Tomcat session的实现：线程安全与管理
等你归去来 人气:3本文所说的session是单机版本的session, 事实上在当前的互联网实践中已经不太存在这种定义了。我们主要讨论的是其安全共享的实现，只从理论上来讨论，不必太过在意实用性问题。
1. session 的意义简说
大概就是一个会话的的定义，客户端有cookie记录，服务端session定义。用于确定你就是你的一个东西。
每个用户在一定范围内共享某个session信息，以实现登录状态，操作的鉴权保持等。
我们将会借助tomcat的实现，剖析session管理的一些实现原理。
2. tomcat 中 session 什么时候创建？
session 信息会在两个地方调用，一是每次请求进来时，框架会尝试去加载原有对应的session信息（不会新建）。二是应用自己调用getSession()时，此时如果不存在session信息，则创建一个新的session对象，代表应用后续会使用此功能。即框架不会自动支持session相关功能，只是在你需要的时候进行辅助操作。
// case1. 框架自行调用session信息，不会主动创建session
// org.springframework.web.servlet.support.SessionFlashMapManager#retrieveFlashMaps
/**
* Retrieves saved FlashMap instances from the HTTP session, if any.
*/
@Override
@SuppressWarnings("unchecked")
protected List<FlashMap> retrieveFlashMaps(HttpServletRequest request) {
HttpSession session = request.getSession(false);
return (session != null ? (List<FlashMap>) session.getAttribute(FLASH_MAPS_SESSION_ATTRIBUTE) : null);
}
// case2. 应用主动调用session信息，不存在时会创建新的session, 以满足业务连续性需要
@GetMapping("sessionTest")
public Object sessionTest(HttpServletRequest request, HttpServletResponse response) {
// 主动获取session信息
HttpSession session = request.getSession();
String sid = session.getId();
System.out.println("sessionId:" + sid);
return ResponseInfoBuilderUtil.success(sid);
}
在tomcat中，HttpServletRequest的实际类都是 RequestFacade, 所以获取session信息也是以其为入口进行。
// org.apache.catalina.connector.RequestFacade#getSession()
@Override
public HttpSession getSession() {
if (request == null) {
throw new IllegalStateException(
sm.getString("requestFacade.nullRequest"));
}
// 如果不存在session则创建一个
// session 的实现有两种：一是基于内存的实现，二是基于文件的实现。
return getSession(true);
}
@Override
public HttpSession getSession(boolean create) {
if (request == null) {
throw new IllegalStateException(
sm.getString("requestFacade.nullRequest"));
}
if (SecurityUtil.isPackageProtectionEnabled()){
return AccessController.
doPrivileged(new GetSessionPrivilegedAction(create));
} else {
// RequestFacade 是个外观模式实现，核心请求还是会传递给 Request处理的
// org.apache.catalina.connector.Request
return request.getSession(create);
}
}
// org.apache.catalina.connector.Request#getSession(boolean)
/**
* @return the session associated with this Request, creating one
* if necessary and requested.
*
* @param create Create a new session if one does not exist
*/
@Override
public HttpSession getSession(boolean create) {
// 由 create 字段决定是否需要创建新的session, 如果不存在的话。
// Session 是tomcat的一个会话实现类，并非对接规范接口类，其会包装一个HttpSession，以便统一交互
// 因为只有 HttpSession 才是 Servlet 的接口规范，在tomcat中会以 StandardSessionFacade 实现接口，其也是一个外观模式的实现，具体工作由 StandardSession 处理。
Session session = doGetSession(create);
if (session == null) {
return null;
}
// 包装 Session 为 HttpSession 规范返回
return session.getSession();
}
// org.apache.catalina.connector.Request#doGetSession
protected Session doGetSession(boolean create) {
// There cannot be a session if no context has been assigned yet
// mappingData.context;
Context context = getContext();
if (context == null) {
return (null);
}
// Return the current session if it exists and is valid
// 此处检查session有效性时，也会做部分清理工作
if ((session != null) && !session.isValid()) {
session = null;
}
if (session != null) {
return (session);
}
// Return the requested session if it exists and is valid
// 获取manager 实例，即真正进行 Session 管理的类，其实主要分两种：1. 基于内存；2. 基于文件的持久化；
Manager manager = context.getManager();
if (manager == null) {
return (null); // Sessions are not supported
}
if (requestedSessionId != null) {
try {
// 如果不是第一次请求，则会带上服务返回的 sessionId, 就会主动查找原来的session
// 从 sessions 中查找即可
session = manager.findSession(requestedSessionId);
} catch (IOException e) {
session = null;
}
if ((session != null) && !session.isValid()) {
session = null;
}
// 后续请求，每次请求都会更新有效时间
if (session != null) {
session.access();
return (session);
}
}
// Create a new session if requested and the response is not committed
// 主动请求session时，才会继续后续逻辑
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.
