How to manually set an authenticated user in Spring Security / SpringMVC

I had the same problem as you a while back. I can't remember the details but the following code got things working for me. This code is used within a Spring Webflow flow, hence the RequestContext and ExternalContext classes. But the part that is most relevant to you is the doAutoLogin method.

public String registerUser(UserRegistrationFormBean userRegistrationFormBean,
                           RequestContext requestContext,
                           ExternalContext externalContext) {

    try {
        Locale userLocale = requestContext.getExternalContext().getLocale();
        this.userService.createNewUser(userRegistrationFormBean, userLocale, Constants.SYSTEM_USER_ID);
        String emailAddress = userRegistrationFormBean.getChooseEmailAddressFormBean().getEmailAddress();
        String password = userRegistrationFormBean.getChoosePasswordFormBean().getPassword();
        doAutoLogin(emailAddress, password, (HttpServletRequest) externalContext.getNativeRequest());
        return "success";

    } catch (EmailAddressNotUniqueException e) {
        MessageResolver messageResolvable 
                = new MessageBuilder().error()
                                      .source(UserRegistrationFormBean.PROPERTYNAME_EMAIL_ADDRESS)
                                      .code("userRegistration.emailAddress.not.unique")
                                      .build();
        requestContext.getMessageContext().addMessage(messageResolvable);
        return "error";
    }

}


private void doAutoLogin(String username, String password, HttpServletRequest request) {

    try {
        // Must be called from request filtered by Spring Security, otherwise SecurityContextHolder is not updated
        UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken(username, password);
        token.setDetails(new WebAuthenticationDetails(request));
        Authentication authentication = this.authenticationProvider.authenticate(token);
        logger.debug("Logging in with [{}]", authentication.getPrincipal());
        SecurityContextHolder.getContext().setAuthentication(authentication);
    } catch (Exception e) {
        SecurityContextHolder.getContext().setAuthentication(null);
        logger.error("Failure in autoLogin", e);
    }

}

I couldn't find any other full solutions so I thought I would post mine. This may be a bit of a hack, but it resolved the issue to the above problem:

public void login(HttpServletRequest request, String userName, String password)
{

    UsernamePasswordAuthenticationToken authRequest = new UsernamePasswordAuthenticationToken(userName, password);

    // Authenticate the user
    Authentication authentication = authenticationManager.authenticate(authRequest);
    SecurityContext securityContext = SecurityContextHolder.getContext();
    securityContext.setAuthentication(authentication);

    // Create a new session and add the security context.
    HttpSession session = request.getSession(true);
    session.setAttribute("SPRING_SECURITY_CONTEXT", securityContext);
}