HttpSession 数据同步

1：第一种方式，把对象的属性定义为immutable，synchronized同步更新

Public class HighScore {
    public final String name;
    public final int score;

    public HighScore(String name, int score) {
        this.name = name;
        this.score = score;
    }
}

public PlayerScore getHighScore() {
    ServletContext ctx = getServletConfig().getServletContext();
    return (PlayerScore) ctx.getAttribute("highScore");
}

public void updateHighScore(PlayerScore newScore) {
    ServletContext ctx = getServletConfig().getServletContext();
    PlayerScore hs = (PlayerScore) ctx.getAttribute("highScore");
    synchronized (lock) {
        if (newScore.score > hs.score) 
            ctx.setAttribute("highScore", newScore);
    }
} 

or
public PlayerScore getHighScore() {
    ServletContext ctx = getServletConfig().getServletContext();
    AtomicReference<PlayerScore> holder 
        = (AtomicReference<PlayerScore>) ctx.getAttribute("highScore");
    return holder.get();
}

public void updateHighScore(PlayerScore newScore) {
    ServletContext ctx = getServletConfig().getServletContext();
    AtomicReference<PlayerScore> holder 
        = (AtomicReference<PlayerScore>) ctx.getAttribute("highScore");
    while (true) {
        HighScore old = holder.get();
        if (old.score >= newScore.score)
            break;
        else if (holder.compareAndSet(old, newScore))
            break;
    } 
}

第二种方式： 一般一个用户同时只有一个请求，如果使用ajax可能出现并行多个请求，这样我们可以再controller层控制同时只有一个请求被处理，SpringMVC AbstractController类提供了相应的实现....

public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)
			throws Exception {

		// Delegate to WebContentGenerator for checking and preparing.
		checkAndPrepare(request, response, this instanceof LastModified);

		// Execute handleRequestInternal in synchronized block if required.
		if (this.synchronizeOnSession) {
			HttpSession session = request.getSession(false);
			if (session != null) {
				Object mutex = WebUtils.getSessionMutex(session);
				synchronized (mutex) {
					return handleRequestInternal(request, response);
				}
			}
		}
		
		return handleRequestInternal(request, response);
	}