Spring Autowired annotation

In my previous post I briefly introduced the concept of Spring’s @Autowired annotation. This post goes more in-depth about this annotation, and its importance when developing with the Spring framework.

Spring uses the @Autowired annotation when it determines where it needs to inject a given @Bean. You can see this in the example below:

This example uses @Autowired on a private member variable referenced via class. In this example, the @Component is on the WeatherService class. The DependencyInjectionWithSpring class references the WeatherService class directly. I’ve annotated this reference with the @Autowired annotation. Spring then knows that it needs to find a bean (component) that matches. In this case, it finds the WeatherService class and injects it into the DependencyInjectionWithSpring class.

You can also reference with @Autowired via interface:

Here, I change the WeatherService class to be named WeatherServiceImpl, and have it implement the WeatherService interface, which I’ve also defined. DependencyInjectionWithSpring now references this interface (WeatherService), and the @Autowired annotation stays. Spring then knows that it needs to find a bean/component that implements the WeatherService interface. It finds the WeatherServiceImpl class and injects it into the DependencyInjectionWithSpring as a dependency (weatherService).

Field X in Y required a single bean, but 2 were found

What happens if we have multiple beans (components) that implement the same interface? Like this:

Both SunnyWeatherServiceImpl and CloudyWeatherServiceImpl implement the WeatherService interface. The DependencyInjectionWithSpring class depends on an implementation of WeatherService. Both of these classes would work, since they implement that interface. Spring is aware and manages both of these classes, since they are annotated with the @Component annotation. How does Spring know which one to choose? Let’s run the application and see what happens:

The application is unable to start. Spring can’t determine which bean to set for the weatherService.

How do I fix this?

To resolve this issue and tell Spring which bean to use in this situation, we have a few options.

The first option is to follow the first suggestion given in the exception:

Consider marking one of the beans as @Primary…

The @Primary annotation can be used to indicate to Spring which bean to use. In this case, I add the @Primary annotation to the SunnyWeatherServiceImpl:

Now when I run the application, it starts up with no errors and runs as expected, injecting the SunnyWeatherServiceImpl (the one with the @Primary annotation) into the DependencyInjectionWithSpring class.

There’s another option that Spring gives us, as mentioned in the error message from above:

or using @Qualifier to identify the bean that should be consumed

@Qualifier is a Spring annotation that allows us to tell Spring which bean we want to inject. See below:

Pay particular attention to the DependencyInjectionWithSpring class. The weatherService dependency in this class is now annotated with @Qualifier(value="cloudyWeatherServiceImpl"). This tells Spring to use the cloudyWeatherServiceImpl bean when injecting this dependency.

Now when I run the application, again it starts up with no errors and runs as expected. This time, I told Spring to use the cloudyWeatherServiceImpl class and it prints out The weather is cloudy with a 90% chance of rain as expected.

There’s yet another way to indicate to Spring which bean to inject. This is by naming the variable of the dependency the name of the class we want to inject. Like so:

Here, the WeatherService interface gets the name cloudyWeatherServiceImpl. This tells Spring that I want to inject the bean with the name cloudyWeatherServiceImpl. When I run this code, the application again starts up with no problems, and I again get the expected output.

Summary

Spring’s @Autowired annotation is used to indicate where to inject a Spring-managed bean. The @Primary annotation allows us to indicate which bean to use when multiple beans that match are present. The @Qualifier annotation allows us to specify a specific bean to inject on a given dependency. Finally, we learned that we can name the dependency with the bean name to also specify which bean to inject.

Dependency Injection using Spring

This post describes the basics of how to do dependency injection with the Spring framework. If you are unfamiliar with dependency injection, you may want to check out my first post on dependency injection before continuing.

Let’s start of with an example

Here we have two classes – DependencyInjectionWithSpring and WeatherService. WeatherService is a dependency of DependencyInjectionWithSpring. DependencyInjectionWithSpring instantiates WeatherService. Its run() method calls the printWeather() method on WeatherService. Pretty simple stuff.

Can you spot the problem? Take a minute and look at DependencyInjectionWithSpring. This class instantiates the WeatherService class, and sets this object to its private member variable weatherService. This is not a problem per se, or rather not compile time or even a runtime problem. This code still compiles. It still runs. It still produces the result we are trying to achieve.

I don’t see any problems with this, can you explain?

Here are a few

  1. Testability – you can’t test DependencyInjectionWithSpring without testing WeatherService as well. What if weather service gets its data from a thermometer or some external system? You wouldn’t want to wait for the service to run for your tests to complete, that could take a while. Unit tests should run fast. Also, you don’t have control over what WeatherService returns. Since you don’t have this control, how can you test for certain conditions, like returning null or throwing an exception.
  2. Reusability – as in the Testability problem, you can’t reuse DependencyInjectionWithSpring without getting WeatherService. This limits how and where we use this class, since wherever we have DependencyInjectionWithSpring we get WeatherService. What if we wanted different behavior than what WeatherService provides? We can’t in this case.
  3. No configuration without recompilation – what if we wanted to change the implementation of WeatherService? What if, instead of reading the weather from a thermometer we wanted to read it from a web service instead? In our current implementation, we are stuck. We would have to write code to change WeatherService to another implementation, recompile, and redeploy. As mentioned before, if we intend to use DependencyInjectionWithSpring we automatically get the WeatherService implementation with it. In other words, they are highly coupled.

So how does Spring help?

Here’s where the Spring framework can help us out.

The Spring framework is known as an IOC container. An IOC container serves two main purposes:

  1. Manage creation (and full lifecycle) of objects to be used in a system.
  2. Configure coupling of components. Spring provides (or injects) the objects it manages to the classes that depend on these objects.

