The URLConnectionHandler is a class in Java that allows developers to create custom handlers for specific URL protocols. It extends the URLStreamHandler class and provides a way to define how to connect to a particular type of resource over the network. This is useful for applications that need to handle non-standard protocols or require special processing for certain types of URLs. When a URL object is created with a specific protocol, the URLConnectionHandler can be registered to manage connections for that protocol. This enables developers to implement tailored behaviors for opening connections, reading data, and handling responses, enhancing the flexibility of network communication in Java applications.