JAX-WS handlers are interceptors that can be plugged into the JAX-WS runtime environment to do additional processing of inbound and outbound messages. They are independent of the application level code and can provide low-level message processing capabilities.
The JAX-WS specification defines two basic handler types:
Logical handlers can process the message payload and the properties of stored in the message context.
Protocol handlers operate on message context properties and protocol-specific messages. They are limited to SOAP-specific configurations and are often refered to as "SOAP handlers".
A handler is assembled into a chain of handlers known as a handler chain. On the client side, you can configure handler chains programmatically.
On the server-side you use the @HandlerChain annotation to reference an XML configuration file that describes the handlers that are included in the chain.