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Service Advisors

Service advice represents a powerful meta-programming facility available to services. In fact, it is a kind of limited Aspect Oriented Programming.

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You could track down the implementation of each service and fix the logic that provides a return value ... or you could advise the methods:

  @Match("*")
  public static void adviseNonNull(MethodAdviceReceiver receiver)
  {
    MethodAdvice advice = new MethodAdvice()
    {
      void advise(Invocation invocation)
      {
        invocation.proceed();

        if (invocation.getResultType().equals(String.class) && invocation.getResult() == null)
          invocation.overrideResult("");
      }
    };

    receiver.adviseAllMethods(advice);
  };

This is a method that is placed in a module class. Note the terminology: advise is the verb ("to advise a method") and advice is the noun ("with this advice"). The MethodAdviceReceiver is a wrapper around the service being advised: you can add advice to some or all methods of the service, and also obtain the interface of the service. It is automatically passed into service advisor methods.

See See Injection in Detail for what can be injected into a service advisor method.

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Advice is pretty efficient, but it is still better to apply it only to methods that make sense. We can improve the service advisor method in our example to only advise methods that return String:

  @Match("*")
  public static void adviseNonNull(MethodAdviceReceiver receiver)
  {
    MethodAdvice advice = new MethodAdvice()
    {
      void advise(Invocation invocation)
      {
        invocation.proceed();

        if (invocation.getResult().equals(null))
          invocation.overrideResult("");
      }
    };

    for (Method m : receiver.getServiceInterface().getMethods())
    {
      if (m.getReturnType().equals(String.class))
        receiver.adviseMethod(m, advice);
    }
  };

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Logging advice is built into Tapestry. You can apply logging advice to your services very easily:

  @Match("*")
  public static void adviseLogging(LoggingAdvisor loggingAdvisor, Logger logger, MethodAdviceReceiver receiver)
  {
    loggingAdvisor.addLoggingAdvice(logger, receiver);
  }

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In many cases, the order in which the advice is given is very important; for example, you may want logging first, then transaction management, then security checks. The @Order annotation allows you to explicitly set the order.

Annotation driven advisors

  @Advise
  @Match("*DAO")
  public static void byServiceId(MethodAdviceReceiver receiver)
  {
    ...
  }

The advice above is applied to any service whose id matches the "*DAO" pattern.

Alternatively, marker annotations can be placed on the advise method to match a specific service.

  @Advise
  @Blue
  public static void byMarkerAnnotation(MethodAdviceReceiver receiver)
  {
    ...
  }

The advice above is applied to any service that is marked by the @Blue annotation.

By default, @Advise annotation applies the advice to any service matched by the @Match or marker annotations. You can limit the matching to a single service interface, as shown in the following example.

  @Advise(serviceInterface=MyService.class)
  @Match("*DAO")
  public static void byMatchAnnotation(MethodAdviceReceiver receiver)
  {
    ...
  }

In the example above, the advice is applied to any implementation of MyService interfaces whose id matches the "*DAO" pattern.

  @Advise(serviceInterface=MyService.class)
  @Blue
  public static void byMarkerAnnotation(MethodAdviceReceiver receiver)
  {
    ...
  }

The advice above is applied to any implementation of the MyService interface that is marked by the @Blue annotation.

Decorators and Advice

Service decorators are another way to achieve the same thing; service advisors are a more recent addition, added in Tapestry 5.1.

It is not recommended that you mix advice and decoration. If you do, decoration take precedence; all decorators will be in effect before any advice (internally, they are two separate steps, with advice being processed and the result of that used by the decorators).

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