Introducing iframe tag 404

 Springboot own learning project, there is a page, there are <iframe> tag with the introduction of another html directory, as follows

 <div id="bd">
        <iframe width="100%" height="100%" id="mainIframe" src="nav.html" frameborder="0"></iframe>
    </div>

Being given a browser to access: 404, I do not know what the reason is not to visit. Then I changed:

<div id="bd">
        <iframe width="100%" height="100%" id="mainIframe" src="nav" frameborder="0"></iframe>
    </div>

 In the controller plus:

  @RequestMapping("/nav")
    public String a() {
        return "nav";
    }

On it.

They did not know the specific reasons, the front is not very good.