In this paper, we present the design methodology, and the software architecture, of J-Sim, an open-source network simulation/emulation environment that has been developed, in part, under the support of the NSF Next Generation Software program. We first give an overview of the component-based software architecture, called the autonomous component architecture (ACA), that is used as the underlying architecture for J-Sim. Then we describe how we lay a generalized packetbased network simulation framework, called Extensible Internetworking Framework (INET), on top of ACA. Both the ACA and the INET have been implemented in Java. The resulting codes, along with an essential suite of network protocols and compoents (for the Internet best-effort/integrated services/differentiated services architecture) its extension for wireless and sensor networks, and a scripting framework and GUI interfaces, is called J-Sim. To demonstrate the composability and extensibility of J-Sim, we elaborate on how we model in J-Sim several different network architectures, namely the Differentiated Services (diffserv) architecture, the Multiprotocol Label Switching (MPLS) architecture, and the wireless sensor network architecture.