o not have to be <code>Serializable</code>.

 <p>
 The <code>createValueExpression</code> and <code>createMethodExpression</code> methods must be thread-safe. That is,
 multiple threads may call these methods on the same <code>ExpressionFactory</code> object simultaneously.
 Implementations should synchronize access if they depend on transient state. Implementations should not, however,
 assume that only one object of each <code>ExpressionFactory</code> type will be instantiated; global caching should
 ther