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做web开发的肯定都知道，cookie和session。不过刚开始，很多人都只是停留在概念上的理解。今天就以看得见的方式再理解一下session。通过查看tomcat源码，可以发现sessions就是一个ConcurrentHashMap。

StandardManger负责管理session的生命周期。如：session过期了，就把它清除掉。tomcat关闭时把session信息持久化到硬盘上。tomcat启动时就把session信息加载进内存。StandardManager.doUnload方法在tomcat关闭时被调用，看一下方法的逻辑：

/*** 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("Unloading persisted sessions");// Open an output stream to the specified pathname, if anyFile file = file();if (file == null)return;if (log.isDebugEnabled())log.debug(sm.getString("standardManager.unloading", pathname));FileOutputStream fos = null;ObjectOutputStream oos = null;try {fos = new FileOutputStream(file.getAbsolutePath());oos = new ObjectOutputStream(new BufferedOutputStream(fos));} catch (IOException e) {log.error(sm.getString("standardManager.unloading.ioe", e), e);if (oos != null) {try {oos.close();} catch (IOException f) {;}oos = null;}throw e;}// Write the number of active sessions, followed by the detailsArrayList list = new ArrayList();synchronized (sessions) {if (log.isDebugEnabled())log.debug("Unloading " + sessions.size() + " sessions");try {oos.writeObject(new Integer(sessions.size()));Iterator elements = sessions.values().iterator();while (elements.hasNext()) {StandardSession session =(StandardSession) elements.next();list.add(session);((StandardSession) session).passivate();session.writeObjectData(oos);}} catch (IOException e) {log.error(sm.getString("standardManager.unloading.ioe", e), e);if (oos != null) {try {oos.close();} catch (IOException f) {;}oos = null;}throw e;}}// Flush and close the output streamtry {oos.flush();oos.close();oos = null;} catch (IOException e) {if (oos != null) {try {oos.close();} catch (IOException f) {;}oos = null;}throw e;}// Expire all the sessions we just wroteif (log.isDebugEnabled())log.debug("Expiring " + list.size() + " persisted sessions");Iterator expires = list.iterator();while (expires.hasNext()) {StandardSession session = (StandardSession) expires.next();try {session.expire(false);} catch (Throwable t) {;} finally {session.recycle();}}if (log.isDebugEnabled())log.debug("Unloading complete");}

tomcat启动的时候，会把读取这个文件，恢复sessions。

/*** 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 structuressessions.clear();// Open an input stream to the specified pathname, if anyFile 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 {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;}// Load the previously unloaded active sessionssynchronized (sessions) {try {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();session.endAccess();}} 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 {// Close the input streamtry {if (ois != null)ois.close();} catch (IOException f) {// ignored}// Delete the persistent storage fileif (file != null && file.exists() )file.delete();}}if (log.isDebugEnabled())log.debug("Finish: Loading persisted sessions");}