thymeleafexamples-stsm | Spring Thyme Seedstarter Manager - Companion application | Learning library

After Spring MVC matches a controller method successfully and before it actually invokes it , it will resolve the method arguments based on argument type and its annotations. You can find all the supported type , annotations and the resolving rules from this table.

In your case , as SeedStarter does not have any annotations on it and its type is not the supported types , it will fall back to default as if @ModelAttribute is annotated on it. (Mentioned by the last rule in that table). And from @ModelAttribute docs , it will instantiate SeedStarter as follows :

• From the model if already added by using Model.
• From the HTTP session by using @SessionAttributes.
• From a URI path variable passed through a Converter (see the next example).
• From the invocation of a default constructor.
• From the invocation of a “primary constructor” with arguments that match to Servlet request parameters. Argument names are determined through JavaBeans @ConstructorProperties or through runtime-retained parameter names in the bytecode.