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 {
// 当session无效时，需要将原来的seesionId置空，删除并新创建一个使用
sessionId = null;
}
// 创建session, StandardManager -> ManagerBase
session = manager.createSession(sessionId);
// Creating a new session cookie based on that session
if (session != null
&& context.getServletContext()
.getEffectiveSessionTrackingModes()
.contains(SessionTrackingMode.COOKIE)) {
// 创建cookie信息，与session对应
Cookie cookie =
ApplicationSessionCookieConfig.createSessionCookie(
context, session.getIdInternal(), isSecure());
// 添加到response中，在响应结果一起返回给客户端
response.addSessionCookieInternal(cookie);
}
if (session == null) {
return null;
}
// 每次请求session时，必然刷新激活时间，以便判定会话是否超时
session.access();
return session;
}
从上面我们可以看到，session的流程大概是这样的：
1. 先查找是否有session信息存在，如果有则判断是否失效；
2. 如果不存在session或已失效，则使用一个新的sessionId（非必须）创建一个session实例；
3. session创建成功，则将sessionId写入到cookie信息中，以便客户端后续使用；
4. 每次请求完session,必定刷新下访问时间以续期；
session的管理主要有两种实现方式，类图如下：
我们先主要以基于内存的实现来理解下session的管理过程。实际上StandardManager基本就依托于 ManagerBase 就实现了Session管理功能，下面我们来看一下其创建session如何？
// org.apache.catalina.session.ManagerBase#createSession
@Override
public Session createSession(String sessionId) {
// 首先来个安全限制，允许同时存在多少会话
// 这个会话实际上代表的是一段时间的有效性，并非真正的用户有效使用在线，所以该值一般要求比预计的数量大些才好
if ((maxActiveSessions >= 0) &&
(getActiveSessions() >= maxActiveSessions)) {
rejectedSessions++;
throw new TooManyActiveSessionsException(
sm.getString("managerBase.createSession.ise"),
maxActiveSessions);
}
// Recycle or create a Session instance
// 创建空的session 容器 return new StandardSession(this);
Session session = createEmptySession();
// Initialize the properties of the new session and return it
// 默认30分钟有效期
session.setNew(true);
session.setValid(true);
session.setCreationTime(System.currentTimeMillis());
session.setMaxInactiveInterval(getContext().getSessionTimeout() * 60);
String id = sessionId;
if (id == null) {
// sessionId 为空时，生成一个，随机id
id = generateSessionId();
}
// 设置sessionId, 注意此处不仅仅是set这么简单，其同时会将自身session注册到全局session管理器中.如下文
session.setId(id);
sessionCounter++;
SessionTiming timing = new SessionTiming(session.getCreationTime(), 0);
synchronized (sessionCreationTiming) {
// LinkedList, 添加一个，删除一个？
sessionCreationTiming.add(timing);
sessionCreationTiming.poll();
}
return (session);
}
// org.apache.catalina.session.StandardSession#setId
/**
* Set the session identifier for this session.
