Since the very early days of Java (and C-like languages overall), the canonical way to start your program has been something like this:

public class A {
  public static void main(String[] args) {
    new A().run(args);
  }
  public void run(String[] args) {
    // Your application starts here
  }
}

If you are still doing this, I’m here to tell you it’s time to stop.

Letting go of ‘new’

First, install Guice in your project:

        <dependency>
          <groupId>com.google.inject</groupId>
           <artifactId>guice</artifactId>
           <version>3.0</version>
        </dependency>

and then, modify your main method as follows:

public class A {
  public static void main(String[] args) {
    Injector.getInstance(A.class).run(args);
  }
}

So, what does this buy you exactly?

You will find a lot of articles explaining the various benefits of Guice, such as being able to substitute different environments on the fly, but I’m going to use a different angle in this article.

Let’s start by assuming the existence of a Config class that contains various configuration parameters. I’ll just hardcode them for now and use fields to make the class smaller:

public class Config {
  String host = "com.example.com";
  int port = 1234;
}

This class is a singleton, it is instantiated somewhere in your main class and not used anywhere else at the moment. One day, you realize you need this instance in another class which happens to be deep in your runtime hierarchy, which we will call Deep. For example, if you put a break point in the method where you need this config object, your debugger would show you stack frames similar to this:

com.example.A.main()
com.example.B.f(int, String)
com.example.C.g(String)
com.example.Deep.h(Foo, int)

The easy and wrong way to solve this problem is to make the Config instance static on some class (probably A) and access it directly from Deep. I’m hoping I don’t need to explain why this is a bad idea: not only do you want to avoid using statics, but you also want to make sure that each object is exposed only to objects that need them, and making the Config object static would make your instance visible to your entire code base. Not a good thing.

The second thought is to pass the object down the stack, so you modify all the signatures as follows:

com.example.A.main()
com.example.B.f(int, String, Config)
com.example.C.g(String, Config)
com.example.Deep.h(Foo, int, Config)

This is a bit better since you have severely restricted the exposure of the Config object, but note that you are still making it available to more methods than really need to: B#f and C#g have really nothing to do with this object, and a little sting of discomfort hits you when you start writing the Javadoc:

public class C {
  ...
  /**
   * @param config This method doesn't really use this parameter,
   * it just passes it down so Deep#h can use it.
   */
  public void g(String s, Config config) {

Unnecessary exposure is actually not the worst part of this approach, the problem is that it changes all these signatures along the way, which is certainly undesirable in a private API and absolutely devastating in a public API. And of course, it’s absolutely not scalable: if you keep adding a parameter to your method whenever you need access to a certain object, you will soon be dealing with methods that take ten parameters, most of which they just pass down the chain.

Here is how we solve this problem with dependency injection (performed by Guice in this example, but this is applicable to any library that implements JSR 330, obviously):

public class Deep {
  @Inject
  private Config config;

and we’re done. That’s it. You don’t need to modify the Config class in any way, nor do you need to make any change in any of the classes that separate Deep from your main class. With this, you have also minimized the exposure of the Config object to just the class that needs it.

Injecting right

There are various ways you can inject object into your class but I’ll just mention the two that, I think, are the most important. I just showed “field injection” in the previous paragraph, but be aware that you can also prefer to use “constructor injection”:

public class Deep {
  private final Config config;
  @Inject
  public Deep(Config config) {
    this.config = config;
  }

This time, you are adding a parameter to the constructor of your Deep class (which shouldn’t worry you too much since you will never invoke it directly, Guice will) and you assign the parameter to the field in the constructor. The benefit is that you can declare your field final. The downside, obviously, is that this approach is much more verbose.

Personally, I see little point in final fields since I have hardly ever encountered a bug that was due to accidentally reassigning a field, so I tend to use field injection whenever I can.

Taking it to the next level

Obviously, the kind of configuration object I used as an example if not very realistic. Typically, a configuration will not hardcode values like I did and will, instead, read them from some external source. Similarly, you will want to inject objects that can’t necessarily be instantiated so early in the lifecycle of your application, such as servlet contexts, database connections, or implementations of your own interfaces.

This topic itself would probably cover several chapters of a book dedicated to dependency injection, so I’ll just summarize it: not all objects can be injected this way, and one benefit of using a dependency injection framework in your code is that it will force you to think about what life cycle category your objects belong to. Having said that, if you want to find out how Guice can inject objects that get created at a later time in your application life cycle, look up the Javadoc for the Provider class.

Wrapping up

I hope this quick introduction to dependency injection piqued your interest and that you will consider using it in your project since it has so much more to offer than what I described in this post. If you want to learn more, I suggest starting with the excellent Guice documentation.