Index: src/main/java/org/apache/logging/log4j/message/FormattedMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/FormattedMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/FormattedMessage.java	(working copy)
@@ -46,12 +46,11 @@
         this.messagePattern = messagePattern;
         this.argArray = arguments;
         this.throwable = throwable;
+        getFormattedMessage(); // LOG4J2-763 take snapshot of parameters at message construction time
     }
 
     public FormattedMessage(final String messagePattern, final Object[] arguments) {
-        this.messagePattern = messagePattern;
-        this.argArray = arguments;
-        this.throwable = null;
+        this(messagePattern, arguments, null);
     }
 
     /**
@@ -60,9 +59,7 @@
      * @param arg The parameter.
      */
     public FormattedMessage(final String messagePattern, final Object arg) {
-        this.messagePattern = messagePattern;
-        this.argArray = new Object[] {arg};
-        this.throwable = null;
+        this(messagePattern, new Object[] {arg}, null);
     }
 
     /**
Index: src/main/java/org/apache/logging/log4j/message/LocalizedMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/LocalizedMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/LocalizedMessage.java	(working copy)
@@ -78,6 +78,7 @@
         this.baseName = baseName;
         this.resourceBundle = null;
         this.locale = locale;
+        getFormattedMessage(); // LOG4J2-763 take snapshot of parameters at message construction time
     }
 
     public LocalizedMessage(final ResourceBundle bundle, final Locale locale, final String key,
@@ -88,6 +89,7 @@
         this.baseName = null;
         this.resourceBundle = bundle;
         this.locale = locale;
+        getFormattedMessage(); // LOG4J2-763 take snapshot of parameters at message construction time
     }
 
     public LocalizedMessage(final Locale locale, final String key, final Object[] arguments) {
Index: src/main/java/org/apache/logging/log4j/message/MapMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/MapMessage.java	(revision 1616259)
+++ src/main/java/org/apache/logging/log4j/message/MapMessage.java	(working copy)
@@ -26,6 +26,11 @@
 
 /**
  * Represents a Message that consists of a Map.
+ * <p>
+ * Thread-safety note: the contents of this message can be modified after construction.
+ * When using asynchronous loggers and appenders it is not recommended to modify this message after the message is
+ * logged, because it is undefined whether the logged message string will contain the old values or the modified
+ * values.
  */
 public class MapMessage implements MultiformatMessage {
     /**
Index: src/main/java/org/apache/logging/log4j/message/MessageFormatMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/MessageFormatMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/MessageFormatMessage.java	(working copy)
@@ -49,6 +49,7 @@
         if (arguments != null && arguments.length > 0 && arguments[arguments.length - 1] instanceof Throwable) {
             this.throwable = (Throwable) arguments[arguments.length - 1];
         }
+        getFormattedMessage(); // LOG4J2-763 take snapshot of parameters at message construction time
     }
 
     /**
Index: src/main/java/org/apache/logging/log4j/message/ObjectMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/ObjectMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/ObjectMessage.java	(working copy)
@@ -29,6 +29,7 @@
     private static final long serialVersionUID = -5903272448334166185L;
 
     private transient Object obj;
+    private final String objectString;
 
     /**
      * Create the ObjectMessage.
@@ -39,6 +40,9 @@
             obj = "null";
         }
         this.obj = obj;
+        
+        // LOG4J2-763: take snapshot of parameters at message construction time
+        objectString = String.valueOf(obj);
     }
 
     /**
@@ -47,7 +51,7 @@
      */
     @Override
     public String getFormattedMessage() {
-        return obj.toString();
+        return objectString;
     }
 
     /**
@@ -56,7 +60,7 @@
      */
     @Override
     public String getFormat() {
-        return obj.toString();
+        return objectString;
     }
 
     /**
@@ -89,7 +93,7 @@
 
     @Override
     public String toString() {
-        return "ObjectMessage[obj=" + obj.toString() + ']';
+        return "ObjectMessage[obj=" + objectString + ']';
     }
 
     private void writeObject(final ObjectOutputStream out) throws IOException {
Index: src/main/java/org/apache/logging/log4j/message/StringFormattedMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/StringFormattedMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/StringFormattedMessage.java	(working copy)
@@ -48,6 +48,7 @@
         if (arguments != null && arguments.length > 0 && arguments[arguments.length - 1] instanceof Throwable) {
             this.throwable = (Throwable) arguments[arguments.length - 1];
         }
+        getFormattedMessage(); // LOG4J2-763 take snapshot of parameters at message construction time
     }
 
     /**
Index: src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/StructuredDataMessage.java	(working copy)
@@ -22,6 +22,11 @@
 
 /**
  * Represents a Message that conforms to an RFC 5424 StructuredData element along with the syslog message.
+ * <p>
+ * Thread-safety note: the contents of this message can be modified after construction.
+ * When using asynchronous loggers and appenders it is not recommended to modify this message after the message is
+ * logged, because it is undefined whether the logged message string will contain the old values or the modified
+ * values.
  *
  * @see <a href="https://tools.ietf.org/html/rfc5424">RFC 5424</a>
  */
Index: src/main/java/org/apache/logging/log4j/message/ThreadDumpMessage.java
===================================================================
--- src/main/java/org/apache/logging/log4j/message/ThreadDumpMessage.java	(revision 1616228)
+++ src/main/java/org/apache/logging/log4j/message/ThreadDumpMessage.java	(working copy)
@@ -38,9 +38,7 @@
     private static final ThreadInfoFactory FACTORY;
 
     private volatile Map<ThreadInformation, StackTraceElement[]> threads;
-
     private final String title;
-
     private String formattedMessage;
 
     static {
@@ -61,7 +59,10 @@
      */
     public ThreadDumpMessage(final String title) {
         this.title = title == null ? Strings.EMPTY : title;
-        threads = FACTORY.createThreadInfo();
+        this.threads = FACTORY.createThreadInfo();
+        
+        // LOG4J2-763: take snapshot of parameters at message construction time
+        this.formattedMessage = getFormattedMessage();
     }
 
     private ThreadDumpMessage(final String formattedMsg, final String title) {