diff --git a/quickfixj-core/src/main/java/quickfix/SocketAcceptor.java b/quickfixj-core/src/main/java/quickfix/SocketAcceptor.java
index 49f27d7f15..23a2769448 100644
--- a/quickfixj-core/src/main/java/quickfix/SocketAcceptor.java
+++ b/quickfixj-core/src/main/java/quickfix/SocketAcceptor.java
@@ -19,6 +19,7 @@
 
 package quickfix;
 
+import java.util.concurrent.atomic.AtomicBoolean;
 import quickfix.mina.EventHandlingStrategy;
 import quickfix.mina.SingleThreadedEventHandlingStrategy;
 import quickfix.mina.acceptor.AbstractSocketAcceptor;
@@ -28,7 +29,7 @@
  * sessions.
  */
 public class SocketAcceptor extends AbstractSocketAcceptor {
-    private volatile Boolean isStarted = Boolean.FALSE;
+    private final AtomicBoolean isStarted = new AtomicBoolean(false);
     private final SingleThreadedEventHandlingStrategy eventHandlingStrategy;
 
     private SocketAcceptor(Builder builder) throws ConfigError {
@@ -103,30 +104,30 @@ public void start() throws ConfigError, RuntimeError {
     }
 
     private void initialize() throws ConfigError {
-        if (isStarted.equals(Boolean.FALSE)) {
-            eventHandlingStrategy.setExecutor(longLivedExecutor);
-            startAcceptingConnections();
-            eventHandlingStrategy.blockInThread();
-            isStarted = Boolean.TRUE;
-        } else {
-            log.warn("Ignored attempt to start already running SocketAcceptor.");
+        synchronized (isStarted) {
+            if (isStarted.compareAndSet(false, true)) {
+                eventHandlingStrategy.setExecutor(longLivedExecutor);
+                startAcceptingConnections();
+                eventHandlingStrategy.blockInThread();
+            }
         }
     }
 
     @Override
     public void stop(boolean forceDisconnect) {
-        if (isStarted.equals(Boolean.TRUE)) {
-            try {
-                logoutAllSessions(forceDisconnect);
-                stopAcceptingConnections();
-                stopSessionTimer();
-            } finally {
+        synchronized (isStarted) {
+            if (isStarted.compareAndSet(true, false)) {
                 try {
-                    eventHandlingStrategy.stopHandlingMessages(true);
+                    logoutAllSessions(forceDisconnect);
+                    stopAcceptingConnections();
+                    stopSessionTimer();
                 } finally {
-                    Session.unregisterSessions(getSessions(), true);
-                    clearConnectorSessions();
-                    isStarted = Boolean.FALSE;
+                    try {
+                        eventHandlingStrategy.stopHandlingMessages(true);
+                    } finally {
+                        Session.unregisterSessions(getSessions(), true);
+                        clearConnectorSessions();
+                    }
                 }
             }
         }
diff --git a/quickfixj-core/src/main/java/quickfix/SocketInitiator.java b/quickfixj-core/src/main/java/quickfix/SocketInitiator.java
index f7c7389cbd..7da2f1b3f7 100644
--- a/quickfixj-core/src/main/java/quickfix/SocketInitiator.java
+++ b/quickfixj-core/src/main/java/quickfix/SocketInitiator.java
@@ -19,6 +19,7 @@
 
 package quickfix;
 
+import java.util.concurrent.atomic.AtomicBoolean;
 import quickfix.mina.EventHandlingStrategy;
 import quickfix.mina.SingleThreadedEventHandlingStrategy;
 import quickfix.mina.initiator.AbstractSocketInitiator;
@@ -28,7 +29,7 @@
  * sessions.
  */
 public class SocketInitiator extends AbstractSocketInitiator {
-    private volatile Boolean isStarted = Boolean.FALSE;
+    private final AtomicBoolean isStarted = new AtomicBoolean(false);
     private final SingleThreadedEventHandlingStrategy eventHandlingStrategy;
 
     private SocketInitiator(Builder builder) throws ConfigError {
@@ -120,33 +121,33 @@ public void start() throws ConfigError, RuntimeError {
     }
     
