How to change the default message of the required field in the popover of form-control in bootstrap?

 <form class="form-asd" role="form">
    <h2 class="form-signin-heading">login</h2><hr />
    <label class="control-label"  for="username">Username</label>
    <input class="form-control" type="email"  required="" placeholder="username"data-error="enter username"></input>
    <label class="control-label"  for="username">password</label>
    <input class="form-control" type="password"  required=" " placeholder="Password"></input>
    <label class="checkbox"></label>
    <button class="btn btn-lg btn-primary " type="submit">submit</button>
</form>

how can we change this default message of popover of the require field "Please fill out this field "to "please enter username"

You can use setCustomValidity function when oninvalid event occurs.

Like below:-

<input class="form-control" type="email" required="" 
    placeholder="username" oninvalid="this.setCustomValidity('Please Enter valid email')">
</input>

Update:-

To clear the message once you start entering use oninput="setCustomValidity('') attribute to clear the message.

<input class="form-control" type="email"  required="" placeholder="username"
 oninvalid="this.setCustomValidity('Please Enter valid email')"
 oninput="setCustomValidity('')"></input>

Combination of Mritunjay and Bartu's answers are full answer to this question. I copying the full example.

<input class="form-control" type="email"  required="" placeholder="username"
 oninvalid="this.setCustomValidity('Please Enter valid email')"
 oninput="setCustomValidity('')"></input>

Here,

this.setCustomValidity('Please Enter valid email')" - Display the custom message on invalidated of the field

oninput="setCustomValidity('')" - Remove the invalidate message on validated filed.

And for all input and select:

$("input[required], select[required]").attr("oninvalid", "this.setCustomValidity('Required!')");
$("input[required], select[required]").attr("oninput", "setCustomValidity('')");

$("input[required]").attr("oninvalid", "this.setCustomValidity('Say Somthing!')");

this work if you move to previous or next field by mouse, but by enter key, this is not work !!!