In this recipe, we will write a very simple modular Hello world program to test our JDK installation. This simple example prints Hello world in XML; after all, it's the world of web services.

You should have JDK installed and the PATH variable updated to point to the JDK installation.

1. Let's define the model object with the relevant properties and annotations that will be serialized into XML:

        @XmlRootElement
        @XmlAccessorType(XmlAccessType.FIELD) 
        class Messages{     
          @XmlElement 
          public final String message = "Hello World in XML"; 
        }

In the preceding code, @XmlRootElement is used to define the root tag, @XmlAccessorType is used to define the type of source for the tag name and tag values, and @XmlElement is used to identify the sources that become the tag name and tag values in the XML.

2. Let's serialize an instance of the Message class into XML using JAXB:

public class HelloWorldXml{
  public static void main(String[] args) throws JAXBException{
    JAXBContext jaxb = JAXBContext.newInstance(Messages.class);
    Marshaller marshaller = jaxb.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FRAGMENT,Boolean.TRUE);
    StringWriter writer = new StringWriter();
    marshaller.marshal(new Messages(), writer);
    System.out.println(writer.toString());
  } 
}

3. We will now create a module named com.packt. To create a module, we need to create a file named module-info.java, which contains the module definition. The module definition contains the dependencies of the module and the packages exported by the module to other modules:

    module com.packt{
      //depends on the java.xml.bind module
      requires java.xml.bind;
      //need this for Messages class to be available to java.xml.bind
      exports  com.packt to java.xml.bind;
    }

The directory structure with the preceding files is as follows:

4. Let's compile and run the code. From the hellowordxml directory, create a new directory in which to place your compiled class files:

      mkdir -p mods/com.packt

Compile the source, HelloWorldXml.java and module-info.java, into the mods/com.packt directory:

      javac -d mods/com.packt/ src/com.packt/module-info.java
      src/com.packt/com/packt/HelloWorldXml.java

5. Run the compiled code using java --module-path mods -m com.packt/com.packt.HelloWorldXml. You will see the following output:

<messages><message>Hello World in XML</message></messages>

Don't worry if you are not able to understand the options passed with the java or javac commands. You will learn about them in Chapter 3, Modular Programming.