这篇文章主要介绍Tomcat9中如何管理session，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

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源码解析

session相关共有两个类：

• StandardSession：默认的session的类，是对session的系统抽象

• StandardManager：默认的session管理类，管理session的创建、加载、持久化等
  

  org.apache.catalina.core.StandardContext.startInternal()

protected synchronized void startInternal() throws LifecycleException {
...省略其他代码...
                Manager contextManager = null;
                Manager manager = getManager();
                if (manager == null) {
                    if (log.isDebugEnabled()) {
                        log.debug(sm.getString("standardContext.cluster.noManager",
                                Boolean.valueOf((getCluster() != null)),
                                Boolean.valueOf(distributable)));
                    }
                    if ((getCluster() != null) && distributable) {
                        try {
                            contextManager = getCluster().createManager(getName());
                        } catch (Exception ex) {
                            log.error(sm.getString("standardContext.cluster.managerError"), ex);
                            ok = false;
                        }
                    } else {
                        contextManager = new StandardManager();
                    }
                }
               // Configure default manager if none was specified
                if (contextManager != null) {
                    if (log.isDebugEnabled()) {
                        log.debug(sm.getString("standardContext.manager",
                                contextManager.getClass().getName()));
                    }
                    setManager(contextManager);
                }
...省略其他代码...
public void setManager(Manager manager) {
        Lock writeLock = managerLock.writeLock();
        writeLock.lock();
        Manager oldManager = null;
        try {
            // Change components if necessary
            oldManager = this.manager;
            if (oldManager == manager)
                return;
            this.manager = manager;
            // Stop the old component if necessary
            if (oldManager instanceof Lifecycle) {
                try {
                    ((Lifecycle) oldManager).stop();
                    ((Lifecycle) oldManager).destroy();
                } catch (LifecycleException e) {
                    log.error(sm.getString("standardContext.setManager.stop"), e);
                }
            }
            // Start the new component if necessary
            if (manager != null) {
                manager.setContext(this);
            }
            if (getState().isAvailable() && manager instanceof Lifecycle) {
                try {
                    ((Lifecycle) manager).start();
                } catch (LifecycleException e) {
                    log.error(sm.getString("standardContext.setManager.start"), e);
                }
            }
        } finally {
            writeLock.unlock();
        }
        // Report this property change to interested listeners
        support.firePropertyChange("manager", oldManager, manager);
    }
}

在StandardContext的start方法中，会对sessionManager进行实例化，这里是支持Tomcat的集群session，本文以单机session管理为例，即StandardManager，在setManager中会调用StandardManager.start

org.apache.catalina.session.StandardManager.start()