*
* @param id The new session identifier
*/
@Override
public void setId(String id) {
setId(id, true);
}
@Override
public void setId(String id, boolean notify) {
// 如果原来的id不为空，则先删除原有的
if ((this.id != null) && (manager != null))
manager.remove(this);
this.id = id;
// 再将自身会话注册到 manager 中，即 sessions 中
if (manager != null)
manager.add(this);
// 通知监听者，这是框架该做好的事（扩展点），不过不是本文的方向，忽略
if (notify) {
tellNew();
}
}
// org.apache.catalina.session.ManagerBase#add
@Override
public void add(Session session) {
// 取出 sessionId, 添加到 sessions 容器，统一管理
sessions.put(session.getIdInternal(), session);
int size = getActiveSessions();
// 刷新最大活跃数，使用双重锁优化更新该值
if( size > maxActive ) {
synchronized(maxActiveUpdateLock) {
if( size > maxActive ) {
maxActive = size;
}
}
}
}
// 查找session也是异常简单，只管从 ConcurrentHashMap 中查找即可
// org.apache.catalina.session.ManagerBase#findSession
@Override
public Session findSession(String id) throws IOException {
if (id == null) {
return null;
}
return sessions.get(id);
}
有兴趣的同学可以看一下sessionId的生成算法：主要保证两点：1. 随机性；2.不可重复性；
// org.apache.catalina.session.ManagerBase#generateSessionId
/**
* Generate and return a new session identifier.
* @return a new session id
*/
protected String generateSessionId() {
String result = null;
do {
if (result != null) {
// Not thread-safe but if one of multiple increments is lost
// that is not a big deal since the fact that there was any
// duplicate is a much bigger issue.
duplicates++;
}
// 使用 sessionIdGenerator 生成sessionId
result = sessionIdGenerator.generateSessionId();
// 如果已经存在该sessionId, 则重新生成一个
// session 是一个 ConcurrentHashMap 结构数据
} while (sessions.containsKey(result));
return result;
}
// org.apache.catalina.util.SessionIdGeneratorBase#generateSessionId
/**
* Generate and return a new session identifier.
*/
@Override
public String generateSessionId() {
return generateSessionId(jvmRoute);
}
// org.apache.catalina.util.StandardSessionIdGenerator#generateSessionId
@Override
public String generateSessionId(String route) {
byte random[] = new byte[16];
// 默认16
int sessionIdLength = getSessionIdLength();
// Render the result as a String of hexadecimal digits
// Start with enough space for sessionIdLength and medium route size
// 创建双倍大小的stringBuilder, 容纳sessionId
StringBuilder buffer = new StringBuilder(2 * sessionIdLength + 20);
int resultLenBytes = 0;
//
while (resultLenBytes < sessionIdLength) {
getRandomBytes(random);
for (int j = 0;
j < random.length && resultLenBytes < sessionIdLength;
j++) {
// 转换为16进制
byte b1 = (byte) ((random[j] & 0xf0) >> 4);
byte b2 = (byte) (random[j] & 0x0f);
if (b1 < 10)
buffer.append((char) ('0' + b1));
else
buffer.append((char) ('A' + (b1 - 10)));
if (b2 < 10)
buffer.append((char) ('0' + b2));
else
buffer.append((char) ('A' + (b2 - 10)));
resultLenBytes++;
}
}
if (route != null && route.length() > 0) {
buffer.append('.').append(route);
} else {
String jvmRoute = getJvmRoute();
if (jvmRoute != null && jvmRoute.length() > 0) {
buffer.append('.').append(jvmRoute);
}
}
return buffer.toString();
}
// org.apache.catalina.util.SessionIdGeneratorBase#getRandomBytes
protected void getRandomBytes(byte bytes[]) {
// 使用 random.nextBytes(), 预生成 random
SecureRandom random = randoms.poll();
if (random == null) {
random = createSecureRandom();
}
random.nextBytes(bytes);
// 添加到 ConcurrentLinkedQueue 队列中，事实上该 random 将会被反复循环使用， poll->add
randoms.add(random);
}
创建好session后，需要进行随时的维护：我们看下tomcat是如何刷新访问时间的？可能比预想的简单，其仅是更新一个访问时间字段，再无其他。
// org.apache.catalina.session.StandardSession#access
/**
* Update the accessed time information for this session. This method
* should be called by the context when a request comes in for a particular
* session, even if the application does not reference it.
