spring - constructor injection and overriding parent definition of nested bean

Question

I\'ve read the Spring 3 reference on inheriting bean definitions, but I\'m confused about what is possible and not possible.

For example, a bean that takes a collaborato

Answer 1

I'm not entirely sure what you are trying to achieve. I don't think you can achieve exactly what you want without creating your own custom schema (which is non-trivial for nested structures), but the following example is probably pretty close without doing that.

First, define an abstract bean to use as a template for your outer bean (my example uses a Car as the outer bean and an Engine as the inner bean), giving it default values that all other beans can inherit:

Since all Honda Civics have the same engine (in my world, where I know nothing about cars), I give it a default nested engine bean. Unfortunately, a bean cannot reference an abstract bean, so the default engine cannot be abstract. I've defined a concrete bean for the engine, but mark it as lazy-init so it will not actually be instantiated unless another bean uses it:

Now I can define my specific car, taking all the default values by referencing the bean where they are defined via parent:

My wife has a car just like mine, except its a different model (again, I know nothing about cars - let's assume the engines are the same even though in real life they probably are not). Instead of redefining a bunch of beans/properties, I just extend the default car definition again, but override one of its properties:

My sister has the same car as my wife (really), but it has a different color. I can extend a concrete bean and override one or more properties on it:

If I liked racing minivans, I might consider getting one with a bigger engine. Here I extend a minivan bean, overriding its default engine with a new engine. This new engine extends the default engine, overriding a few properties:

You can also do this more concisely by using nested properties:

This will use the "defaultEngine". However, if you were to create two cars this way, each with different property values, the behavior will not be correct. This is because the two cars would be sharing the same engine instance, with the second car overriding the property settings set on the first car. This can be remedied by marking the defaultEngine as a "prototype", which instantiates a new one each time it is referenced:

I think this example gives the basic idea. If your data structure is complex, you might define multiple abstract beans, or create several different abstract hierarchies - especially if your bean hierarchy is deeper than two beans.

Side note: my example uses properties, which I believe are much clearer to understand, both in Spring xml and in Java code. However, the exact same technique works for constructors, factory methods, etc.