Java provides several libraries and frameworks that can be used to design protocols. A protocol is a set of rules and standards that govern the communication between two or more entities. Here are some common techniques and libraries used in Java to design protocols:

1. Java Networking API: Java Networking API provides a set of classes and interfaces for creating network-enabled applications. It includes classes like Socket, ServerSocket, DatagramSocket, DatagramPacket, etc. These classes can be used to implement custom protocols for communication between two or more entities.

2. Java RMI (Remote Method Invocation): Java RMI is a distributed computing framework that allows objects in one JVM to invoke methods on objects in another JVM. It provides a protocol for communication between different JVMs, which is based on the Java Networking API. RMI can be used to implement custom protocols for communication between different JVMs.

3. Java NIO (New Input/Output): Java NIO provides a set of classes and interfaces for performing non-blocking I/O operations. It includes classes like Selector, SocketChannel, ServerSocketChannel, etc. NIO can be used to implement custom protocols that require high-performance, non-blocking I/O.

4. RESTful Web Services: REST (Representational State Transfer) is a web service protocol that uses HTTP for communication between a client and a server. RESTful web services are stateless, meaning that each request from the client includes all the necessary information for the server to fulfill the request. Java provides several libraries and frameworks for implementing RESTful web services, including JAX-RS (Java API for RESTful Web Services), Spring MVC, and Dropwizard.

5. WebSocket: WebSocket is a protocol that provides a full-duplex communication channel between a client and a server over a single TCP connection. It allows both the client and server to initiate communication, making it ideal for real-time applications like online games or chat applications. Java provides several libraries and frameworks for implementing WebSocket, including Java API for WebSocket and Spring WebSocket.

When designing a protocol in Java, it is important to consider factors such as the performance, scalability, and security of the protocol. It is also important to follow established best practices for protocol design, such as using well-defined message formats, using timeouts to avoid blocking, and properly handling errors and exceptions.