*/
@Override
public void access() {
// 更新访问时间
this.thisAccessedTime = System.currentTimeMillis();
// 访问次数统计，默认不启用
if (ACTIVITY_CHECK) {
accessCount.incrementAndGet();
}
}
最后，还需要看下 HttpSession 是如何被包装返回的？
// org.apache.catalina.session.StandardSession#getSession
/**
* Return the <code>HttpSession</code> for which this object
* is the facade.
*/
@Override
public HttpSession getSession() {
if (facade == null){
if (SecurityUtil.isPackageProtectionEnabled()){
final StandardSession fsession = this;
facade = AccessController.doPrivileged(
new PrivilegedAction<StandardSessionFacade>(){
@Override
public StandardSessionFacade run(){
return new StandardSessionFacade(fsession);
}
});
} else {
// 直接使用 StandardSessionFacade 包装即可
facade = new StandardSessionFacade(this);
}
}
return (facade);
}
再最后，要说明的是，整个sessions的管理使用一个 ConcurrentHashMap 来存放全局会话信息，sessionId->session实例。
对于同一次http请求中，该session会被存储在当前的Request栈org.apache.catalina.connector.Request#session字段中，从而无需每次深入获取。每个请求进来后，会将session保存在当前的request信息中。
3. 过期session清理？
会话不可能不过期，不过期的也不叫会话了。
会话过期的触发时机主要有三个：1. 每次进行会话调用时，会主动有效性isValid()验证，此时如果发现过期可以主动清理： 2. 后台定时任务触发清理； 3. 启动或停止应用的时候清理；（这对于非内存式的存储会更有用些）
// case1. 请求时验证，如前面所述
// org.apache.catalina.connector.Request#doGetSession
protected Session doGetSession(boolean create) {
...
// Return the current session if it exists and is valid
if ((session != null) && !session.isValid()) {
session = null;
}
if (session != null) {
return (session);
}
...
}
// case2. 后台定时任务清理
// org.apache.catalina.session.ManagerBase#backgroundProcess
@Override
public void backgroundProcess() {
// 并非每次定时任务到达时都会进行清理，而是要根据其清理频率设置来运行
// 默认是 6
count = (count + 1) % processExpiresFrequency;
if (count == 0)
processExpires();
}
/**
* Invalidate all sessions that have expired.
*/
public void processExpires() {
long timeNow = System.currentTimeMillis();
// 找出所有的sessions, 转化为数组遍历
Session sessions[] = findSessions();
int expireHere = 0 ;
if(log.isDebugEnabled())
log.debug("Start expire sessions " + getName() + " at " + timeNow + " sessioncount " + sessions.length);
for (int i = 0; i < sessions.length; i++) {
// 事实上后台任务也是调用 isValid() 方法 进行过期任务清理的
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 );
}
//case3. start/stop 时触发过期清理(生命周期事件)
// org.apache.catalina.session.StandardManager#startInternal
/**
* Start this component and implement the requirements
* of {@link org.apache.catalina.util.LifecycleBase#startInternal()}.
*
* @exception LifecycleException if this component detects a fatal error
* that prevents this component from being used
*/
@Override
protected synchronized void startInternal() throws LifecycleException {
super.startInternal();
// Load unloaded sessions, if any
try {
// doLoad() 调用
load();
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
log.error(sm.getString("standardManager.managerLoad"), t);
}
setState(LifecycleState.STARTING);
}
/**
* Load any currently active sessions that were previously unloaded
* to the appropriate persistence mechanism, if any. If persistence is not
* supported, this method returns without doing anything.
*
* @exception ClassNotFoundException if a serialized class cannot be
* found during the reload
* @exception IOException if an input/output error occurs
*/
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.
// 主动调用 expire
session.setValid(true);
session.expire();
}
sessionCounter++;
}
} finally {
// Delete the persistent storage file
if (file.exists()) {
file.delete();
}
}
}
} catch (FileNotFoundException e) {
if (log.isDebugEnabled()) {
log.debug("No persisted data file found");
}
return;
}
if (log.isDebugEnabled()) {
log.debug("Finish: Loading persisted sessions");
}
}
// stopInternal() 事件到达时清理 sessions
/**
* Save any currently active sessions in the appropriate persistence
* mechanism, if any. If persistence is not supported, this method
* returns without doing anything.
