By default when {@link #getMessageHeaders()} is called, {@code "this"}
{@code MessageHeaderAccessor} instance can no longer be used to modify the
underlying message headers and the returned {@code MessageHeaders} is immutable.
<p>However when this is set to {@code true}, the returned (underlying)
{@code MessageHeaders} instance remains mutable. To make further modifications
continue to use the same accessor instance or re-obtain it via:<br>
{@link MessageHeaderAccessor#getAccessor(Message, Class)
MessageHeaderAccessor.getAccessor(Message, Class)}
<p>When modifications are complete use {@link #setImmutable()} to prevent
further changes. The intended use case for this mechanism is initialization
of a Message within a single thread.
<p>By default this is set to {@code false}.
@since 4.1
By default when {@link #getMessageHeaders()} is called, {@code "this"} {@code MessageHeaderAccessor} instance can no longer be used to modify the underlying message headers and the returned {@code MessageHeaders} is immutable. <p>However when this is set to {@code true}, the returned (underlying) {@code MessageHeaders} instance remains mutable. To make further modifications continue to use the same accessor instance or re-obtain it via:<br> {@link MessageHeaderAccessor#getAccessor(Message, Class) MessageHeaderAccessor.getAccessor(Message, Class)} <p>When modifications are complete use {@link #setImmutable()} to prevent further changes. The intended use case for this mechanism is initialization of a Message within a single thread. <p>By default this is set to {@code false}. @since 4.1