tomcat集群时统计session与在线人数

　　tomcat集群时，原来通过HttpSessionListener实现类监听session的创建和销毁来统计在线人数的方法不再有效，因为不是每个人登陆都会在同一个tomcat服务器上，而在另一台tomcat上登陆的人的session是通过session复制创建的，而复制过程不会调用HttpSessionListener接口的方法，也一直没找着如何监听session复制的方法，所以就没法统计在线人了。

今天突然回想起tomcat下的manager应用上面就能看到session数和session的内容，于是本文的实现原理就是，做一个类似这样的servlet，此servlet把tomcat上负责管理应用的对象保存下来，供我任意使用。在tomcat上看应用的信息时，使用的是http://localhost:8080/manager/html/list这个路径，页面信息见下图：



　　于是把源码下来看看 (apache-tomcat-6.0.39-src)，细看tomcat下webapps/manager/WEB-INF/web.xml文件配置，发现原来tomcat是通过org.apache.catalina.manager.ManagerServlet这个类来提供以上服务的，跟踪此类doGet方法代码

// --------------------------------------------------------- Public Methods

/**
* Process a GET request for the specified resource.
*
* @param request
*            The servlet request we are processing
* @param response
*            The servlet response we are creating
*
* @exception IOException
*                if an input/output error occurs
* @exception ServletException
*                if a servlet-specified error occurs
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {

// Identify the request parameters that we need
// 取得访问路径，
String command = request.getPathInfo();

String path = request.getParameter("path");
String deployPath = request.getParameter("deployPath");
String deployConfig = request.getParameter("deployConfig");
String deployWar = request.getParameter("deployWar");

// Prepare our output writer to generate the response message
response.setContentType("text/html; charset=" + Constants.CHARSET);

String message = "";
// Process the requested command
if (command == null || command.equals("/")) {
} else if (command.equals("/deploy")) {
message = deployInternal(deployConfig, deployPath, deployWar);
// 找到了，就是这个路径，往下看list方法
} else if (command.equals("/list")) {
} else if (command.equals("/reload")) {
message = reload(path);
} else if (command.equals("/undeploy")) {
message = undeploy(path);
} else if (command.equals("/expire")) {
message = expireSessions(path, request);
} else if (command.equals("/sessions")) {
try {
doSessions(path, request, response);
return;
} catch (Exception e) {
log("HTMLManagerServlet.sessions[" + path + "]", e);
message = sm
.getString("managerServlet.exception", e.toString());
}
} else if (command.equals("/start")) {
message = start(path);
} else if (command.equals("/stop")) {
message = stop(path);
} else if (command.equals("/findleaks")) {
message = findleaks();
} else {
message = sm.getString("managerServlet.unknownCommand", command);
}
// 就是这个方法生成上面的那个页面
list(request, response, message);
}

list

/**
* Render a HTML list of the currently active Contexts in our virtual host,
* and memory and server status information.
*
* @param request
*            The request
* @param response
*            The response
* @param message
*            a message to display
*/
public void list(HttpServletRequest request, HttpServletResponse response,
String message) throws IOException {

if (debug >= 1)
log("list: Listing contexts for virtual host '" + host.getName()
+ "'");

PrintWriter writer = response.getWriter();

// HTML Header Section
writer.print(Constants.HTML_HEADER_SECTION);

// Body Header Section
Object[] args = new Object[2];
args[0] = request.getContextPath();
args[1] = sm.getString("htmlManagerServlet.title");
writer.print(MessageFormat.format(Constants.BODY_HEADER_SECTION, args));

// Message Section
args = new Object[3];
args[0] = sm.getString("htmlManagerServlet.messageLabel");
if (message == null || message.length() == 0) {
args[1] = "OK";
} else {
args[1] = RequestUtil.filter(message);
}
writer.print(MessageFormat.format(Constants.MESSAGE_SECTION, args));