*
* @exception IOException if an input/output error occurs
*/
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
ArrayList<StandardSession> 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
// 将所有session失效，实际上应用即将关闭，失不失效的应该也无所谓了
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");
}
}
接下来我们看下具体如何清理过期的会话？实际应该就是一个remove的事。
// org.apache.catalina.session.StandardSession#isValid
/**
* Return the <code>isValid</code> flag for this session.
*/
@Override
public boolean isValid() {
if (!this.isValid) {
return false;
}
if (this.expiring) {
return true;
}
if (ACTIVITY_CHECK && accessCount.get() > 0) {
return true;
}
// 超过有效期，主动触发清理
if (maxInactiveInterval > 0) {
int timeIdle = (int) (getIdleTimeInternal() / 1000L);
if (timeIdle >= maxInactiveInterval) {
expire(true);
}
}
return this.isValid;
}
// org.apache.catalina.session.StandardSession#expire(boolean)
/**
* Perform the internal processing required to invalidate this session,
* without triggering an exception if the session has already expired.
*
* @param notify Should we notify listeners about the demise of
* this session?
*/
public void expire(boolean notify) {
// Check to see if session has already been invalidated.
// Do not check expiring at this point as expire should not return until
// isValid is false
if (!isValid)
return;
// 上锁保证线程安全
synchronized (this) {
// Check again, now we are inside the sync so this code only runs once
// Double check locking - isValid needs to be volatile
// The check of expiring is to ensure that an infinite loop is not
// entered as per bug 56339
if (expiring || !isValid)
return;
if (manager == null)
return;
// Mark this session as "being expired"
expiring = true;
// Notify interested application event listeners
// FIXME - Assumes we call listeners in reverse order
Context context = manager.getContext();
// The call to expire() may not have been triggered by the webapp.
// Make sure the webapp's class loader is set when calling the
// listeners
if (notify) {
ClassLoader oldContextClassLoader = null;
try {
oldContextClassLoader = context.bind(Globals.IS_SECURITY_ENABLED, null);
Object listeners[] = context.getApplicationLifecycleListeners();
if (listeners != null && listeners.length > 0) {
HttpSessionEvent event =
new HttpSessionEvent(getSession());
for (int i = 0; i < listeners.length; i++) {
int j = (listeners.length - 1) - i;
if (!(listeners[j] instanceof HttpSessionListener))
continue;
HttpSessionListener listener =
(HttpSessionListener) listeners[j];
try {
context.fireContainerEvent("beforeSessionDestroyed",
listener);
listener.sessionDestroyed(event);
context.fireContainerEvent("afterSessionDestroyed",
listener);
} catch (Throwable t) {
ExceptionUtils.handleThrowable(t);
try {
context.fireContainerEvent(
"afterSessionDestroyed", listener);
} catch (Exception e) {
// Ignore
}
manager.getContext().getLogger().error
(sm.getString("standardSession.sessionEvent"), t);
}
}
}
} finally {
context.unbind(Globals.IS_SECURITY_ENABLED, oldContextClassLoader);
}
}
if (ACTIVITY_CHECK) {
accessCount.set(0);
}
// Remove this session from our manager's active sessions
// 从ManagerBase 中删除
manager.remove(this, true);
// Notify interested session event listeners
if (notify) {
fireSessionEvent(Session.SESSION_DESTROYED_EVENT, null);
}
// Call the logout method
if (principal instanceof TomcatPrincipal) {
TomcatPrincipal gp = (TomcatPrincipal) principal;
try {
gp.logout();
} catch (Exception e) {
manager.getContext().getLogger().error(
sm.getString("standardSession.logoutfail"),
e);
}
}
// We have completed expire of this session
setValid(false);
expiring = false;
// Unbind any objects associated with this session
String keys[] = keys();
ClassLoader oldContextClassLoader = null;
