config/src/main/java/org/springframework/security/config/annotation/web/configurers/AbstractAuthenticationFilterConfigurer.java
protected T loginPage(String loginPage) {
		setLoginPage(loginPage);
		updateAuthenticationDefaults();
		this.customLoginPage = true;
		return getSelf();
	}
protected T loginPage(String loginPage) {
		setLoginPage(loginPage);
		updateAuthenticationDefaults();
		this.customLoginPage = true;
		return getSelf();
	}
<p>
Specifies the URL to send users to if login is required. If used with
{@link WebSecurityConfigurerAdapter} a default login page will be generated when
this attribute is not specified.
</p>

<p>
If a URL is specified or this is not being used in conjuction with
{@link WebSecurityConfigurerAdapter}, users are required to process the specified
URL to generate a login page.
</p>