Spring series, part 4: @Lazy on injection points

Last time, when I talked about lazily loaded beans I forgot to mention one interesting feature of Spring 4 - lazily loaded injection points. This post introduces the combination of @Lazy and @Autowire as a counterpart to the standard approach (JSR-330) using @Inject and Provider.

As of Spring 4.0 @Lazy can be applied on injection points, such as @Autowire or @Inject. From the docs:

In addition to its role for component initialization, this annotation may also be placed on injection points marked with Autowired or Inject: In that context, it leads to the creation of a lazy-resolution proxy for all affected dependencies, as an alternative to using ObjectFactory or Provider.

Let's start with a bit of a background. The Provider interface is part of JSR-330 and, among other usages, it allows for breaking circular dependencies and deals with lazy instantiation in general.

A quick explanatory example, a mere copy-paste from the docs:

class Car {  
   @Inject Car(Provider seatProvider) {  
     Seat driver = seatProvider.get();  
     Seat passenger = seatProvider.get();  
     ...  
   }  
}

Okay, but how exactly would it be implemented outside of a JEE container? Well, here is a simple Spring application.

Start with adding a Maven dependency:

 <dependency>  
   <groupid>javax.inject</groupid>  
   <artifactid>javax.inject</artifactid>  
   <version>1</version>  
 </dependency>

Next, define the model with indirection in mind (Provider, @Inject):

 public class Seat {..}

import javax.inject.Inject;  
import javax.inject.Provider;  
public class Car {  
   private Seat driver;  
   private Seat passenger;  
   @Inject  
   public Car(Provider<Seat> seatProvider) {  
     driver =  seatProvider.get();  
     passenger = seatProvider.get();  
   }  
}

Now, and here is where the magic happens, make use of the Spring application context capabilities:

@Component  
public class Car {..}  
  import org.springframework.context.annotation.Bean;  
  import org.springframework.context.annotation.Configuration;  
  import org.springframework.context.annotation.Scope;  
  import javax.inject.Provider;  
  
  @Configuration  
  public class SeatProvider implements Provider<Seat> {  
    @Override  
    @Bean  
    @Scope(value = "prototype")  
    public Seat get() {  
      return new Seat();  
    }  
}

Finally, try it out:

// Unit tests  
  ..  
   @Autowired  
   private Car car;  
   ..  
   // Instantiation should happen  
   assertNotNull(car.getDriver());  
   assertNotNull(car.getPassenger());  
   // The passenger and the driver shouldn't  
   // share the same (prototype-scoped) seat  
   assertFalse(car.getDriver().equals(car.getPassenger()));  
 }

All of the tests pass and, with a little help of Spring (application context, prototypes), the dependency injection is mediated via the Provider.

As of Spring 4.0, the same goal can be achieved by simply using @Lazy alongside with @Autowire. Here is how our example would change when using Spring only:

 import org.springframework.context.annotation.Scope;  
 import org.springframework.stereotype.Component;  
   
 @Component(value = "lazySeat")  
 @Scope(value = "prototype")  
 public class Seat {..}

 import org.springframework.beans.factory.annotation.Autowired;  
 import org.springframework.context.annotation.Lazy;  
 import org.springframework.stereotype.Component;  
   
 @Component(value = "lazyCar")  
 public class Car {  
   @Autowired @Lazy  
   private Seat driver;  
   
   @Autowired @Lazy  
   private Seat passenger;  
 }

That's it. The same tests would pass again and more importantly the eager load of Seats would never happen, thanks to adding @Lazy to the @Autowire injection point.

Source Code

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