Wednesday, July 25, 2007

dependency injection -- %%intro

Inspired by James Shore,

Q: What's a dependency? What are the 2 parties in a dependency? What's bad?
A: a "dependency" is one of class A's instance variables b, of type B.

"injecting the dependency into A" = assigning an object to the instance var b with a public setter method. This can be done in a third-party class C, through an interface.
"setting the dependency" = A autonomously assigning object to its own instance var b. Common practice


Q: why do people mention factory pattern, service-locator pattern?
A: http://jcp.org/en/jsr/proposalDetails?id=330
http://jcp.org/en/jsr/proposalDetails?id=330 has a simple example (stop watch) demonstrating constructor injection. also used in dotnet.

--http://www.jamesshore.com/Blog/Dependency-Injection-Demystified.html says

Some call those "dependencies." Most people call them "variables." Sometimes, when they're feeling fancy, they call them "instance variables."

Why Do We Do This?
Among other things, it's handy for isolating classes during testing.


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