Wednesday, November 21, 2012

Google Guice Quick Reference



Google Guice Quick Reference

 bold marked words in code are derived from guice library

Google Guice Cheat Sheet
Basic @Inject
  1. Service Consumer Class
 
@Inject

  RealBillingService(CreditCardProcessor processor,TransactionLog transactionLog) {
this.processor = processor;

    this.transactionLog= transactionLog;

  }
 
  1. Guice Configuration Class
 
public class BillingModule extends AbstractModule {

  @Override 

  protected void configure() {


// Class binding
       bind(TransactionLog.class).to(DatabaseTransactionLog.class).in(Singleton.class);
bind(CreditCardProcessor.class).to(PaypalCreditCardProcessor.class);
 
//or instance binding
bind(CreditCardProcessor.class).toInstance(new PaypalCreditCardProcessor());
 
  }

}
  1. Client Class
 
    Injector injector = Guice.createInjector(new BillingModule());



    /*

     * Now that we've got the injector, we can build objects.

     */

    RealBillingService billingService= injector.getInstance(RealBillingService.class);
 
Extention @Inject
1.Linked Binding – Link DatabaseTransactionLog to subclass MySqlDatabaseTransactionLog. You can chain this linking. Injector will return instance of MySqlDatabaseTransactionLog
public class BillingModule extends AbstractModule {

  @Override 

  protected void configure() {

    bind(TransactionLog.class).to(DatabaseTransactionLog.class);

    bind(DatabaseTransactionLog.class).to(MySqlDatabaseTransactionLog.class);

  }

}
2.Binding Annotation
Ø  2a . @Named
  @Inject

  public RealBillingService(@Named("PayPal") CreditCardProcessor processor)
- Guice Configuration
    bind(CreditCardProcessor.class)

        .annotatedWith(Names.named("PayPal"))

        .to(PayPalCreditCardProcessor.class);

Ø  2b.Custom Annotation @PayPal
a.Client Guice Configuration
import com.google.inject.BindingAnnotation;

import java.lang.annotation.Target;

import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import static java.lang.annotation.ElementType.PARAMETER;

import static java.lang.annotation.ElementType.FIELD;

import static java.lang.annotation.ElementType.METHOD;



@BindingAnnotation @Target({ FIELD, PARAMETER, METHOD }) @Retention(RUNTIME)

public @interface PayPal {}
b.Service Consumer
  @Inject

  public RealBillingService(@PayPal CreditCardProcessor processor)
c. Client
    bind(CreditCardProcessor.class)

        .annotatedWith(PayPal.class)

        .to(PayPalCreditCardProcessor.class);

Ø  2c.@Provides –create an object with some code instade of single line toInstance method
Guice Configuration
public class BillingModule extends AbstractModule {



 // Add any subannotation if required like @PayPal

  @Provides

  TransactionLog provideTransactionLog() {

    DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();

    transactionLog.setJdbcUrl("jdbc:mysql://localhost/pizza");

    transactionLog.setThreadPoolSize(30);

    return transactionLog;

  }

}
Ø  2d.Provider Binding – Alternative to @Provides by moving instance production logic to a separate class
a.Guice Configuration
public class BillingModule extends AbstractModule {

  @Override

  protected void configure() {

    bind(TransactionLog.class)

        .toProvider(DatabaseTransactionLogProvider.class);

  }



b.Provider as instance creation class
public interface Provider<T> {

  T get();

}
 
public class DatabaseTransactionLogProvider implements Provider<TransactionLog> {

 



  public TransactionLog get() {

    DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();

    transactionLog.setConnection(connection);

    return transactionLog;

  }

}

Ø  2e. @ImplementedBy and @ProvidedBy – Provides default implementation, can be overriden by the bind() method in guice configuration module.
@ImplementedBy(PayPalCreditCardProcessor.class)

public interface CreditCardProcessor{…}
Equivalient to
bind(CreditCardProcessor.class).to(PayPalCreditCardProcessor.class);

@ProvidedBy(DatabaseTransactionLogProvider.class)

public interface TransactionLog {…}
Equivalent to
    bind(TransactionLog.class).toProvider(DatabaseTransactionLogProvider.class);

Ø  2f. Inject into an existing instance – (for objects that are already instatiated not by guice)useful in servlets or instances that you can’t instantiate or already instatiated by container
    Injector injector = Guice.createInjector(...);

    

    CreditCardProcessor creditCardProcessor = new PayPalCreditCardProcessor();

    injector.injectMembers(creditCardProcessor);




Injecting by Providers (alternative to direct @Inject)
Usability Criteria - Whenever you need to scope mixing (like accessing a request scoped object in session scope or singleton scope) or get instances on demand (lazy loading) or inject more than one instances per type.
Below instead of injecting directly CreditcardProcessor, we use Provider< CreditcardProcessor > and its get() method call provides an instance when required. You can call the get() method to provide as many instances you like and wherever you need inside the code
public class RealBillingService implements BillingService {

  private final Provider<CreditCardProcessor> processorProvider;

  private final Provider<TransactionLog> transactionLogProvider;



