JAVA EE – CDI Interceptor Example

In this post I will explain you one simple way of using interceptors in Java EE.

We have a simple java project that only displays a name wich is setted in the Controller class by the init method. But as you will see next, the final result it’s not the name that is setted by the controler, because we have an interceptor that changes the name behind the scenes.

The index page index.xhtm makes a server request to display the name. But that request is going to be intercepted by our interceptor MyInterceptor.java. Then who makes the request to the controller is the interceptor and if the response is of the type String it changes it to another string. When the page renders it will show not the response from the controller but the object that our interceptor returns to it.

And that’s it. Even without the final user knows nothing about it, the response has been manipulated by our interceptor.

  • Project Scheme
interceptor_project
interceptor_project

 

  • index.xhtml

 

  • Controller.java

Note that in line 10 we have a special annotation. This marks this bean to be intercepted, as you will see next.  The code in the 20th line, just sets the variable name to John Doe.

  • LogToConsoleAndModifyResult.java

This is a simple annotation class (out of the scope of this post, for now). The important thing here is the @InterceptorBinding annotation.

  • MyInterceptor.java

In the MyInterceptor class we have 3 important annotations:

@Interceptor  (line 10),
@LogToConsoleAndModifyResult  (line 11) and the
@AroundInvoke  (line 14).

The first one makes this bean an interceptor. The second, marks this interceptor to intercept all methods in classes  annotated with the the same annotation. The last one, marks the method that is going to manipulate all the data.

Note that this method must returns an Object and as a parameter it takes the invocation context.

First we have a logger that prints out to the console a message before the interception. Then with the help of the invocation context, it lets the original method to proceed and keeps the result. Then it prints again to the console the real name returned by the controller (John Doe). After this, it changes the return value to Jane Doe, that is exactly what is going to be displayed in the page as the final result.

  • beans.xml

Finally, in order to use interceptors we must register them in the beans.xml file (line 3). Another possibility is to annotate the controller with @Interceptors({MyInterceptor.class}) but this approach is not recommended.

  •  The Log

the log messages

Here are the three messages sent to the console.

  • The result page

web page

As you can see here the displayed name isn’t John Doe, as you might expect, but the modified result: Jane Doe.


Source code

Feel free to comment.

Logger CDI Injection

In this post I will show you how to inject an instance of Logger class using CDI (Context Dependency Injection) and SLF4J (Simple Logging Facade for Java).

To keep it simple, we only have a xhtml page (index), a managed bean (Controller.java) and the producer (LoggerProducer.java). The project configuration is as follows:

project
project

 

As we are using JSF with facelets, the index page looks like:

In this file we simply display the name that is setted in the controller (line 10).

 

In the controller we inject (@Inject) an instance of the Logger class and print to the console the name before and after the name is setted (lines 23 and 25). CDI knows which Logger to inject because we have a class that works as a factory of Loggers (LoggerProducer) with some special annotations.

 

In the LoggerProducer class we have an annotation (@Produces) in a method that returns a Logger instance. This is enough to CDI inject a Logger when needed. Notice that in line 13 we pass as a parameter the injection point (ip) in order to get the class where the logger is being injected (in this specific case ‘Controller.class’). This is only necessary beacuse the getLogger method needs it.

Finally, here are the results (the log and the web page):

log

log

As you can see here, we have two lines displaying the information that we expected: first the name is null and after the set of the variable name, name = John Doe.

the result page
the result page

 

That’s it. It’s just that simple!

Conclusion

First we create a factory of loggers, then we inject them whenever we need them:
@Produces to indicate CDI what we are producing and
@Inject to inject the instance ready to use.

Notice that this methodology works with other types rather than Logger. You may inject whatever you want by simply following the above technic.

Feel free to comment.

Source code