session到底是什么

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 any
File 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 details
ArrayList 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 stream
try {
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 wrote
if (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");

}

逻辑很简单，就是通过文件流将sessions写到硬盘上。这个文件到底是什么样子的呢？可以通过tomcat自带的host-manager来看一下，打开${tomcat_root}/work/Catalina/${host}/host-manager路径，SESSIONS.ser就存储着session中的数据。tomcat启动完成后，会自动删除这个文件。如下图：



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 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));
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) {
4000
;
}
ois = null;
}
throw e;
}

// Load the previously unloaded active sessions
synchronized (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 stream
try {
if (ois != null)
ois.close();
} catch (IOException f) {
// ignored
}

// Delete the persistent storage file
if (file != null && file.exists() )
file.delete();
}
}

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

以上整体的逻辑就是，通过文件流读取SESSIONS.ser并恢复sessions。我这里提到的启动、关闭tomcat是指通过startup、shutdown命令。如果直接kill掉tomcat进程，以上操作还没来得及执行，进程就挂掉了。