Excalibur is an open source software project of The Apache Software Foundation. Our primary product is a lightweight, embeddable Inversion of Control container named Fortress that is written in java.
Inversion of control, also known as the hollywood principle ("don't call us, we'll call you"), is a simple but powerful concept. The idea is that we don't "wire up" all the pieces that make up an application (the "components") by writing lots of this-component-uses-that-one-like-so code, nor do we use some kind of lookup directory (like JNDI, for example) where each component decides what components to interact with itself. Instead, we instruct a smart piece of software, the container, to tell the components how to interact.
Fortress (and also its predecessor, "ECM") is such a container. It is lightweight, by which we mean that it doesn't need a lot of resources, take a lot of disk or memory, or impose all sorts of demands on its environment. Fortress is also embeddable, by which we mean that you can use fortress inside just about every java environment. More concretely, you can use it as the basis of a large standalone development platform (like the Keel project), at the core of a servlet-based web application (like Cocoon) or even as the basis of a GUI application (like GuiApp).
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