Session对象的创建

Session对象的创建一般是源于这样的一条语句： 
Session session = request.getSession(false);或者Session session = request.getSession();如果不在乎服务器压力可能多那么一点点的话。 

在Tomcat的实现中，这个request是org.apache.catalina.connector.Request类的包装类org.apache.catalina.connector.RequestFacade的对象，它的两个#getSession()方法如下： 

public HttpSession getSession(boolean create) {
    if (request == null) {
        throw new IllegalStateException(
                        sm.getString("requestFacade.nullRequest"));
    }

    if (SecurityUtil.isPackageProtectionEnabled()){
        return (HttpSession)AccessController.
            doPrivileged(new GetSessionPrivilegedAction(create));
    } else {
        return request.getSession(create);
    }
}

public HttpSession getSession() {
    if (request == null) {
        throw new IllegalStateException(
                        sm.getString("requestFacade.nullRequest"));
    }

    return getSession(true);
}

其实差不太多，最后都会进入org.apache.catalina.connector.Request的#getSession()方法。这个方法的源代码如下： 

public HttpSession getSession(boolean create) {
    Session session = doGetSession(create);
    if (session != null) {
        return session.getSession();
    } else {
        return null;
    }
}

然后调用就到了#doGetSession()这个方法了。源代码如下 

protected Session doGetSession(boolean create) {
    // 没有Context的话直接返回null
    if (context == null)
        return (null);

    // 判断Session是否有效
    if ((session != null) && !session.isValid())
        session = null;
    if (session != null)
        return (session);

    // 返回Manager对象，这里是StandardManager类的对象
    Manager manager = null;
    if (context != null)
        manager = context.getManager();
    if (manager == null)
        return (null); // Sessions are not supported
    // 判断是否有SessionID
    if (requestedSessionId != null) {
        try {
            // 在Manager中根据SessionID查找Session
            session = manager.findSession(requestedSessionId);
        } catch (IOException e) {
            session = null;
        }
        if ((session != null) && !session.isValid())
            session = null;
        if (session != null) {
            // 更新访问时间
            session.access();
            return (session);
        }
    }

    // 创建新的Session
    if (!create)
        return (null);
    if ((context != null) && (response != null) && context.getCookies()
            && response.getResponse().isCommitted()) {
        throw new IllegalStateException(sm.getString("coyoteRequest.sessionCreateCommitted"));
    }

    // 判断是否使用 "/" 作为Session Cookie的存储路径 并且 是否SessionID来自Cookie
    if (connector.getEmptySessionPath() && isRequestedSessionIdFromCookie()) {
        // 创建Session
        session = manager.createSession(getRequestedSessionId());
    } else {
        session = manager.createSession(null);
    }

    // 创建一个新的Session Cookies
    if ((session != null) && (getContext() != null) && getContext().getCookies()) {
        Cookie cookie = new Cookie(Globals.SESSION_COOKIE_NAME, session.getIdInternal());
        // 配置Session Cookie
        configureSessionCookie(cookie);
        // 在响应中加入Session Cookie
        response.addCookieInternal(cookie);
    }

    if (session != null) {
        // 更新访问时间
        session.access();
        return (session);
    } else {
        return (null);
    }

}

这个方法说明了Session创建的大致过程，首先判断requestedSessionId是否存在，如果存在，那么根据这个ID去查找Session对象。如果requestedSessionId不存在或者没有取到Session，并且传递给#getSession(boolean)的参数为真，那么要创建一个新的Session，并且给客户端写回去一个Session Cookie。 

