JSESSIONID的说明

JSESSIONID的说明

Tans 1,352 2022-03-20

JSESSIONID的说明

JSESSIONID作用

因为HTTP协议是无状态的, 所以我们的服务器不知道对方是谁, 所以事实上, 当用户访问服务器的时候, 服务器会为每一个用户开启一个session, 那么浏览器怎么判断这个session到底属于哪个用户呢, 那么Jsessionid就显现出他的优势了, 服务器可以通过它来找到用户session信息, 它可以帮助服务期判断对方的身份(比如说对方是否已经认证过了....), 用代码表示就是:

jsessionid ==request.getSession().getId()

image-20210316140845408

基本流程

  1. 第一次访问服务器的时候, 会在响应头看到Set-Cookie的信息(只有在首次访问的时候在响应头会出现该信息), 随后浏览器会将浏览器的cookie并保存.但是因为没有设置时间, 所以关闭浏览器后就会消失.

    Set-Cookie: JSESSIONID=**********;

    image-20210316140218266

  2. 当再次请求的时候,浏览器会把请求头里面的cookie发送给服务器(每次请求都是这样)

Session对象

Session类图

image-20210316202105932

  • HttpSession是我们大家可以直接使用的Session
  • StandardSession是标准的HttpSession的实现, 同时它实现了Session接口, 用于Tomcat内部管理

Manager类图

image-20210316202327687

  • Manager是用来储存的Session的类, 它会使用Map来储存Session类

        /**
     * The set of currently active Sessions for this Manager, keyed by
     * session identifier.
     */
    protected Map<String, Session> sessions = new ConcurrentHashMap<>();

  • 其实现类有StandardManager和PersistentManager类; 两者的区别:

    • 前者在Tomcat执行的时候, 会把Session储存在内存中, Tomcat关闭的时候,(正常关闭), 会把Session写入到硬盘中, 等到Tomcat重启后将Session写入进来
    • 后者直接将Session存入到硬盘中

Manager与Context

在Tomcat中，一個Context就是部署到Tomcat中的一個應用（Webapp）。每一個Context都有一個單獨的Manager物件來管理這個應用的會話資訊。

image-20210316202824305

image-20210316205748479

Session的生命周期

这里我们使用Tomcat作为我们的Web服务器, 那么

  1. 解析获取requestedSession和requestSessionID:

    request.getSeesion()获得Session, 主要调用doGetSession方法

    //Parameter: create: 在没有获取到Session的情况下, 是否创建一个新的Session
protected Session doGetSession(boolean create) {

    // There cannot be a session if no context has been assigned yet
    //如果没有上下文容器, 那么没有session
    Context context = getContext();
    if (context == null) {
        return (null);
    }

    // Return the current session if it exists and is valid
    //如果存在session但是失效, 返回现有的session = null
    if ((session != null) && !session.isValid()) {
        session = null;
    }
    if (session != null) {
        return (session);
    }

    // Return the requested session if it exists and is valid
    //如果session可用并且存在
    Manager manager = context.getManager();
    if (manager == null) {
        return null;        // Sessions are not supported
    }
    if (requestedSessionId != null) {
        try {
            //通过我们所说的sessionId来寻找session
            session = manager.findSession(requestedSessionId);
        } catch (IOException e) {
            session = null;
        }
        if ((session != null) && !session.isValid()) {
            session = null;
        }
        if (session != null) {
            session.access();
            return (session);
        }
    }

    // Create a new session if requested and the response is not committed
    //否则创建一个Seesion
    if (!create) {
        return (null);
    }
    if ((response != null) &&
            context.getServletContext().getEffectiveSessionTrackingModes().
            contains(SessionTrackingMode.COOKIE) &&
            response.getResponse().isCommitted()) {
        throw new IllegalStateException
        (sm.getString("coyoteRequest.sessionCreateCommitted"));
    }

    // Re-use session IDs provided by the client in very limited
    // circumstances.
    //重用session IDs 被客户端提供的
    String sessionId = getRequestedSessionId();
    if (requestedSessionSSL) {
        // If the session ID has been obtained from the SSL handshake then
        // use it.
    } else if (("/".equals(context.getSessionCookiePath())
            && isRequestedSessionIdFromCookie())) {
        /* This is the common(ish) use case: using the same session ID with
         * multiple web applications on the same host. Typically this is
         * used by Portlet implementations. It only works if sessions are
         * tracked via cookies. The cookie must have a path of "/" else it
         * won't be provided for requests to all web applications.
         *
         * Any session ID provided by the client should be for a session
         * that already exists somewhere on the host. Check if the context
         * is configured for this to be confirmed.
         */
        if (context.getValidateClientProvidedNewSessionId()) {
            boolean found = false;
            for (Container container : getHost().findChildren()) {
                Manager m = ((Context) container).getManager();
                if (m != null) {
                    try {
                        if (m.findSession(sessionId) != null) {
                            found = true;
                            break;
                        }
                    } catch (IOException e) {
                        // Ignore. Problems with this manager will be
                        // handled elsewhere.
                    }
                }
            }
            if (!found) {
                sessionId = null;
            }
        }
    } else {
        sessionId = null;
    }
    //通过manager创建Seesion, 后面会讲到
    session = manager.createSession(sessionId);