// Manager Section
args = new Object[9];
args[0] = sm.getString("htmlManagerServlet.manager");
args[1] = response.encodeURL(request.getContextPath() + "/html/list");
args[2] = sm.getString("htmlManagerServlet.list");
args[3] = response.encodeURL(request.getContextPath() + "/"
+ sm.getString("htmlManagerServlet.helpHtmlManagerFile"));
args[4] = sm.getString("htmlManagerServlet.helpHtmlManager");
args[5] = response.encodeURL(request.getContextPath() + "/"
+ sm.getString("htmlManagerServlet.helpManagerFile"));
args[6] = sm.getString("htmlManagerServlet.helpManager");
args[7] = response.encodeURL(request.getContextPath() + "/status");
args[8] = sm.getString("statusServlet.title");
writer.print(MessageFormat.format(Constants.MANAGER_SECTION, args));

// Apps Header Section
args = new Object[6];
args[0] = sm.getString("htmlManagerServlet.appsTitle");
args[1] = sm.getString("htmlManagerServlet.appsPath");
args[2] = sm.getString("htmlManagerServlet.appsName");
args[3] = sm.getString("htmlManagerServlet.appsAvailable");
args[4] = sm.getString("htmlManagerServlet.appsSessions");
args[5] = sm.getString("htmlManagerServlet.appsTasks");
writer.print(MessageFormat.format(APPS_HEADER_SECTION, args));

// Apps Row Section
// Create sorted map of deployed applications context paths.

// host就当是当前的tomcat吧，那么contexts就此tomcat下的所有应用
Container children[] = host.findChildren();
String contextPaths[] = new String[children.length];
// 循环每个应用
for (int i = 0; i < children.length; i++)
// 应用名称
contextPaths[i] = children[i].getName();

TreeMap sortedContextPathsMap = new TreeMap();

for (int i = 0; i < contextPaths.length; i++) {
// 应用部署路径
String displayPath = contextPaths[i];
sortedContextPathsMap.put(displayPath, contextPaths[i]);
}

String appsStart = sm.getString("htmlManagerServlet.appsStart");
String appsStop = sm.getString("htmlManagerServlet.appsStop");
String appsReload = sm.getString("htmlManagerServlet.appsReload");
String appsUndeploy = sm.getString("htmlManagerServlet.appsUndeploy");
String appsExpire = sm.getString("htmlManagerServlet.appsExpire");

Iterator iterator = sortedContextPathsMap.entrySet().iterator();
boolean isHighlighted = true;
boolean isDeployed = true;
String highlightColor = null;

while (iterator.hasNext()) {
// Bugzilla 34818, alternating row colors
isHighlighted = !isHighlighted;
if (isHighlighted) {
highlightColor = "#C3F3C3";
} else {
highlightColor = "#FFFFFF";
}

Map.Entry entry = (Map.Entry) iterator.next();
String displayPath = (String) entry.getKey();
String contextPath = (String) entry.getValue();
Context context = (Context) host.findChild(contextPath);
if (displayPath.equals("")) {
displayPath = "/";
}

if (context != null) {
try {
isDeployed = isDeployed(contextPath);
} catch (Exception e) {
// Assume false on failure for safety
isDeployed = false;
}

args = new Object[7];
args[0] = URL_ENCODER.encode(contextPath + "/");
args[1] = RequestUtil.filter(displayPath);
if (context.getDisplayName() == null) {
args[2] = " ";
} else {
args[2] = RequestUtil.filter(context.getDisplayName());
}
managerServlet
// 应用是否已启动
args[3] = new Boolean(context.getAvailable());
args[4] = response.encodeURL(request.getContextPath()
+ "/html/sessions?path="
+ URL_ENCODER.encode(displayPath));
if (context.getManager() != null) {
args[5] = new Integer(context.getManager()
.getActiveSessions());
} else {
args[5] = new Integer(0);
}

args[6] = highlightColor;
//打印出一行关于此应用的信息，应用的URL，当前状态，session数等，具体见上图
writer.print(MessageFormat.format(APPS_ROW_DETAILS_SECTION,
args));

