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Chapter 7. Advanced Dependency Injection and IoC

Today, Dependency injection (DI), also called Inversion of Control (IoC), lies at the core of many frameworks that embrace the notion of a container or a component model. Component models were discussed in a previous chapter. JBoss JMX kernel, the precursor to the Microcontainer, provided only lightweight DI/IoC support, primarily due to the limitations of accessing MBeans through the MBeans server. However with the new POJO-based component model, several new and interesting features are available.
This chapter shows how you can apply different DI concepts with the help of the JBoss Microcontainer. These concepts will be expressed via XML code, but you can also apply most of these features using annotations.

7.1. Value Factory
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A value factory is a bean which has one or more methods devoted to generating values for you. See Example 7.1, “Value Factory”.

Example 7.1. Value Factory

<bean name="Binding" class="org.jboss.demos.ioc.vf.PortBindingManager">
  <constructor>
    <parameter>
      <map keyClass="java.lang.String" valueClass="java.lang.Integer">
	<entry>
	  <key>http</key>
	  <value>80</value>
	</entry>
	<entry>
	  <key>ssh</key>
	  <value>22</value>
	</entry>
      </map>
    </parameter>
  </constructor>
</bean>
<bean name="PortsConfig" class="org.jboss.demos.ioc.vf.PortsConfig">
  <property name="http"><value-factory bean="Binding" method="getPort" parameter="http"/></property>
  <property name="ssh"><value-factory bean="Binding" method="getPort" parameter="ssh"/></property>
  <property name="ftp">
    <value-factory bean="Binding" method="getPort">
      <parameter>ftp</parameter>
      <parameter>21</parameter>
    </value-factory>
  </property>
  <property name="mail">
    <value-factory bean="Binding" method="getPort">
      <parameter>mail</parameter>
      <parameter>25</parameter>
    </value-factory>
  </property>
</bean>
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Example 7.2, “PortsConfig” shows how the PortsConfig bean uses Binding bean to get its values via the getPort method invocation.

Example 7.2. PortsConfig

public class PortBindingManager {
    private Map<String, Integer> bindings;
    public PortBindingManager(Map<String, Integer> bindings)
    {
	this.bindings = bindings;
    }
    public Integer getPort(String key)
    {
	return getPort(key, null);
    }
    public Integer getPort(String key, Integer defaultValue)
    {
	if (bindings == null)
	    return defaultValue;
	Integer value = bindings.get(key);
	if (value != null)
	    return value;
	if (defaultValue != null)
	    bindings.put(key, defaultValue);
	return defaultValue;
    }
}
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