by Miško Hevery

Dependency injection asks us to separate the new operators from the application logic. This separation forces your code to have factories which are responsible for wiring your application together. However, better than writing factories,  we want to use automatic dependency injection such as GUICE to do the wiring for us. But can DI really save us from all of the new operators?

Lets look at two extremes. Say you have a class MusicPlayer which needs to get a hold of AudioDevice. Here we want to use DI and ask for the AudioDevice in the constructor of the MusicPlayer. This will allow us to inject a test friendly AudioDevice which we can use to assert that correct sound is coming out of our MusicPlayer. If we were to use the new operator to instantiate the BuiltInSpeakerAudioDevice we would have hard time testing. So lets call objects such as AudioDevice or MusicPlayer “Injectables.” Injectables are objects which you will ask for in the constructors and expect the DI framework to supply.

Now, lets look at the other extreme. Suppose you have primitive “int” but you want to auto-box it into an “Integer” the simplest thing is to call new Integer(5) and we are done. But if DI is the new “new” why are we calling the new in-line? Will this hurt our testing? Turns out that DI frameworks can’t really give you the Integer you are looking for since it does not know which Integer you are referring to. This is a bit of a toy example so lets look at something more complex.

Lets say the user entered the email address into the log-in box and you need to call new Email(”a@b.com”). Is that OK, or should we ask for the Email in our constructor. Again, the DI framework has no way of supplying you with the Email since it first needs to get a hold of a String where the email is. And there are a lot of Strings to chose from. As you can see there are a lot of objects out there which DI framework will never be able to supply. Lets call these “Newables” since you will be forced to call new on them manually.

First, lets lay down some ground rules. An Injectable class can ask for other Injectables in its constructor. (Sometimes I refer to Injectables as Service Objects, but that term is overloaded.) Injectables tend to have interfaces since chances are we may have to replace them with an implementation friendly to testing. However, Injectable can never ask for a non-Injectable (Newable) in its constructor. This is because DI framework does not know how to produce a Newable. Here are some examples of classes I would expect to get from my DI framework: CreditCardProcessor, MusicPlayer, MailSender, OfflineQueue. Similarly Newables can ask for other Newables in their constructor, but not for Injectables (Sometimes I refer to Newables as Value Object, but again, the term is overloaded). Some examples of Newables are: Email, MailMessage, User, CreditCard, Song. If you keep this distinctions your code will be easy to test and work with. If you break this rule your code will be hard to test.

Lets look at an example of a MusicPlayer and a Song

class Song {
  Song(String name, byte[] content);
}
class MusicPlayer {
  @Injectable
  MusicPlayer(AudioDevice device);
  play(Song song);
}

Notice that Song only asks for objects which are Newables. This makes it very easy to construct a Song in a test. Music player is fully Injectable, and so is its argument the AudioDevice, therefore, it can be gotten from DI framework.

Now lets see what happens if the MusicPlayer breaks the rule and asks for Newable in its constructor.

class Song {
  String name;
  byte[] content;
  Song(String name, byte[] content);
}
class MusicPlayer {
  AudioDevice device;
  Song song;
  @Injectable
  MusicPlayer(AudioDevice device, Song song);
  play();
}

Here the Song is still Newable and it is easy to construct in your test or in your code. The MusicPlayer is the problem. If you ask DI framework for MusicPlayer it will fail, since the DI framework will not know which Song you are referring to. Most people new to DI frameworks rarely make this mistake since it is so easy to see: your code will not run.

Now lets see what happens if the Song breaks the rule and ask for Injectable in its constructor.

class MusicPlayer {
  AudioDevice device;
  @Injectable
  MusicPlayer(AudioDevice device);
}
class Song {
  String name;
  byte[] content;
  MusicPlayer palyer;
  Song(String name, byte[] content, MusicPlayer player);
  play();
}
class SongReader {
  MusicPlayer player
  @Injectable
  SongReader(MusicPlayer player) {
    this.player = player;
  }
  Song read(File file) {
    return new Song(file.getName(),
                    readBytes(file),
                    player);
  }
}

At first the world looks OK. But think about how the Songs will get created. Presumably the songs are stored on a disk and so we will need a SongReader. The SongReader will have to ask for MusicPlayer so that when it calls the new on a Song it can satisfy the dependencies of Song on MusicPlayer. See anything wrong here? Why in the world does SongReader need to know about the MusicPlayer. This is a violation of Law of Demeter. The SongReader does not need to know about MusicPlayer. You can tell since SongReader does not call any method on the MusicPlayer. It only knows about the MusicPlayer because the Song has violated the Newable/Injectable separation. The SongReader pays the price for a mistake in Song. Since the place where the mistake is made and where the pain is felt are not the same this mistake is very subtle and hard to diagnose. It also means that a lot of people make this mistake.

Now from the testing point of view this is a real pain. Suppose you have a SongWriter and you want to verify that it correctly serializes the Song to disk. Why do you have to create a MockMusicPlayer so that you can pass it into a Song so that you can pass it into the SongWritter. Why is MusicPlayer in the picture? Lets look at it from a different angle. Song is something you may want to serialize, and simplest way to do that is to use Java serialization. This will serialize not only the Song but also the MusicPlayer and the AudioDevice. Neither MusicPlayer nor the AudioDevice need to be serialized. As you can see a subtle change makes a whole lot of difference in the easy of testability.

As you can see the code is easiest to work with if we keep these two kinds objects distinct. If you mix them your code will be hard to test.  Newables are objects which are at the end of your application object graph. Newables may depend on other Newables as in CreditCard may depend on Address which may depend on a City but these things are leafs of the application graph. Since they are leafs, and they don’t talk to any external services (external services are Injectables) there is no need to mock them. Nothing behaves more like a String like than a String. Why would I mock User if I can just new User, Why mock any of these: Email, MailMessage, User, CreditCard, Song? Just call new and be done with it.

Now here is something very subtle. It is OK for Newable to know about Injectable. What is not OK is for the Newable to have a field reference to Injectable. In other words it is OK for Song to know about MusicPlayer. For example it is OK for an Injectable MusicPlayer to be passed in through the stack to a Newable Song. This is because the stack passing is independent of DI framework. As in this example:

class Song {
  Song(String name, byte[] content);
  boolean isPlayable(MusicPlayer player);
}

The problem becomes when the Song has a field reference to MusicPlayer. Field references are set through the constructor which will force a Law of Demeter violation for the caller and we will have hard time to test.