Previous examples show how Nucleus creates and initializes components from properties files. Nucleus also allows components to point to each other through configuration file properties.

For example, a Weather component might be defined in Nucleus, and the Person component needs a pointer to that Weather. The Weather class might look like this:

public class Weather {
  String currentWeather;

  public Weather () {
    System.out.println ("constructing Weather");
  }
  public String getCurrentWeather () {
    return currentWeather;
  }
  public void setCurrentWeather (String currentWeather) {
    System.out.println ("setting currentWeather to " + currentWeather);
    this.currentWeather = currentWeather;
  }
}

This example requires instantiation of a Nucleus Weather component, /services/Weather. You should compile the Weather Java class and create a Weather class component with a Weather.properties file in the same directory as Person.properties. The properties file might look like this:

$class=Weather
currentWeather=sunny

Next, modify the Person class so it defines a property that is set to a Weather component:

public class Person {
  String name;
  int age;
  Weather weather;

  public Person () {
    System.out.println ("constructing Person");
  }
  public String getName () { return name; }
  public void setName (String name) {
    System.out.println ("setting name to " + name);
    this.name = name;
  }
  public int getAge () { return age; }
  public void setAge (int age) {
    System.out.println ("setting age to " + age);
    this.age = age;
  }
  public Weather getWeather () { return weather; }
  public void setWeather (Weather weather) {
    System.out.println ("setting weather to " + weather.getCurrentWeather());
    this.weather = weather;
  }
}

Finally, modify the Person component’s properties file so it has a weather property that points to the weather component:

$class=Person
name=Stephen
age=20
weather=Weather

If you include the Person component as an initial service (described in the earlier section Starting a Nucleus Component), when you start your application, the Person component is created and initialized. Its name and age properties are set from the values found in the properties file. In order to set the weather property, Nucleus resolves the name Weather by creating and initializing the Weather component before assigning it to the Person property. The output should look something like this:

constructing Person
setting name to Stephen
setting age to 20
constructing Weather
setting currentWeather to sunny
setting weather to sunny

The first two lines of the output show that Nucleus created the /services/Person component and set the age property. Then Nucleus attempts to set the weather property. In doing so, it searches for the component named Weather. This is a relative name, and so it is resolved relative to the current context /services, resulting in /services/Weather.

Nucleus searches its existing components and, finding that there is no /services/Weather, it tries to create one from the configuration file services/Weather.properties. This causes Nucleus to construct an instance of the Weather class and initialize its currentWeather property, thereby resulting in the third and fourth lines of output.

Now that a /services/Weather component is created and initialized, Nucleus can initialize the rest of the Person component, by setting its weather and name properties. This results in the last two lines of output.

Nucleus does not limit the number of components that refer to each other through properties. For example, component 1 can refer to component 2, which refers to component 3, and so on. Nucleus can even resolve circular references without spiraling into infinite loops. For example, component 1 might have a property that points to component 2, which has a property that points back to component 1. However, you should try to avoid circular references as they can result in deadlocks. See Enabling Deadlock Detection for information about avoiding deadlocks.

Application errors can also occur if you reference a property of a component before that component is completely configured. To diagnose this type of error, set the loggingInfo property of the / Nucleus service to true, and the Oracle Commerce Platform prints information messages for this situation.

Arrays of components can also be specified in the same way that other array values are specified: as a comma-separated list. For example, the Person component might have a property called cityWeathers that contains an array of Weather components:

public Weather [] getCityWeathers ();
public void setCityWeathers (Weather [] cityWeathers);

This property might be initialized in the configuration file like this:

cityWeathers=\
        /services/weather/cities/atlanta,\
        /services/weather/cities/boston,\
        /services/weather/cities/tampa,\
        /services/weather/cities/phoenix

Nucleus handles this by finding each of the components in the list, arranging the found components into a 4-element array, then assigning that array to the cityWeathers property of the Person component.