97 Things Every Java Programmer Should Know

Chapter 89. Use Custom Identity Annotations Liberally

Mark Richards

Annotations in Java are easy to write, easy to use, and very powerful—at least, some are. Traditionally, annotations in Java have provided a convenient way to implement aspect-oriented programming (AOP), a technique intended to separate out common behavioral concerns by injecting behavior at specified points in the code. However, most developers have largely abandoned AOP due to undesirable side effects as well as the desire to have all the code in one place—the class file.

Identity annotations are entirely different in that they don’t contain any functionality. Instead, they only provide programmatic information that can be used to govern, analyze, or document some aspect of the code or architecture. You can use identity annotations to identify transaction boundaries or a domain or subdomain, describe a service taxonomy, denote framework code, and employ them in dozens of other use cases.

For example, identifying classes that are part of the underlying framework (or template code in microservices) is often important so changes can be closely monitored or guarded. The following annotation does just this:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Framework {}

@Framework
public class Logger {...}

Wait—this annotation does nothing! Or does it? It denotes this class as a framework-related ...