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Bean Subtypes

<p>
	In addition to the bean type name support described above, simplified support is provided
	for bean subtypes.
</p>
<p>
	Bean subtypes are similar in concept to bean type names, except for the following differences:
</p>
<ul>
	<li>You specify the list of possible subclasses through an annotation on a parent bean class.
	<li>You do not need to register the subtype classes on the bean dictionary of the parser.
</ul>
<p>
	In the following example, the abstract class has two subclasses:
</p>
<p class='bpcode w800'>
	<jc>// Abstract superclass</jc>
	<ja>@Bean</ja>(
		dictionary={A1.<jk>class</jk>, A2.<jk>class</jk>}
	)
	<jk>public abstract class</jk> A {
		<jk>public</jk> String <jf>f0</jf> = <js>"f0"</js>;
	}
	 
	<jc>// Subclass 1</jc>
	<ja>@Bean</ja>(typeName=<js>"A1"</js>)
	<jk>public class</jk> A1 <jk>extends</jk> A {
		<jk>public</jk> String <jf>f1</jf>;
	}
	 
	<jc>// Subclass 2</jc>
	<ja>@Bean</ja>(typeName=<js>"A2"</js>)
	<jk>public class</jk> A2 <jk>extends</jk> A {
		<jk>public</jk> String <jf>f2</jf>;
	}
</p>
<p>
	When serialized, the subtype is serialized as a virtual <js>"_type"</js> property:
</p>	
<p class='bpcode w800'>
	JsonSerializer s = SimpleJsonSerializer.<jsf>DEFAULT</jsf>;
	
	A1 a1 = <jk>new</jk> A1();
	a1.<jf>f1</jf> = <js>"f1"</js>;
	String r = s.serialize(a1);
	<jsm>assertEquals</jsm>(<js>"{_type:'A1',f1:'f1',f0:'f0'}"</js>, r);
</p>
<p>
	The following shows what happens when parsing back into the original object.
</p>
<p class='bpcode w800'>
	JsonParser p = JsonParser.<jsf>DEFAULT</jsf>;
	A a = p.parse(r, A.<jk>class</jk>);
	<jsm>assertTrue</jsm>(a <jk>instanceof</jk> A1);
</p>