Following the evolution of Cloud Computing and Service-Oriented Architecture (SOA), microservices (MS) have naturally emerged as the next trend due to the advantages they provide. These advantages include increased maintainability, better scalability, and an overall better synergy with DevOps techniques. This makes migrating legacy software towards a microservice-oriented architecture (MSA) an attractive prospect for organizations. The migration process is a complex and consequently risky endeavor that can be decomposed into two phases (1) the microservice-based architecture recovery phase and (2) the transformation (i.e. materialization) phase. Several studies have been done to automate the microservice architecture recovery phase. However, to the best of our knowledge, no work has been completed to automate the transformation phase. In this paper, we propose a systematic approach to refactor the existing code of an object-oriented monolithic application towards an MS-oriented one by using the target architecture from the recovery phase as a guide. By defining and applying a set of transformation patterns, we are able to generate a set of deployable microservices. Finally, we validate our approach by automating it through our tool MonoToMicro, and we apply it to a set of monolithic Java applications to generate a set of MSAs.