 protected synchronized void startInternal() throws LifecycleException {
        super.startInternal();
        // Load unloaded sessions, if any
        try {
            load();
        } catch (Throwable t) {
            ExceptionUtils.handleThrowable(t);
            log.error(sm.getString("standardManager.managerLoad"), t);
        }
        setState(LifecycleState.STARTING);
    }
protected String pathname = "SESSIONS.ser";
protected void doLoad() throws ClassNotFoundException, IOException {
        if (log.isDebugEnabled()) {
            log.debug("Start: Loading persisted sessions");
        }
        // Initialize our internal data structures
        sessions.clear();
        // Open an input stream to the specified pathname, if any
        File file = file();
        if (file == null) {
            return;
        }
        if (log.isDebugEnabled()) {
            log.debug(sm.getString("standardManager.loading", pathname));
        }
        Loader loader = null;
        ClassLoader classLoader = null;
        Log logger = null;
        try (FileInputStream fis = new FileInputStream(file.getAbsolutePath());
                BufferedInputStream bis = new BufferedInputStream(fis)) {
            Context c = getContext();
            loader = c.getLoader();
            logger = c.getLogger();
            if (loader != null) {
                classLoader = loader.getClassLoader();
            }
            if (classLoader == null) {
                classLoader = getClass().getClassLoader();
            }
            // Load the previously unloaded active sessions
            synchronized (sessions) {
                try (ObjectInputStream ois = new CustomObjectInputStream(bis, classLoader, logger,
                        getSessionAttributeValueClassNamePattern(),
                        getWarnOnSessionAttributeFilterFailure())) {
                    Integer count = (Integer) ois.readObject();
                    int n = count.intValue();
                    if (log.isDebugEnabled())
                        log.debug("Loading " + n + " persisted sessions");
                    for (int i = 0; i < n; i++) {
                        StandardSession session = getNewSession();
                        session.readObjectData(ois);
                        session.setManager(this);
                        sessions.put(session.getIdInternal(), session);
                        session.activate();
                        if (!session.isValidInternal()) {
                            // If session is already invalid,
                            // expire session to prevent memory leak.
                            session.setValid(true);
                            session.expire();
                        }
                        sessionCounter++;
                    }
                } finally {
                    // Delete the persistent storage file
                    if (file.exists()) {
                        if (!file.delete()) {
                            log.warn(sm.getString("standardManager.deletePersistedFileFail", file));
                        }
                    }
                }
            }
        } catch (FileNotFoundException e) {
            if (log.isDebugEnabled()) {
                log.debug("No persisted data file found");
            }
            return;
        }
        if (log.isDebugEnabled()) {
            log.debug("Finish: Loading persisted sessions");
        }
    }
protected void doUnload() throws IOException {
        if (log.isDebugEnabled())
            log.debug(sm.getString("standardManager.unloading.debug"));
        if (sessions.isEmpty()) {
            log.debug(sm.getString("standardManager.unloading.nosessions"));
            return; // nothing to do
        }
        // Open an output stream to the specified pathname, if any
        File file = file();
        if (file == null) {
            return;
        }
        if (log.isDebugEnabled()) {
            log.debug(sm.getString("standardManager.unloading", pathname));
        }
        // Keep a note of sessions that are expired
        List list = new ArrayList<>();
        try (FileOutputStream fos = new FileOutputStream(file.getAbsolutePath());
                BufferedOutputStream bos = new BufferedOutputStream(fos);
                ObjectOutputStream oos = new ObjectOutputStream(bos)) {
            synchronized (sessions) {
                if (log.isDebugEnabled()) {
                    log.debug("Unloading " + sessions.size() + " sessions");
                }
                // Write the number of active sessions, followed by the details
                oos.writeObject(Integer.valueOf(sessions.size()));
                for (Session s : sessions.values()) {
                    StandardSession session = (StandardSession) s;
                    list.add(session);
                    session.passivate();
                    session.writeObjectData(oos);
                }
            }
        }
        // Expire all the sessions we just wrote
        if (log.isDebugEnabled()) {
            log.debug("Expiring " + list.size() + " persisted sessions");
        }
        for (StandardSession session : list) {
            try {
                session.expire(false);
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
            } finally {
                session.recycle();
            }
        }
        if (log.isDebugEnabled()) {
            log.debug("Unloading complete");
        }
    }

在start方法中，会调用load方法，load方法最终调用doLoad方法，会读取文件名为SESSIONS.ser的文件，该文件存储了Tomcat关闭时session的内容，具体存储逻辑在doUnload方法中

Request.getSession()

public HttpSession getSession() {
        Session session = doGetSession(true);
        if (session == null) {
            return null;
        }
        return session.getSession();
    }
protected Session doGetSession(boolean create) {
        // There cannot be a session if no context has been assigned yet
        Context context = getContext();
        if (context == null) {
            return null;
        }
        // Return the current session if it exists and is valid
        if ((session != null) && !session.isValid()) {
            session = null;
        }
        if (session != null) {
            return session;
        }
        // Return the requested session if it exists and is valid
        Manager manager = context.getManager();
        if (manager == null) {
            return null;      // Sessions are not supported
        }
        if (requestedSessionId != null) {
            try {
                session = manager.findSession(requestedSessionId); //读取session
            } 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
        if (!create) {
            return null;
        }
        boolean trackModesIncludesCookie =
                context.getServletContext().getEffectiveSessionTrackingModes().contains(SessionTrackingMode.COOKIE);
        if (trackModesIncludesCookie && response.getResponse().isCommitted()) {
            throw new IllegalStateException(sm.getString("coyoteRequest.sessionCreateCommitted"));
        }
        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())) {
            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;
        }
        session = manager.createSession(sessionId); //创建新的session
        // Creating a new session cookie based on that session
        if (session != null && trackModesIncludesCookie) {
            Cookie cookie = ApplicationSessionCookieConfig.createSessionCookie(
                    context, session.getIdInternal(), isSecure());
            response.addSessionCookieInternal(cookie);
        }
        if (session == null) {
            return null;
        }
        session.access();
        return session;
    }

在getSession方法中，会先根据sessionId在调用manager.findSession查找，如果查找不存在则会调用manager.createSession创建

findSession() & createSession()

public Session findSession(String id) throws IOException {
        if (id == null) {
            return null;
        }
        return sessions.get(id);
    }
public Session createSession(String sessionId) {
        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(getContext().getSessionTimeout() * 60);
        String id = sessionId;
        if (id == null) {
            id = generateSessionId();
        }
        session.setId(id);
        sessionCounter++;
        SessionTiming timing = new SessionTiming(session.getCreationTime(), 0);
        synchronized (sessionCreationTiming) {
            sessionCreationTiming.add(timing);
            sessionCreationTiming.poll();
        }
        return session;
    }

所有的session为维护在一个HashMap中，在创建时会想Map中维护，读取是从Map中查找。

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