    // Creating a new session cookie based on that session
    if ((session != null) && (getContext() != null)
            && getContext().getServletContext().
            getEffectiveSessionTrackingModes().contains(
                    SessionTrackingMode.COOKIE)) {
        Cookie cookie =
                ApplicationSessionCookieConfig.createSessionCookie(
                        context, session.getIdInternal(), isSecure());
        //返回一个带有 SessionID的Cookie
        response.addSessionCookieInternal(cookie);
    }

    if (session == null) {
        return null;
    }

    session.access();
    return session;
}

    关于requestedSessionId是如何获取的, Tomcat内部可以支持从 Cookie和URL中获取, 具体查看CoyoteAdapter类中的 postPraseRequest:

    String sessionID;
if (request.getServletContext().getEffectiveSessionTrackingModes()
        .contains(SessionTrackingMode.URL)) {

    //如果浏览器的Cookie被禁用
    //首先尝试在URL中获取, 这里getSessionUriParamName = jsessionid
    sessionID = request.getPathParameter(
            SessionConfig.getSessionUriParamName(
                    request.getContext()));
    if (sessionID != null) {
        request.setRequestedSessionId(sessionID);
        request.setRequestedSessionURL(true);
    }
}

//在sessionID中获取
parseSessionCookiesId(req, request);

    具体的parseSessionCookiesId(req, request)方法:

    //SessionCookieName = JSESSIONID
String sessionCookieName = SessionConfig.getSessionCookieName(context);

//遍历Cookie, 取出属性名为 JSESSIONID 的字段,
for (int i = 0; i < count; i++) {
    ServerCookie scookie = serverCookies.getCookie(i);
    if (scookie.getName().equals(sessionCookieName)) {
        // Override anything requested in the URL
        if (!request.isRequestedSessionIdFromCookie()) {
            // Accept only the first session id cookie
            convertMB(scookie.getValue());
            //将读出来的 JSESSIONID 设置在 request上..
            request.setRequestedSessionId
                (scookie.getValue().toString());
            request.setRequestedSessionCookie(true);
            request.setRequestedSessionURL(false);
            if (log.isDebugEnabled()) {
                log.debug(" Requested cookie session id is " +
                    request.getRequestedSessionId());
            }
        } else {
            if (!request.isRequestedSessionIdValid()) {
                // Replace the session id until one is valid
                convertMB(scookie.getValue());
                request.setRequestedSessionId
                    (scookie.getValue().toString());
            }
        }
    }
}

  2. 通过findSession()查询Session

    获取到 SessionId后, 回去寻找session, 不同的管理器获取方式不一样, 默认的是StandardManager:

    protected Map<String, Session> sessions = new ConcurrentHashMap<String, Session>();

public Session findSession(String id) throws IOException {
    if (id == null) {
        return null;
    }
    return sessions.get(id);
}

  3. 通过creatSession()创建Session

    public Session createSession(String sessionId) {

    //拒绝会话
    if ((maxActiveSessions >= 0) &&
            (getActiveSessions() >= maxActiveSessions)) {
        rejectedSessions++;
        throw new TooManyActiveSessionsException(
                sm.getString("managerBase.createSession.ise"),
                maxActiveSessions);
    }

    // Recycle or create a Session instance
    Session session = createEmptySession();

    // Initialize the properties of the new session and return it
    session.setNew(true);
    session.setValid(true);
    session.setCreationTime(System.currentTimeMillis());
    session.setMaxInactiveInterval(((Context) getContainer()).getSessionTimeout() * 60);
    String id = sessionId;
    //如果没有 SessionId 那么会生成一个唯一的 SessionId
    if (id == null) {
        id = generateSessionId();
    }
    //这里我们没有把Session放入到ConcurrentHashMap中, 其实已经在下面这条语句执行过了,可看下一段代码分析
    session.setId(id);
    sessionCounter++;

    SessionTiming timing = new SessionTiming(session.getCreationTime(), 0);
    synchronized (sessionCreationTiming) {
        sessionCreationTiming.add(timing);
        sessionCreationTiming.poll();
    }
    return (session);

}

    //SessionID设置完成, 自动放入到 Manager
public void setId(String id, boolean notify) {

    if ((this.id != null) && (manager != null))
        manager.remove(this);

    this.id = id;

    if (manager != null)
        manager.add(this);

    if (notify) {
        tellNew();
    }
}

  4. **销毁 Session **

    Tomcat会定期检测出不活跃的session，然后将其删除，一方面session占用内存，另一方面是安全性的考虑；启动tomcat的同时会启动一个后台线程用来检测过期的session，具体可以查看ContainerBase的内部类ContainerBackgroundProcessor：

