Inject a Method Interceptor in Guice

I recently made a mistake to new an object (MethodInterceptor) in an plain old way in a Guice configuration, which caused the object’s @Inject-annotated fields, like Logger for example, were initialized with null value.

public class FooInterceptor implements MethodInterceptor{
  @Inject
  private Logger logger;

  @Override
  public Object invoke(MethodInvocation invocation) throws Throwable {
    // NPE if logger not inject correctly
  	logger.info("start to invoke"); 
  	return invocation.proceed();
  }
}

public class BarModule extends AbstractModule {
	@Override
	protected void configure() {	
    bindInterceptor(Matchers.subclassesOf(OrderApi.class), 
      Matchers.any(), 
      // new the interceptor in the plain old way
      new FooInterceptor()); 
  }
}

Solution

Guice wiki clearly states that requestInjection() should be used for injection of a “method interceptor”.

How do I inject a method interceptor?

In order to inject dependencies in an AOP MethodInterceptor, use requestInjection() alongside the standard bindInterceptor() call.

public class NotOnWeekendsModule extends AbstractModule {
  protected void configure() {
    MethodInterceptor interceptor = new WeekendBlocker();
    // for injection of a "method interceptor"
    requestInjection(interceptor);
    bindInterceptor(any(), annotatedWith(NotOnWeekends.class), interceptor);
  }
}

Some Thoughts

Once you decide to use a dependency injection framework, like Guice here, please DO NOT new Java objects in the plain old way any more. Otherwise, you may very possibly fail to set up the objects’ dependencies correctly.

Secondly, use constructor-injection for mandatory dependency, like Logger here. It’s impossible to forget to inject a dependency using constructor-injection, even if that object is constructed in the plain old way. (However, too many dependencies injected via the constructor makes the constructor look a bit ugly.)