Lookup

From APIDesign

Lookup is a library that provides API that unifies component injection with adaptable pattern. It is based on long time JDK standard for registering instances of services in JAR files which has been formalized as ServiceLoader in JDK 6. Lookup extends this area and allows local pools of objects that can be queries for certain capability. Lookup is also suited for dynamic environment as it supports notification of changes. Lookup bridges the classical injection pattern with local adaptable pattern.

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The basic idea, in its ServiceLoader incarnation is that it is enough to compose an application from various JAR files and let these JAR files communicate. This is achieved by defining well-known location inside the JAR file META-INF/services/nameoftype that is read from all available libraries JARs on the classpath.

This greatly simplifies final assembly and deployment of the application. It is enough to drop the JARs with an implementation available in the system and all the pieces can work suddenly cooperate by mutually discovering themselves via Lookup.getDefault().lookup(...).

On the other hand, looking from a local point of view, there can be many pools to fish for an interface. Almost any object can implement Lookup.Provider interface and thus give access to its adaptable features to everyone.

Using the same interface to play two different roles (component injection and Adaptable pattern) shows the common properties of both. The Lookup can serve as a assembly language to provide pool of objects to use during real Dependency Injection. The bridge between Lookup and Spring provides more details about this area.

Overuse of Lookup

Sometimes people call Lookup magical bag. The pejorative flavor of the nickname is based on observation that you can put your hand in it and pray to find the object you are seeking for. Sometimes people believe there shall be a better way. Yes, in many cases there is. Lookup is not handy for everything. It is basically a tool to implement a teleinterfaces. In case a teleinterface is not what you want (like in case of CumulativeFactory), you'd rather prefer type safety and design proper API without magical bag - aka Lookup.