - synchronized (responderSync) {
  + responderSync.lock();
  + try {
  if (!hasResponder()) { getLog().onEvent( "Attempt to send while not connected (message stored until connected)."); return false; }
  return getResponder().send(messageString);
  + } finally {+ responderSync.unlock(); }

  }

Attached QFJ-484.patch based on using a re-entrant lock as discussed above and the change recommended by Parwinder Sekhon at http://www.quickfixj.org/jira/browse/QFJ-433?focusedCommentId=11014&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_11014

Parwdiner's change is critical when using synchronous writes when you have multiple sessions running.

Copy of patch as applied to 1_5_0 Session source.

committed on integration branch rev #1024

After discussion, the usage of Reentrant Lock would likely cause some throughput/latency performance.
Also, the scenario that lead to that issue is not a common usecase (very slow file writing operation).
However, it highlighted a regression following the issue QFJ-421 which has been reopened.