Reflect (Generic) UDF
A Java class and method often exists to handle the exact function a user would like to use in Hive. Rather than having to write a wrapper UDF to call this method, the majority of these methods can be called using reflect UDF. Reflect uses Java reflection to instantiate and call methods of objects; it can also call static functions. The method must return a primitive type or a type that Hive knows how to serialize.
As of Hive 0.9.0, java_method() is a synonym for reflect(). See Misc. Functions in Hive Operators and UDFs.
Note that Reflect UDF is non-deterministic since there is no guarantee what a specific method will return given the same parameters. So be cautious when using Reflect on the WHERE clause because that may invalidate Predicate Pushdown optimization.