Render Facelet programmatically. See: http://www.bradchen.com/blog/2011/07/17/render-facelets-programmatically

FaceletRenderer.java

public class FaceletRenderer {

	private FacesContext context;

	public FaceletRenderer(FacesContext context) {
		this.context = context;
	}

	/**
	 * Render the Facelets template specified.
	 *
	 * @param template path to the Facelets template
	 * @return rendered content of the Facelets template
	 */
	public String renderView(String template) {
		try {
			// store the original response writer
			ResponseWriter originalWriter = context.getResponseWriter();

			// put in a StringWriter to capture the output
			StringWriter stringWriter = new StringWriter();
			ResponseWriter writer = createResponseWriter(context, stringWriter);
			context.setResponseWriter(writer);

			// create a UIViewRoot instance using the template specified
			ViewHandler viewHandler = context.getApplication().getViewHandler();
			UIViewRoot view = viewHandler.createView(context, template);

			// the fun part -- do the actual rendering here
			ViewDeclarationLanguage vdl = viewHandler
					.getViewDeclarationLanguage(context, template);
			vdl.buildView(context, view);
			renderChildren(context, view);

			// restore the response writer
			context.setResponseWriter(originalWriter);

			return stringWriter.toString();
		} catch (IOException exception) {
			throw new RuntimeException(exception);
		}
	}

	/**
	 * Create ResponseWriter. Taken from FaceletViewDeclarationLanguage.java of
	 * MyFaces.
	 */
	private ResponseWriter createResponseWriter(FacesContext context,
			Writer writer) {
		ExternalContext extContext = context.getExternalContext();
		Map<String, Object> requestMap = extContext.getRequestMap();
		String contentType = (String)requestMap.get("facelets.ContentType");
		String encoding = (String)requestMap.get("facelets.Encoding");
		RenderKit renderKit = context.getRenderKit();
		return renderKit.createResponseWriter(writer, contentType, encoding);
	}

	/**
	 * Render a UIComponent. Taken from JSF.java of Seam 2.2.
	 */
	private void renderChildren(FacesContext context, UIComponent component)
			throws IOException {
		List<UIComponent> children = component.getChildren();
		for (int i = 0, size = component.getChildCount(); i < size; i++) {
			UIComponent child = (UIComponent)children.get(i);
			renderChild(context, child);
		}
	}

	/**
	 * Render the child and all its children components.
	 */
	private void renderChild(FacesContext context, UIComponent child)
			throws IOException {
		if (child.isRendered()) {
			child.encodeAll(context);
		}
	}

}