web/src/main/java/org/springframework/security/web/authentication/switchuser/SwitchUserFilter.java
protected Authentication attemptSwitchUser(HttpServletRequest request)
			throws AuthenticationException {
		UsernamePasswordAuthenticationToken targetUserRequest;

		String username = request.getParameter(this.usernameParameter);

		if (username == null) {
			username = "";
		}

		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Attempt to switch to user [" + username + "]");
		}

		UserDetails targetUser = this.userDetailsService.loadUserByUsername(username);
		this.userDetailsChecker.check(targetUser);

		// OK, create the switch user token
		targetUserRequest = createSwitchUserToken(request, targetUser);

		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Switch User Token [" + targetUserRequest + "]");
		}

		// publish event
		if (this.eventPublisher != null) {
			this.eventPublisher.publishEvent(new AuthenticationSwitchUserEvent(
					SecurityContextHolder.getContext().getAuthentication(), targetUser));
		}

		return targetUserRequest;
	}
protected Authentication attemptSwitchUser(HttpServletRequest request)
			throws AuthenticationException {
		UsernamePasswordAuthenticationToken targetUserRequest;

		String username = request.getParameter(this.usernameParameter);

		if (username == null) {
			username = "";
		}

		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Attempt to switch to user [" + username + "]");
		}

		UserDetails targetUser = this.userDetailsService.loadUserByUsername(username);
		this.userDetailsChecker.check(targetUser);

		// OK, create the switch user token
		targetUserRequest = createSwitchUserToken(request, targetUser);

		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Switch User Token [" + targetUserRequest + "]");
		}

		// publish event
		if (this.eventPublisher != null) {
			this.eventPublisher.publishEvent(new AuthenticationSwitchUserEvent(
					SecurityContextHolder.getContext().getAuthentication(), targetUser));
		}

		return targetUserRequest;
	}
Attempt to switch to another user. If the user does not exist or is not active,
return null.

@return The new <code>Authentication</code> request if successfully switched to
another user, <code>null</code> otherwise.

@throws UsernameNotFoundException If the target user is not found.
@throws LockedException if the account is locked.
@throws DisabledException If the target user is disabled.
@throws AccountExpiredException If the target user account is expired.
@throws CredentialsExpiredException If the target user credentials are expired.