首先，我感兴趣的是requestedSessionId的赋值，它到底是什么时候被赋值的呢？ 

还要向回看Tomcat的请求处理过程，请求曾到过这一步，org.apache.catalina.connector.CoyoteAdapter的#service()方法。里边有这样一句方法调用：postParseRequest(req, request, res, response)。就是这一步处理了SessionID的获取，这个方法调用了#parseSessionId()和parseSessionCookiesId()这两个方法，就是它对Session ID进行了提取，源代码分别如下： 

protected void parseSessionId(org.apache.coyote.Request req, Request request) {

    ByteChunk uriBC = req.requestURI().getByteChunk();
    // 判断URL中是不是有";jsessionid="这个字符串
    int semicolon = uriBC.indexOf(match, 0, match.length(), 0);

    if (semicolon > 0) {
        // Parse session ID, and extract it from the decoded request URI
        // 在URL中提取Session ID
        int start = uriBC.getStart();
        int end = uriBC.getEnd();

        int sessionIdStart = semicolon + match.length();
        int semicolon2 = uriBC.indexOf(';', sessionIdStart);
        if (semicolon2 >= 0) {
            request.setRequestedSessionId(new String(uriBC.getBuffer(), start + sessionIdStart,
                    semicolon2 - sessionIdStart));
            byte[] buf = uriBC.getBuffer();
            for (int i = 0; i < end - start - semicolon2; i++) {
                buf[start + semicolon + i] = buf[start + i + semicolon2];
            }
            uriBC.setBytes(buf, start, end - start - semicolon2 + semicolon);
        } else {
            request.setRequestedSessionId(new String(uriBC.getBuffer(), start + sessionIdStart,
                    (end - start) - sessionIdStart));
            uriBC.setEnd(start + semicolon);
        }
        // 设定Session ID来自于URL
        request.setRequestedSessionURL(true);

    } else {
        request.setRequestedSessionId(null);
        request.setRequestedSessionURL(false);
    }

}

protected void parseSessionCookiesId(org.apache.coyote.Request req, Request request) {
    Context context = (Context) request.getMappingData().context;
    if (context != null && !context.getCookies())
        return;

    // 返回Cookie
    Cookies serverCookies = req.getCookies();
    int count = serverCookies.getCookieCount();
    if (count <= 0)
        return;

    for (int i = 0; i < count; i++) {
        ServerCookie scookie = serverCookies.getCookie(i);
        // 判断是否有JSESSIONID这个名字的Cookie
        if (scookie.getName().equals(Globals.SESSION_COOKIE_NAME)) {
            // Override anything requested in the URL
            if (!request.isRequestedSessionIdFromCookie()) {
                // 设定Session ID
                convertMB(scookie.getValue());
                request.setRequestedSessionId(scookie.getValue().toString());
                // 如果之前在URL中读到了SessionID，那么会覆盖它
                request.setRequestedSessionCookie(true);
                request.setRequestedSessionURL(false);
                if (log.isDebugEnabled())
                    log.debug(" Requested cookie session id is " + request.getRequestedSessionId());
            } else {
                if (!request.isRequestedSessionIdValid()) {
                    convertMB(scookie.getValue());
                    request.setRequestedSessionId(scookie.getValue().toString());
                }
            }
        }
    }

}

Tomcat就是通过上边的两个方法读到URL或者Cookie中存放的Session ID的。 

了解了Session ID的获取，下面要看一下Session的查找过程，就是org.apache.catalina.session.StandardManager的#findSession()方法。这个方法是在它的基类中定义的，源代码如下： 

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

代码很短，其中sessions是一个ConcurrentHashMap<String, Session>对象。那么这个sessions的对象是什么时候载入的Session呢？ 

启动的时候！可以看一下StandardManager#start()方法。最后调用了#load()方法，这个就是载入Session的方法了： 

public void load() throws ClassNotFoundException, IOException {
    if (SecurityUtil.isPackageProtectionEnabled()) {
        try {
            AccessController.doPrivileged(new PrivilegedDoLoad());
        } catch (PrivilegedActionException ex) {
            Exception exception = ex.getException();
            if (exception instanceof ClassNotFoundException) {
                throw (ClassNotFoundException) exception;
            } else if (exception instanceof IOException) {
                throw (IOException) exception;
            }
            if (log.isDebugEnabled())
                log.debug("Unreported exception in load() " + exception);
        }
    } else {
        doLoad();
    }
}