try {
oldContextClassLoader = context.bind(Globals.IS_SECURITY_ENABLED, null);
for (int i = 0; i < keys.length; i++) {
removeAttributeInternal(keys[i], notify);
}
} finally {
context.unbind(Globals.IS_SECURITY_ENABLED, oldContextClassLoader);
}
}
}
// org.apache.catalina.session.ManagerBase#remove(org.apache.catalina.Session, boolean)
@Override
public void remove(Session session, boolean update) {
// If the session has expired - as opposed to just being removed from
// the manager because it is being persisted - update the expired stats
if (update) {
long timeNow = System.currentTimeMillis();
int timeAlive =
(int) (timeNow - session.getCreationTimeInternal())/1000;
updateSessionMaxAliveTime(timeAlive);
expiredSessions.incrementAndGet();
SessionTiming timing = new SessionTiming(timeNow, timeAlive);
synchronized (sessionExpirationTiming) {
sessionExpirationTiming.add(timing);
sessionExpirationTiming.poll();
}
}
// 从sessions中移除session
if (session.getIdInternal() != null) {
sessions.remove(session.getIdInternal());
}
}
清理工作的核心任务没猜错，还是进行remove对应的session, 但作为框架必然会设置很多的扩展点，为各监听器接入的机会。这些点的设计，直接关系到整个功能的好坏了。
4. session如何保证线程安全？
实际是废话，前面已经明显看出，其使用一个 ConcurrentHashMap 作为session的管理容器，而ConcurrentHashMap本身就是线程安全的，自然也就保证了线程安全了。
不过需要注意的是，上面的线程安全是指的不同客户端间的数据是互不影响的。然而对于同一个客户端的重复请求，以上实现并未处理，即可能会生成一次session，也可能生成n次session，不过实际影响不大，因为客户端的状态与服务端的状态都是一致的。
5. 使用持久化方案的session管理实现
默认情况使用内存作为session管理工具，一是方便，二是速度相当快。但是最大的缺点是，其无法实现持久化，即可能停机后信息就丢失了（虽然上面有在停机时做了持久化操作，但仍然是不可靠的）。
所以就有了与之相对的存储方案了：Persistent，它有一个基类 PersistentManagerBase 继承了 ManagerBase，做了些特别的实现：
// 1. session的添加
// 复用 ManagerBase
// 2. session的查找
// org.apache.catalina.session.PersistentManagerBase#findSession
/**
* {@inheritDoc}
* <p>
* This method checks the persistence store if persistence is enabled,
* otherwise just uses the functionality from ManagerBase.
*/
@Override
public Session findSession(String id) throws IOException {
// 复用ManagerBase, 获取Session实例
Session session = super.findSession(id);
// OK, at this point, we're not sure if another thread is trying to
// remove the session or not so the only way around this is to lock it
// (or attempt to) and then try to get it by this session id again. If
// the other code ran swapOut, then we should get a null back during
// this run, and if not, we lock it out so we can access the session
// safely.
if(session != null) {
synchronized(session){
session = super.findSession(session.getIdInternal());
if(session != null){
// To keep any external calling code from messing up the
// concurrency.
session.access();
session.endAccess();
}
}
}
if (session != null)
return session;
// See if the Session is in the Store
// 如果内存中找不到会话信息，从存储中查找，这是主要的区别
session = swapIn(id);
return session;
}
// org.apache.catalina.session.PersistentManagerBase#swapIn
/**
* Look for a session in the Store and, if found, restore
* it in the Manager's list of active sessions if appropriate.
* The session will be removed from the Store after swapping
* in, but will not be added to the active session list if it
* is invalid or past its expiration.
*
* @param id The id of the session that should be swapped in
* @return restored session, or {@code null}, if none is found
* @throws IOException an IO error occurred
*/
protected Session swapIn(String id) throws IOException {
if (store == null)
return null;
Object swapInLock = null;
/*
* The purpose of this sync and these locks is to make sure that a
* session is only loaded once. It doesn't matter if the lock is removed
* and then another thread enters this method and tries to load the same
* session. That thread will re-create a swapIn lock for that session,
* quickly find that the session is already in sessions, use it and
* carry on.