     private void initialize() throws ConfigError {
-        if (isStarted.equals(Boolean.FALSE)) {
-            eventHandlingStrategy.setExecutor(longLivedExecutor);
-            createSessionInitiators();
-            for (Session session : getSessionMap().values()) {
-                Session.registerSession(session);
+        synchronized (isStarted) {
+            if (isStarted.compareAndSet(false, true)) {
+                eventHandlingStrategy.setExecutor(longLivedExecutor);
+                createSessionInitiators();
+                for (Session session : getSessionMap().values()) {
+                    Session.registerSession(session);
+                }
+                startInitiators();
+                eventHandlingStrategy.blockInThread();
             }
-            startInitiators();
-            eventHandlingStrategy.blockInThread();
-            isStarted = Boolean.TRUE;
-        } else {
-            log.warn("Ignored attempt to start already running SocketInitiator.");
         }
     }
 
     @Override
     public void stop(boolean forceDisconnect) {
-        if (isStarted.equals(Boolean.TRUE)) {
-            try {
-                logoutAllSessions(forceDisconnect);
-                stopInitiators();
-            } finally {
+        synchronized (isStarted) {
+            if (isStarted.compareAndSet(true, false)) {
                 try {
-                    eventHandlingStrategy.stopHandlingMessages(true);
+                    logoutAllSessions(forceDisconnect);
+                    stopInitiators();
                 } finally {
-                    Session.unregisterSessions(getSessions(), true);
-                    clearConnectorSessions();
-                    isStarted = Boolean.FALSE;
+                    try {
+                        eventHandlingStrategy.stopHandlingMessages(true);
+                    } finally {
+                        Session.unregisterSessions(getSessions(), true);
+                        clearConnectorSessions();
+                    }
                 }
             }
         }
diff --git a/quickfixj-core/src/main/java/quickfix/mina/acceptor/AbstractSocketAcceptor.java b/quickfixj-core/src/main/java/quickfix/mina/acceptor/AbstractSocketAcceptor.java
index e20b76d84b..556a24c047 100644
--- a/quickfixj-core/src/main/java/quickfix/mina/acceptor/AbstractSocketAcceptor.java
+++ b/quickfixj-core/src/main/java/quickfix/mina/acceptor/AbstractSocketAcceptor.java
@@ -56,6 +56,8 @@
 import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
 /**
  * Abstract base class for socket acceptors.
@@ -64,7 +66,7 @@ public abstract class AbstractSocketAcceptor extends SessionConnector implements
     private final Map<SocketAddress, AcceptorSessionProvider> sessionProviders = new HashMap<>();
     private final SessionFactory sessionFactory;
     private final Map<SocketAddress, AcceptorSocketDescriptor> socketDescriptorForAddress = new HashMap<>();
-    private final Map<AcceptorSocketDescriptor, IoAcceptor> ioAcceptors = new HashMap<>();
+    private final ConcurrentMap<AcceptorSocketDescriptor, IoAcceptor> ioAcceptors = new ConcurrentHashMap<>();
 
     protected AbstractSocketAcceptor(SessionSettings settings, SessionFactory sessionFactory)
             throws ConfigError {
diff --git a/quickfixj-core/src/main/java/quickfix/mina/initiator/AbstractSocketInitiator.java b/quickfixj-core/src/main/java/quickfix/mina/initiator/AbstractSocketInitiator.java
index 09c07cbc6b..893c3cac1e 100644
--- a/quickfixj-core/src/main/java/quickfix/mina/initiator/AbstractSocketInitiator.java
+++ b/quickfixj-core/src/main/java/quickfix/mina/initiator/AbstractSocketInitiator.java
@@ -47,10 +47,10 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ThreadFactory;
@@ -63,7 +63,7 @@
 public abstract class AbstractSocketInitiator extends SessionConnector implements Initiator {
 
     protected final Logger log = LoggerFactory.getLogger(getClass());
-    private final Set<IoSessionInitiator> initiators = new HashSet<>();
+    private final Set<IoSessionInitiator> initiators = ConcurrentHashMap.newKeySet();
     private final ScheduledExecutorService scheduledReconnectExecutor;
     public static final String QFJ_RECONNECT_THREAD_PREFIX = "QFJ Reconnect Thread-";