Google Guice - AOP

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AOP, Aspect oriented programming entails breaking down program logic into distinct parts called so-called concerns. The functions that span multiple points of an application are called cross-cutting concerns and these cross-cutting concerns are conceptually separate from the application's business logic. There are various common good examples of aspects like logging, auditing, declarative transactions, security, caching, etc.

The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Dependency Injection helps you decouple your application objects from each other and AOP helps you decouple cross-cutting concerns from the objects that they affect. AOP is like triggers in programming languages such as Perl, .NET, Java, and others. Guice provides interceptors to intercept an application. For example, when a method is executed, you can add extra functionality before or after the method execution.

Important Classes

  • Matcher − Matcher is an interface to either accept or reject a value. In Guice AOP, we need two matchers: one to define which classes participate, and another for the methods of those classes.

  • MethodInterceptor − MethodInterceptors are executed when a matching method is called. They can inspect the call: the method, its arguments, and the receiving instance. We can perform cross-cutting logic and then delegate to the underlying method. Finally, we may inspect the return value or exception and return.

Example

Create a java class named GuiceTester.

GuiceTester.java

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Inject;
import com.google.inject.Injector;
import com.google.inject.matcher.Matchers;

public class GuiceTester {
   public static void main(String[] args) {
      Injector injector = Guice.createInjector(new TextEditorModule());
      TextEditor editor = injector.getInstance(TextEditor.class);
      editor.makeSpellCheck(); 
   } 
}
class TextEditor {
   private SpellChecker spellChecker;

   @Inject
   public TextEditor(SpellChecker spellChecker) {
      this.spellChecker = spellChecker;
   }
   public void makeSpellCheck() {
      spellChecker.checkSpelling();
   }
}

//Binding Module
class TextEditorModule extends AbstractModule {
   @Override
   
   protected void configure() {
      bind(SpellChecker.class).to(SpellCheckerImpl.class);
      bindInterceptor(Matchers.any(), 
         Matchers.annotatedWith(CallTracker.class), 
         new CallTrackerService());
   } 
}

//spell checker interface
interface SpellChecker {
   public void checkSpelling();
}

//spell checker implementation
class SpellCheckerImpl implements SpellChecker {
   @Override @CallTracker
   
   public void checkSpelling() {
      System.out.println("Inside checkSpelling." );
   } 
}
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)
@interface CallTracker {}

class CallTrackerService implements MethodInterceptor  {
   @Override
   
   public Object invoke(MethodInvocation invocation) throws Throwable {
      System.out.println("Before " + invocation.getMethod().getName());
      Object result = invocation.proceed();
      System.out.println("After " + invocation.getMethod().getName());
      return result;
   }
}

Output

Compile and run the file, you may see the following output.

Before checkSpelling
Inside checkSpelling.
After checkSpelling
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