So I sometimes really miss old school Dependency Injection. Back when Spring was still "lightweight" we happily configured all our beans in an application.xml file with the "learn-in-a-day" Spring bean xml configuration. The downsides to this were of course a loss of type safety. I can think of quite a few test cases whose sole purpose was to bootstrap the Spring configuration file and just see if the ApplicationContext starts up without going belly-up due to mis-wiring and correct resolution of included bean xml configuration files.

I may be a minority but I never liked the Spring Schema configuration. To me it feels a bit like configuration for configuration.

Annotations came along and improved things, with the caveat in that you have to import libraries for all those annotations. I like annotations but  there is a good case for having all your DI information in a central place so you can actually see how your app hangs together. Finally you sometimes need to create managed objects you can't annotate.

Java Spring configuration makes things better with compile time safety, but I had to rethink the way I did a lot of my wiring, as I had to be careful how I did my wiring as I  lost some of the lazy eval that you get in a Spring context as your Java code evaluates immediately when the ApplicationContext starts up.

So, Java based DI is nice but how can we use Java 8.0 to improve it?

Apply that Lambda Hammer

Right so this is the part of the post that starts applying the new hammer in Java 8.0:Lambdas.

Firstly Lambdas give a type safe way of deferring execution till it's needed.

So, lets first create a wrapper object called "ObjectDefinition" who's job it is to define how an object should be created and wired with various values. It works by instantiating the class we want to create and object from (In this case we have a class called "MyObject"). We also give it a list of java.util.function.BiConsumer interfaces which are mapped to a specific value. This list will be used to perform the actual task of setting values on the object.

ObjectDefintion then instantiates the object using normal reflection and then runs though this list of BiConsumer interfaces, passing the the instance of the concrete object and the mapped value.

Assuming we give our ObjectDefinition a fluent DSL we can do the following to define the object by adding the set() method which takes a BiConsumer and the value to set and populates the BiConsumer list as follows:

  MyObject result = new ObjectDefinition() 
    .type(MyObject.class)
    .set((myObject, value)-> myObject.setA(value), "hello world")
    .set((myObject, value)-> myObject.setB(value), "hallo welt")
    .create();

   MyObject result = new ObjectDefinition()
     .type(MyObject.class)
     .set(MyObject::setA, "hello world")
     .set(MyObject::setB, "hallo welt")
     .create();
   String myString = container.get(MyObject.class);  

     Container container = create((builder) -> {
          builder
              .define(MyObject.class)
              .set(MyObject::setA, "hello world");
     }); 

   Container container = create((builder) -> {
      builder.define(String.class)
             .args("hello world");

      builder.define(MyObject.class)
             .inject(MyObject::setA,String.class);
   });
   MyObject myString = container.get(MyObject.class);

    Container container = create((builder) -> {
      builder.define(MyObject.class)
             .set(MyObject::setA,"hello world")
             .initWith(MyObject::start);
    });
    MyObject myString = container.get(MyObject.class);