Jakarta Contexts and Dependency Injection (CDI) is certainly one of the most important APIs for the Jakarta EE platform. In version 2.0, it also works with Java SE.

Nowadays, CDI has an impact on many other APIs in the Jakarta EE platform as said in an interview for Java EE 8 – The Next Frontier project:

There are a lot of new features in CDI 2.0. This recipe will cover observer ordering to give you a quick start.

We need to add the right CDI 2.0 dependency to your project. To make things easier at this point, we are going to use CDI SE, the dependency that allows you to use CDI without a Jakarta EE server:

<dependency>
    <groupId>org.jboss.weld.se</groupId>
    <artifactId>weld-se-shaded</artifactId>
    <version>3.1.0.Final</version>
</dependency>

This recipe will show you one of the main features introduced by CDI 2.0: ordered observers. Now, you can turn the observer's job into something predictable:

1. First, let's create an event to be observed:

public class MyEvent {
    
    private final String value;
    
    public MyEvent(String value){
        this.value = value;
    }
    
    public String getValue(){
        return value;
    }
}

2. Now, we build our observers and the server that will fire them:

public class OrderedObserver {

    public static void main(String[] args){
        try(SeContainer container =    
        SeContainerInitializer.newInstance().initialize()){
            container
                .getBeanManager()
                .fireEvent(new MyEvent("event: " + 
                System.currentTimeMillis()));
        }
    }
 
    public void thisEventBefore(
            @Observes @Priority(Interceptor.Priority
            .APPLICATION - 200) 
            MyEvent event){
        
        System.out.println("thisEventBefore: " + event.getValue());
    }
 
    public void thisEventAfter(
            @Observes @Priority(Interceptor.Priority
           .APPLICATION + 200) 
            MyEvent event){
        
        System.out.println("thisEventAfter: " + event.getValue());
    }  
}

3. Also, don't forget to add the beans.xml file to the META-INF folder:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://xmlns.jcp.org/xml/ns/javaee"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
       http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd"
       bean-discovery-mode="all">
</beans>

4. Once you run it, you should see a result like this:

INFO: WELD-ENV-002003: Weld SE container 
353db40d-e670-431d-b7be-4275b1813782 initialized
 thisEventBefore: event -> 1501818268764
 thisEventAfter: event -> 1501818268764

Now, let's see how this works.

First, we are building a server to manage our event and observers:

public static void main(String[] args){
    try(SeContainer container =  
    SeContainerInitializer.newInstance().initialize()){
        container
            .getBeanManager()
            .fireEvent(new ExampleEvent("event: " 
            + System.currentTimeMillis()));
    }
}

This will give us all of the resources needed to run the recipe as if it were a Jakarta EE server.

Then, we build an observer, as follows:

public void thisEventBefore(
        @Observes @Priority(Interceptor.Priority.APPLICATION - 200) 
        MyEvent event){
  
    System.out.println("thisEventBefore: " + event.getValue());
}

So, we have three important topics:

• @Observes: This annotation is used to tell the server that it needs to watch the events fired with MyEvent.
• @Priority: This annotation informs in which priority order this observer needs to run; it receives an int parameter, and the execution order is ascendant.
• MyEvent event: This is the event being observed.

In the thisEventBefore method and thisEventAfter, we only changed the @Priority value and the server took care of running it in the right order.

The behavior would be exactly the same in a Jakarta EE 8 server. You just wouldn't need SeContainerInitializer and would need to change the dependencies to the following:

        <dependency>
            <groupId>jakarta.platform</groupId>
            <artifactId>jakarta.jakartaee-api</artifactId>
            <version>8.0.0</version>
        </dependency>

• You can stay tuned with everything related to the Jakarta CDI specification at http://www.cdi-spec.org/.
• The source code of this recipe is at https://github.com/eldermoraes/javaee8-cookbook/tree/master/chapter01/ch01-cdi