Protocol and content handlers separate the data that's retrieved from an Internet server from the protocol used to retrieve it. They also allow browsers to be dynamically extended at runtime to handle new and unexpected protocols and content types.
Content handlers are subclasses of the
java.net.ContentHandler
class. Protocol handlers are a
combination of the subclasses of the
java.net.URLConnection
and
java.net.URLStreamHandler
classes.
The best available description of protocol and content handlers is probably chapters 11 and 12 of my own Java Network Programming. (O'Reilly & Associates, 2004).