For #1, that means we can get rid of the new WeatherService() piece. Like this:

If you try to run this, you’ll get a NullPointerException because weatherService has not been instantiated.

How does Spring fix this?

Since Spring is an IOC container it can instantiate objects on our behalf. The objects that get instantiated are known as Beans in the Spring framework. The way that you flag a class that you want Spring to be aware of (that is, you want it to be a Spring bean) is through the @Component annotation, like this:

This annotation lets Spring know that it should create and manage the lifecycle of this object.

However, if we run our code now, we’ll still run into a NullPointerException. Why is this so?

That’s where #2 from above comes into play. The Spring Framework also configures how components are coupled together. It injects objects into the classes that depend on them.

@Autowired

Spring injects objects via the @Autowired annotation. There are multiple ways that @Autowired can be used. To keep things simple, we’ll annotate a private member variable with @Autowired, and let Spring inject it for us. See below:

Pay particular attention to lines 23-24. This is where the WeatherService dependency gets injected into DependencyInjectionWithSpring.

Now, when we run it, we no longer see a NullPointerException, and the text The weather is sunny with a 20% chance of rain gets printed out, as expected.

To reiterate. Spring instantiates the weatherService for us. It also injects it where requested (via the @Autowired annotation).

CommandLineRunner

If you notice in the gist above, and if you run the code, you’ll see that this application is not a web application, but a command line application. In particular, take a look at line 21 above implements CommandLineRunner. CommandLineRunner is an interface from Spring Boot that has one method with the following signature: public void run(String... args) throws Exception. In this example, our DependencyInjectionWithSpring class implements this interface, and in the run method calls the weatherService.printWeather() method.

Summary

In this post I described the basics of dependency injection with the Spring framework. Project examples can be found on my Spring Beginner Tutorial on github.

Getting Started with Spring (via Spring Boot)

How do I get started with Spring?

Software developers often ask me what the easiest way to get started with Spring is. There are, of course, multiple options for getting started. This post lays out a simple yet effective way to get started with Spring.

One of the best things to happen to Spring is a project called ‘Spring Boot’. In my opinion, this is the quickest and easiest way to get started with a Spring project.

Spring Boot

Spring Boot is a project from Pivotal Software that is designed to get you up and running fast. As the project website states, “Spring Boot makes it easy to create stand-alone, production-grade Spring based Applications that you can ‘just run’. We take an opinionated view of the Spring platform and third-party libraries so you can get started with minimum fuss. Most Spring Boot applications need very little Spring configuration.”

Spring Initializr

What’s even more exciting about Spring Boot is the Spring Initializr site at start.spring.io This site will generate a Spring Boot project skeleton (choose either Gradle or Maven), let you set project metadata as well as which dependencies or Spring Boot Starters you want to include in your project. You then click the ‘Generate Project’ button to download a zip archive of your skeleton project. How cool is that??

There’s also a ‘full version’ from the same start.spring.io page if you want to choose options like Java Version, Language, and specific Spring Boot Starter dependencies.

OK, what’s the first step?

To get started, head over to start.spring.io From there, choose ‘Gradle Project’ and ‘Spring Boot 1.4.0’:

Spring Boot with Gradle

Or, if you’d prefer, you can get this project directly from my github repository – simply visit my Spring Beginner Tutorial repo where you can clone or download this Spring Boot project.

Specify Project Metadata:

Project Metadata

Under dependencies, just leave this blank for now.

Click Generate Project and download a zip of your project skeleton. Extract this into your project workspace and head over to Eclipse.

Generate Project

Open your project in Eclipse

From the Eclipse menu, choose File > Import > Gradle Project > Next

Specify the root directory of your project. Eclipse will then import your project.

Running a Spring Boot application

Now let’s go ahead and run our newly created Spring Boot application. Right-click on the project and choose ‘Debug As’ > ‘Java Application’. If, at this point, you get a dialog asking ‘Select Java Application’, go ahead and choose ‘ProjectNameApplication’, where ProjectName is the name you gave your project. Usually it’s the top option in the list.

In the console output, you’ll see several lines printed out. Once you see a line like the one below printed out, you know that your Spring Boot project is up and running:

INFO 10076 --- [ main] c.d.s.SpringBootIntroApplication : Started SpringBootIntroApplication in 2.833 seconds (JVM running for 3.426)

What does this project actually do?

If you followed my instructions exactly, you ended up with the basics of a Spring project, with a few essentials. Take a look at the debug output, in particular the following lines:

INFO 10076 --- [ main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat initialized with port(s): 8080 (http)
INFO 10076 --- [ main] o.apache.catalina.core.StandardService : Starting service Tomcat
INFO 10076 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/8.5.4
INFO 10076 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
INFO 10076 --- [ main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat started on port(s): 8080 (http)

As you can see, the following gets launched as a part of running our Spring Boot application:

  1. Tomcat
  2. Spring WebApplicationContext

I’ll get into more details around the different context options in later blog posts. For now, just know that the Spring WebApplicationContext, along with the Tomcat Servlet Engine, are what allows your application to accept web requests. With your project still running, open up a web browser to http://localhost:8080 and see what response you get. You should see something similar to the following:

404

Don’t be alarmed with the 404 response, it’s actually expected. The 404 is actually coming from your Spring Boot application. A 404 is being returned because we still have more work to do. We need to map URLs to methods in our Spring Boot application to handle the request that come to the application. In future blog posts, I’ll get into those details.

Congratulations!

Good job! You’ve now created your first Spring (via Spring Boot) application! It’s a simple application, but has all the pieces necessary for a Spring project. Spring Boot helped us along the way; it’s a great way to get a Spring project up and running.