  @Inject

  public RealBillingService(Provider<CreditCardProcessor> processorProvider,

      Provider<TransactionLog> transactionLogProvider) {

    this.processorProvider = processorProvider;

    this.transactionLogProvider = transactionLogProvider;

  }



  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {

    CreditCardProcessor processor = processorProvider.get();

    TransactionLog transactionLog = transactionLogProvider.get();



    /* use the processor and transaction log here */

  }

}

Guice configuration
 
public class BillingModule extends AbstractModule {

  @Override 

  protected void configure() {


// Class binding
       bind(TransactionLog.class).to(DatabaseTransactionLog.class).in(Singleton.class);
bind(CreditCardProcessor.class).to(PaypalCreditCardProcessor.class);
 
//or instance binding
bind(CreditCardProcessor.class).toInstance(new PaypalCreditCardProcessor());
 
  }

}

AOP with GUICE
Two concepts – Matcher and MethodInterceptors
Matchers matches classes to apply pointcuts (cross cutting concerns), MethodInterceptors implement the cross cutting concern.

Example in steps – stop pizza orders on weekends
1.create an annotation or use builtin @Named annotation
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD)

@interface NotOnWeekends {}
2.Annotate the method where you want to introduce pointcut
public class RealBillingService implements BillingService {



  @NotOnWeekends

  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {

    ...

  }
3.Implement MethodInterceptors
public class WeekendBlocker implements MethodInterceptor {

  public Object invoke(MethodInvocation invocation) throws Throwable {

    Calendar today = new GregorianCalendar();

    if (today.getDisplayName(DAY_OF_WEEK, LONG, ENGLISH).startsWith("S")) {

      throw new IllegalStateException(

          invocation.getMethod().getName() + " not allowed on weekends!");

    }

    return invocation.proceed();

  }

}

4.Wire or configure all
public class NotOnWeekendsModule extends AbstractModule {

  protected void configure() {

    bindInterceptor(Matchers.only(RealBillingService.class), Matchers.annotatedWith(NotOnWeekends.class), 

        new WeekendBlocker());

  }

}

You can use Matchers.any() or Matchers.subclassesof(BillingService.class)
or if you want the interceptor to be injected with some objects
public class NotOnWeekendsModule extends AbstractModule {

  protected void configure() {

    WeekendBlocker weekendBlocker = new WeekendBlocker();

    requestInjection(weekendBlocker);

    bindInterceptor(Matchers.any(), Matchers.annotatedWith(NotOnWeekends.class), 

       weekendBlocker);

  }

}




Bootstrap for Injecting in http servlets –
Bootstrap guice servlet –
1.
 
<listener>

  <listener-class>com.example.MyGuiceServletConfig</listener-class>

</listener>
2.
public class MyGuiceServletConfig extends GuiceServletContextListener {



  @Override

  protected Injector getInjector() {

    return Guice.createInjector(new ServletModule());

  }

}
or You can subclass ServletModule class like above
   Guice.createInjector(..., new ServletModule() {



     @Override

     protected void configureServlets() {
filter("/*").through(MyFilter.class);

       serve("*.html").with(MyServlet.class);

     }

   }

3.
  <filter>

    <filter-name>guiceFilter</filter-name>

    <filter-class>com.google.inject.servlet.GuiceFilter</filter-class>

  </filter>



  <filter-mapping>

    <filter-name>guiceFilter</filter-name>

    <url-pattern>/*</url-pattern>

  </filter-mapping>

  1. Injecting inside servlet if ServletModdule class is installed as above
@Singleton

public class MyServlet extends HttpServlet {
@Inject private Injector injector;
...
}

Or Inside servlet init() method, use the bootstrap class
    public void init(ServletConfig config) throws ServletException {
      super.init(config);
      ServletContext servletContext = config.getServletContext();
      Injector injector =
          (Injector) servletContext.getAttribute(Injector.class.getName());
      injector.injectMembers(this);
    }



You can inject ServletContext. You can inject Request parameters by,
@Inject @RequestParameters Map<String, String[]> params;
 
 



GUICE with JERSEY REST Service
Setup –
Step 1 – Use maven dependency  jersey-guice with google guice as shown below.
              <!-- Google Guice -->
              <dependency>
                     <groupId>com.google.inject</groupId>
                     <artifactId>guice</artifactId>
                     <version>3.0</version>
              </dependency>

              <!-- Jersey Guice Module -->
              <dependency>
                     <groupId>com.sun.jersey.contribs</groupId>
                     <artifactId>jersey-guice</artifactId>
                     <version>1.15</version>
              </dependency>


Step 2 - Follow the same setup as web application, but Instead of creating injector from ServletModule class, use JerseyServletModule and use jersey GuiceContainer class to bind to request that will be server by Jersey as REST resources as shown below.

   Guice.createInjector(..., new JerseyServletModule() {



     @Override

     protected void configureServlets() {
filter("/*").through(MyFilter.class);

       serve("*.html").with(MyServlet.class);
serve("/rest/*").with(GuiceContainer.class);


     }

   }

That’s it, then you are read to inject guice dependency to REST services by jersey framework.
JSP injection - look here
http://turbomanage.wordpress.com/2009/12/11/how-to-inject-guice-objects-in-a-jsp/
 

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