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Using with Spring and Injection frameworks

<p>
	The Juneau REST server API is compatible with dependency injection frameworks such as Spring.
</p>
<p>
	The important class is the {@link oajr.RestResourceResolver} class which is used
	to resolve child servlet/resource implementation classes inside parent contexts.
	In other words, it's used for resolving {@link oajr.annotation.Rest#children() @Rest(children)} instances.
</p>
<p>
	The general approach starts with defining a resolver that uses the Spring application context for resolution:
</p>
<p class='bpcode w800'>
	<jk>public class</jk> SpringRestResourceResolver <jk>extends</jk> RestResourceResolverSimple {

		<jk>private final</jk> ApplicationContext <jf>appContext</jf>;
		
		<jk>public</jk> SpringRestResourceResolver(ApplicationContext appContext) {
			<jk>this</jk>.<jf>appContext</jf> = appContext;
		}
		
		<jk>@Override</jk> <jc>/* RestResourceResolverSimple */</jc>
		<jk>public</jk> Object resolve(Class&lt;?&gt; resourceType, RestContextBuilder builder) <jk>throws</jk> Exception {
			Object resource = <jf>appContext.</jf>getBean(type);
			
			<jc>// If Spring can't resolve it, use default resolution (just look for no-arg constructor).</jc>
			<jk>if</jk> (resource == <jk>null</jk>) {
				resource = <jk>super</jk>.resolve(resourceType, builder);
			}
			<jk>return</jk> resource;
		}
	}
</p>
<p>
	Next, define the Spring configuration to return our resolver:
</p>
<p class='bpcode w800'>
	<ja>@Configuration</ja>
	<jk>public abstract class</jk> MySpringConfiguration {
		
		<ja>@Autowired</ja>
		<jk>private static</jk> ApplicationContext <jsf>appContext</jsf>;
		
		<jk>public static</jk> ApplicationContext getAppContext(){
			<jk>return</jk> <jsf>appContext</jsf>;
		}
		
		<jk>public static void</jk> setAppContext(ApplicationContext appContext){
			MySpringConfiguration.<jsf>appContext</jsf> = appContext;
		}
		
		<ja>@Bean</ja>
		<jk>public</jk> RestResourceResolver restResourceResolver(ApplicationContext appContext) {
			<jk>return new</jk> SpringRestResourceResolver(appContext);
		}
	}
</p>
<p>
	Finally, define your <c>Root</c> resource with a constructor that takes in our rest resource resolver and
	sets it on the config object during initialization.
</p>
<p class='bpcode w800'>
	<ja>@Rest</ja>(
		children={
			...
		}
	)
	<jk>public class</jk> Root <jk>extends</jk> BasicRestServletGroup {
		
		<jk>private final</jk> RestResourceResolver <jf>resolver</jf>;
		
		<ja>@Inject</ja>
		<jk>public</jk> Root(RestResourceResolver resolver) {
			<jk>this</jk>.<jf>resolver</jf> = resolver;
		}
		
		<ja>@RestHook</ja>(<jsf>INIT</jsf>)
		<jk>public void</jk> initSpring(RestContextBuilder builder) <jk>throws</jk> Exception {
			builder.setResourceResolver(<jf>resolver</jf>);
		}
	}
</p>
<p>
	After that, just define constructors on your child resources to take in Spring beans:
</p>
<p class='bpcode w800'>
	<ja>@Rest</ja>(
		path=<js>"/child"</js>
	)
	<jk>public class</jk> MyChildResource <jk>extends</jk> BasicRestServlet {
	
		<jk>private final</jk> Bean1 <jf>bean1</jf>;
		<jk>private final</jk> Bean2 <jf>bean2</jf>;
		<jk>private final</jk> Bean3 <jf>bean3</jf>;
		
		<ja>@Inject</ja>
		<jk>public</jk> MyChildResource(Bean1 bean1, Bean2 bean2, Bean3 bean3) {
			<jk>this</jk>.<jf>bean1</jf> = bean1;
			<jk>this</jk>.<jf>bean2</jf> = bean2;
			<jk>this</jk>.<jf>bean3</jf> = bean3;
		}
</p>