*/
// 额，总之就是有点复杂
synchronized (this) {
swapInLock = sessionSwapInLocks.get(id);
if (swapInLock == null) {
swapInLock = new Object();
sessionSwapInLocks.put(id, swapInLock);
}
}
Session session = null;
synchronized (swapInLock) {
// First check to see if another thread has loaded the session into
// the manager
session = sessions.get(id);
if (session == null) {
Session currentSwapInSession = sessionToSwapIn.get();
try {
if (currentSwapInSession == null || !id.equals(currentSwapInSession.getId())) {
// 从存储中查找session
session = loadSessionFromStore(id);
sessionToSwapIn.set(session);
if (session != null && !session.isValid()) {
log.error(sm.getString("persistentManager.swapInInvalid", id));
session.expire();
removeSession(id);
session = null;
}
// 重新加入到内存 sessions 中
if (session != null) {
reactivateLoadedSession(id, session);
}
}
} finally {
sessionToSwapIn.remove();
}
}
}
// Make sure the lock is removed
synchronized (this) {
sessionSwapInLocks.remove(id);
}
return session;
}
private Session loadSessionFromStore(String id) throws IOException {
try {
if (SecurityUtil.isPackageProtectionEnabled()){
return securedStoreLoad(id);
} else {
// 依赖于store的实现了，比如 file, jdbc...
return store.load(id);
}
} catch (ClassNotFoundException e) {
String msg = sm.getString(
"persistentManager.deserializeError", id);
log.error(msg, e);
throw new IllegalStateException(msg, e);
}
}
// store 实现样例: fileStore
// org.apache.catalina.session.FileStore#load
/**
* Load and return the Session associated with the specified session
* identifier from this Store, without removing it. If there is no
* such stored Session, return <code>null</code>.
*
* @param id Session identifier of the session to load
*
* @exception ClassNotFoundException if a deserialization error occurs
* @exception IOException if an input/output error occurs
*/
@Override
public Session load(String id) throws ClassNotFoundException, IOException {
// Open an input stream to the specified pathname, if any
File file = file(id);
if (file == null) {
return null;
}
if (!file.exists()) {
return null;
}
Context context = getManager().getContext();
Log contextLog = context.getLogger();
if (contextLog.isDebugEnabled()) {
contextLog.debug(sm.getString(getStoreName()+".loading", id, file.getAbsolutePath()));
}
ClassLoader oldThreadContextCL = context.bind(Globals.IS_SECURITY_ENABLED, null);
try (FileInputStream fis = new FileInputStream(file.getAbsolutePath());
ObjectInputStream ois = getObjectInputStream(fis)) {
StandardSession session = (StandardSession) manager.createEmptySession();
session.readObjectData(ois);
session.setManager(manager);
return session;
} catch (FileNotFoundException e) {
if (contextLog.isDebugEnabled()) {
contextLog.debug("No persisted data file found");
}
return null;
} finally {
context.unbind(Globals.IS_SECURITY_ENABLED, oldThreadContextCL);
}
}
private void reactivateLoadedSession(String id, Session session) {
if(log.isDebugEnabled())
log.debug(sm.getString("persistentManager.swapIn", id));
session.setManager(this);
// make sure the listeners know about it.
((StandardSession)session).tellNew();
// 添加回sessions
add(session);
((StandardSession)session).activate();
// endAccess() to ensure timeouts happen correctly.
// access() to keep access count correct or it will end up
// negative
session.access();
session.endAccess();
}
// 3. session 的移除
@Override
public void remove(Session session, boolean update) {
super.remove (session, update);
// 和内存的实现差别就是，还要多一个对外部存储的管理维护
if (store != null){
removeSession(session.getIdInternal());
}
}
可以看到， PersistentManager 的实现还是有点复杂的，主要是在安全性和性能之间的平衡，它和 StandardManager 基本是一种包含关系，即除了要维护内存session外，还要维护外部存储的状态。
而现实情况是，既然已经需要自行维护外部状态了，为何还要去使用tomcat自带的session管理呢？而如果站在框架session管理的设计者的角度，这可能也是无可奈何的事。
而在我们自己的session管理实现中，一般的思路还是相通的，创建 -> 查找 -> 维持 -> 删除 。 可以基于数据库，缓存，或者其他，而且相信也不是件难事。
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