forum.domen=forum.domen.com
forum.alink=<a href="http://${forum.domen}/{0}">{1}<a/>

int id = 12;
MessageSource messages = (MessageSource) ctx.getBean("messageSource");
System.out.println(messages.getMessage("forum.alink", new Object[] {"thread.jsp?id=" + id, "тут"}, Locale.getDefault()));

<a href="http://forum.domen.com/thread.jsp?id=12">тут<a/>

<bean id="messageSource" class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
        <property name="basename" value="classpath:messages"/>
        <property name="defaultEncoding" value="UTF-8"/>
</bean>

Exception in thread "main" java.lang.IllegalArgumentException: can't parse argument number forum.domen

Exception in thread "main" java.lang.IllegalArgumentException: can't parse argument number forum.domen
	at java.text.MessageFormat.makeFormat(MessageFormat.java:1339)
	at java.text.MessageFormat.applyPattern(MessageFormat.java:458)
	at java.text.MessageFormat.<init>(MessageFormat.java:368)
	at org.springframework.context.support.MessageSourceSupport.createMessageFormat(MessageSourceSupport.java:151)
	at org.springframework.context.support.ReloadableResourceBundleMessageSource$PropertiesHolder.getMessageFormat(ReloadableResourceBundleMessageSource.java:653)
	at org.springframework.context.support.ReloadableResourceBundleMessageSource.resolveCode(ReloadableResourceBundleMessageSource.java:291)
	at org.springframework.context.support.AbstractMessageSource.getMessageInternal(AbstractMessageSource.java:205)
	at org.springframework.context.support.AbstractMessageSource.getMessage(AbstractMessageSource.java:127)
	at ru.onec.TestClass.main(TestClass.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)