code: public static void toWriter(String templateFileName, VelocityContext context, Writer writer) {
  		assertInit();
  
  		final Template template = Velocity.getTemplate(templateFileName);
  		merge(template, context, writer);
  	}
docs: 生成内容写入流<br>
  会自动关闭Writer
  
  @param templateFileName 模板文件名
  @param context 上下文
  @param writer 流