AbstractSoftware architectures and modular composition help in constructing large-scale software systems. Current programming languages provide only insufficient support for software architecture. “Architectural programming” overcomes the problem of architectural erosion in implementations by integrating concepts of software architecture into programming languages. We present the new programming language Java/A as an instance for Java-based architectural programming and show how Java/A integrates architectural notions such as components, connectors, and assemblies into Java. A main asset of Java/A is its underlying abstract component model which provides the basis for reasoning about software components and assemblies. We give a formalisation of the abstract component model in terms of transition systems and states as algebras, and prove a consistency result for assemblies.