    protected class ContainerBackgroundProcessor implements Runnable {

     @Override
     public void run() {
         Throwable t = null;
         String unexpectedDeathMessage = sm.getString(
                 "containerBase.backgroundProcess.unexpectedThreadDeath",
                 Thread.currentThread().getName());
         try {
             while (!threadDone) {
                 try {
                     //backgroundProcessorDelay默认值为10, 也就是10秒检测一次
                     Thread.sleep(backgroundProcessorDelay * 1000L);
                 } catch (InterruptedException e) {
                     // Ignore
                 }
                 if (!threadDone) {
                     Container parent = (Container) getMappingObject();
                     ClassLoader cl =
                         Thread.currentThread().getContextClassLoader();
                     if (parent.getLoader() != null) {
                         cl = parent.getLoader().getClassLoader();
                     }
                     //调用processChildren方法
                     processChildren(parent, cl);
                 }
             }
         } catch (RuntimeException e) {
             t = e;
             throw e;
         } catch (Error e) {
             t = e;
             throw e;
         } finally {
             if (!threadDone) {
                 log.error(unexpectedDeathMessage, t);
             }
         }
     }

     protected void processChildren(Container container, ClassLoader cl) {
         try {
             if (container.getLoader() != null) {
                 Thread.currentThread().setContextClassLoader
                     (container.getLoader().getClassLoader());
             }
             //调用backgroundProcess();方法, 看下段代码分析
             container.backgroundProcess();
         } catch (Throwable t) {
             ExceptionUtils.handleThrowable(t);
             log.error("Exception invoking periodic operation: ", t);
         } finally {
             Thread.currentThread().setContextClassLoader(cl);
         }
         Container[] children = container.findChildren();
         for (int i = 0; i < children.length; i++) {
             if (children[i].getBackgroundProcessorDelay() <= 0) {
                 processChildren(children[i], cl);
             }
         }
  }
 }

    //Manager的方法
//接上47行代码 Container类的backgroundProcess() 方法
public void backgroundProcess() {
    //
    count = (count + 1) % processExpiresFrequency;
    if (count == 0) //如果達到檢查頻率則開始檢查
        processExpires();
}

/**
 * Invalidate all sessions that have expired.
 */
public void processExpires() {

    long timeNow = System.currentTimeMillis();
    Session sessions[] = findSessions();//獲取所有session物件
    int expireHere = 0 ;//过期session的数量，不要被這個變數名騙了

    if(log.isDebugEnabled())
        log.debug("Start expire sessions " + getName() + " at " + timeNow + " sessioncount " + sessions.length);
    //逐个检测Session查看是否过期
    for (int i = 0; i < sessions.length; i++) {
        //如何检测session是否过期呢?
        if (sessions[i]!=null && !sessions[i].isValid()) {
            expireHere++;
        }
    }
    long timeEnd = System.currentTimeMillis();
    if(log.isDebugEnabled())
         log.debug("End expire sessions " + getName() + " processingTime " + (timeEnd - timeNow) + " expired sessions: " + expireHere);
    processingTime += ( timeEnd - timeNow );

}

    /**
 * Return the <code>isValid</code> flag for this session.
 */
@Override
//判断Session是否过期
public boolean isValid() {

    if (!this.isValid) {
        return false;
    }

    if (this.expiring) {
        return true;
    }

    if (ACTIVITY_CHECK && accessCount.get() > 0) {
        return true;
    }

    //關鍵所在
    //如果有設定最大空閒時間
    //就獲取此Session的空閒時間進行判斷
    //如果已超時，則執行expire操作
    if (maxInactiveInterval > 0) { 
        int timeIdle = (int) (getIdleTimeInternal() / 1000L);
        if (timeIdle >= maxInactiveInterval) {
            expire(true);
        }
    }

    return this.isValid;
}

Spring Session

Spring Session官方解释

Spring Session provides an API and implementations for managing a user’s session information. It also provides transparent integration with:

  • HttpSession

- allows replacing the HttpSession in an application container (i.e. Tomcat) neutral way. Additional features include:

  • Clustered Sessions() - Spring Session makes it trivial to support clustered sessions without being tied to an application container specific solution.

  • Multiple Browser Sessions - Spring Session supports managing multiple users’ sessions in a single browser instance (i.e. multiple authenticated accounts similar to Google).

  • RESTful APIs - Spring Session allows providing session ids in headers to work with RESTful APIs

  • WebSocket - provides the ability to keep the HttpSession alive when receiving WebSocket messages

其具体的特性非常之多，具体的内容可以从文档中了解到，笔者做一点自己的总结，Spring Session的特性包括但不限于以下：

  • 使用GemFire来构建C/S架构的HttpSession（不关注）
  • 使用第三方仓储来实现集群session管理，也就是常说的分布式session容器，替换应用容器（如tomcat的session容器）。仓储的实现，Spring Session提供了三个实现（Redis，mongodb，jdbc），其中Redis使我们最常用的。程序的实现，使用AOP技术，几乎可以做到透明化地替换。（核心）
  • 可以非常方便的扩展Cookie和自定义Session相关的Listener，Filter。
  • 可以很方便的与Spring Security集成，增加诸如findSessionsByUserName，rememberMe，限制同一个账号可以同时在线的Session数（如设置成1，即可达到把前一次登录顶掉的效果）等等