Axon is an open-source Java framework for building systems in CQRS (Command Query Responsibility Segregation), DDD (Domain Driven Design), and Event Sourcing manner. Axon provides a high level of location transparency, which gives the opportunity to easily split the system into several microservices. You can download the fully open-source package here. The Axon Framework consists of message-based commands, events, and queries that are supported types of messages. For each of these messages, we can define a handler (a method or constructor) to handle it. In some cases, the message itself has enough information to be handled, but in many others, we have a dependency on other components and/or variables (message metadata such as correlation information would be a good example). Axon provides a really powerful mechanism to inject these dependencies into message handlers — Parameter Resolvers. This is the story about them. At the end of this article, you can find a cheat sheet of all resolvers provided by the Axon Framework.
Anatomy
There are two important components that support this mechanism: the ParameterResolver and the ParameterResolverFactory (see Figure 1).