最后调用了#doLoad()方法来具体的载入Session，源代码如下： 

protected void doLoad() throws ClassNotFoundException, IOException {
    if (log.isDebugEnabled())
        log.debug("Start: Loading persisted sessions");

    // 清空Map
    sessions.clear();

    // 对应work/Catalina/localhost/%app name%/SESSIONS.ser文件
    File file = file();
    if (file == null)
        return;
    if (log.isDebugEnabled())
        log.debug(sm.getString("standardManager.loading", pathname));
    FileInputStream fis = null;
    ObjectInputStream ois = null;
    Loader loader = null;
    ClassLoader classLoader = null;
    try {
        // 载入Session缓存文件
        fis = new FileInputStream(file.getAbsolutePath());
        BufferedInputStream bis = new BufferedInputStream(fis);
        if (container != null)
            loader = container.getLoader();
        if (loader != null)
            classLoader = loader.getClassLoader();
        if (classLoader != null) {
            if (log.isDebugEnabled())
                log.debug("Creating custom object input stream for class loader ");
            ois = new CustomObjectInputStream(bis, classLoader);
        } else {
            if (log.isDebugEnabled())
                log.debug("Creating standard object input stream");
            ois = new ObjectInputStream(bis);
        }
    } catch (FileNotFoundException e) {
        if (log.isDebugEnabled())
            log.debug("No persisted data file found");
        return;
    } catch (IOException e) {
        log.error(sm.getString("standardManager.loading.ioe", e), e);
        if (ois != null) {
            try {
                ois.close();
            } catch (IOException f) {
                ;
            }
            ois = null;
        }
        throw e;
    }

    synchronized (sessions) {
        try {
            // 读出Session个数
            Integer count = (Integer) ois.readObject();
            int n = count.intValue();
            if (log.isDebugEnabled())
                log.debug("Loading " + n + " persisted sessions");
            //  读入Session
            for (int i = 0; i < n; i++) {
                StandardSession session = getNewSession();
                session.readObjectData(ois);
                session.setManager(this);
                sessions.put(session.getIdInternal(), session);
                session.activate();
                sessionCounter++;
            }
        } catch (ClassNotFoundException e) {
            log.error(sm.getString("standardManager.loading.cnfe", e), e);
            if (ois != null) {
                try {
                    ois.close();
                } catch (IOException f) {
                    ;
                }
                ois = null;
            }
            throw e;
        } catch (IOException e) {
            log.error(sm.getString("standardManager.loading.ioe", e), e);
            if (ois != null) {
                try {
                    ois.close();
                } catch (IOException f) {
                    ;
                }
                ois = null;
            }
            throw e;
        } finally {
            try {
                if (ois != null)
                    ois.close();
            } catch (IOException f) {
            }

            // 删除Session缓存文件
            if (file != null && file.exists())
                file.delete();
        }
    }

    if (log.isDebugEnabled())
        log.debug("Finish: Loading persisted sessions");
}

大致知道了Session的读取过程，后面就是Session没找到时创建Session的过程了。具体就是org.apache.catalina.session.StandardManager的#createSession()方法： 

public Session createSession(String sessionId) {
    if ((maxActiveSessions >= 0) && (sessions.size() >= maxActiveSessions)) {
        rejectedSessions++;
        throw new IllegalStateException(sm.getString("standardManager.createSession.ise"));
    }
    return (super.createSession(sessionId));
}

最后调用到了它的基类的#createSession()方法了。 

public Session createSession(String sessionId) {
    // 创建一个新的Session
    Session session = createEmptySession();

    // 初始化Session的属性
    session.setNew(true);
    session.setValid(true);
    session.setCreationTime(System.currentTimeMillis());
    session.setMaxInactiveInterval(this.maxInactiveInterval);
    // 如果Session ID为null，那么就生成一个
    if (sessionId == null) {
        sessionId = generateSessionId();
    }
    session.setId(sessionId);
    sessionCounter++;

    return (session);

}

通过上述过程，一个新的Session就创建出来了。