싱글톤(Single Ton) 알고리즘을 사용한 중복 로그인 체크

	public class LoginManager implements HttpSessionBindingListener{
	private static LoginManager loginManager = null;
	// Users Hash Table
	private static Hashtable<HttpSession, String> loginUsers = new Hashtable<HttpSession, String>();
	// single ton
	public static synchronized LoginManager getInstance(){
	if(loginManager == null ){
	loginManager = new LoginManager();
	}
	return loginManager;
	}
	/**
	* session.setAttribute 시 호출.
	* HashTable User Save
	* **/
	@Override
	public void valueBound(HttpSessionBindingEvent event) {
	// TODO Auto-generated method stub
	loginUsers.put(event.getSession(), event.getName());
	}
	/**
	* session invalidate
	* hashTable user Delete
	* **/
	@Override
	public void valueUnbound(HttpSessionBindingEvent event) {
	// TODO Auto-generated method stub
	loginUsers.remove(event.getSession());
	}
	@Autowired
	public void removeSession(String userId){
	Enumeration e =loginUsers.keys();
	HttpSession session = null;
	while(e.hasMoreElements()){
	session = (HttpSession)e.nextElement();
	if(loginUsers.get(session).equals(userId)){
	session.invalidate();
	}
	}
	}
	/**
	* 해당 사용자 사용중 체크.
	* **/
	public boolean isUsing(String userId){
	return loginUsers.contains(userId);
	}
	/**
	* 해당 사용자 사용중 체크.
	* **/
	public void setSession(HttpSession session, String userId){
	session.setAttribute(userId, this);
	}
	/**
	* 입력받은 세션의 정보 리턴
	* **/
	public String getUserId(HttpSession session){
	return (String)loginUsers.get(session);
	}
	}

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출처 : http://teck10.tistory.com/75