那么，tomcat中多个会话对应的session是由谁来维护的呢？ManagerBase类，查看其代码，可以发现其有一个sessions成员属性，存储着各个会话的session信息：
/** * The set of currently active Sessions for this Manager, keyed by * session identifier. */ protected Map<String, Session> sessions = new ConcurrentHashMap<String, Session>();
接下来，看一下几个重要的方法，
服务器查找Session对象的方法
客户端每次的请求，tomcat都会在HashMap中查找对应的key为JSESSIONID的Session对象是否存在，可以查看Request的doGetSession方法源码，如下源码：
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);
} 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);
}
if ((context != null) && (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 to 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;
}
sessionId = getRequestedSessionId();
}
} else {
sessionId = null;
}
session = manager.createSession(sessionId);
// Creating a new session cookie based on that session
if ((session != null) && (getContext() != null)
&& getContext().getServletContext().
getEffectiveSessionTrackingModes().contains(
SessionTrackingMode.COOKIE)) {
Cookie cookie =
ApplicationSessionCookieConfig.createSessionCookie(
context, session.getIdInternal(), isSecure());
response.addSessionCookieInternal(cookie);
}
if (session == null) {
return null;
}
session.access();
return session;
}