args = new Object[14];
args[0] = response
.encodeURL(request.getContextPath()
+ "/html/start?path="
+ URL_ENCODER.encode(displayPath));
args[1] = appsStart;
args[2] = response.encodeURL(request.getContextPath()
+ "/html/stop?path=" + URL_ENCODER.encode(displayPath));
args[3] = appsStop;
args[4] = response.encodeURL(request.getContextPath()
+ "/html/reload?path="
+ URL_ENCODER.encode(displayPath));
args[5] = appsReload;
args[6] = response.encodeURL(request.getContextPath()
+ "/html/undeploy?path="
+ URL_ENCODER.encode(displayPath));
args[7] = appsUndeploy;

args[8] = response.encodeURL(request.getContextPath()
+ "/html/expire?path="
+ URL_ENCODER.encode(displayPath));
args[9] = appsExpire;
args[10] = sm.getString("htmlManagerServlet.expire.explain");
Manager manager = context.getManager();
if (manager == null) {
args[11] = sm.getString("htmlManagerServlet.noManager");
} else {
args[11] = new Integer(context.getManager()
.getMaxInactiveInterval() / 60);
}
args[12] = sm.getString("htmlManagerServlet.expire.unit");

args[13] = highlightColor;

if (context.getPath().equals(this.context.getPath())) {
writer.print(MessageFormat.format(
MANAGER_APP_ROW_BUTTON_SECTION, args));
} else if (context.getAvailable() && isDeployed) {
writer.print(MessageFormat.format(
STARTED_DEPLOYED_APPS_ROW_BUTTON_SECTION, args));
} else if (context.getAvailable() && !isDeployed) {
writer.print(MessageFormat.format(
STARTED_NONDEPLOYED_APPS_ROW_BUTTON_SECTION, args));
} else if (!context.getAvailable() && isDeployed) {
writer.print(MessageFormat.format(
STOPPED_DEPLOYED_APPS_ROW_BUTTON_SECTION, args));
} else {
writer.print(MessageFormat.format(
STOPPED_NONDEPLOYED_APPS_ROW_BUTTON_SECTION, args));
}

}
}

// Deploy Section
args = new Object[7];
args[0] = sm.getString("htmlManagerServlet.deployTitle");
args[1] = sm.getString("htmlManagerServlet.deployServer");
args[2] = response.encodeURL(request.getContextPath() + "/html/deploy");
args[3] = sm.getString("htmlManagerServlet.deployPath");
args[4] = sm.getString("htmlManagerServlet.deployConfig");
args[5] = sm.getString("htmlManagerServlet.deployWar");
args[6] = sm.getString("htmlManagerServlet.deployButton");
writer.print(MessageFormat.format(DEPLOY_SECTION, args));

args = new Object[4];
args[0] = sm.getString("htmlManagerServlet.deployUpload");
args[1] = response.encodeURL(request.getContextPath() + "/html/upload");
args[2] = sm.getString("htmlManagerServlet.deployUploadFile");
args[3] = sm.getString("htmlManagerServlet.deployButton");
writer.print(MessageFormat.format(UPLOAD_SECTION, args));

// Diagnostics section
args = new Object[5];
args[0] = sm.getString("htmlManagerServlet.diagnosticsTitle");
args[1] = sm.getString("htmlManagerServlet.diagnosticsLeak");
args[2] = response.encodeURL(request.getContextPath()
+ "/html/findleaks");
args[3] = sm.getString("htmlManagerServlet.diagnosticsLeakWarning");
args[4] = sm.getString("htmlManagerServlet.diagnosticsLeakButton");
writer.print(MessageFormat.format(DIAGNOSTICS_SECTION, args));

// Server Header Section
args = new Object[7];
args[0] = sm.getString("htmlManagerServlet.serverTitle");
args[1] = sm.getString("htmlManagerServlet.serverVersion");
args[2] = sm.getString("htmlManagerServlet.serverJVMVersion");
args[3] = sm.getString("htmlManagerServlet.serverJVMVendor");
args[4] = sm.getString("htmlManagerServlet.serverOSName");
args[5] = sm.getString("htmlManagerServlet.serverOSVersion");
args[6] = sm.getString("htmlManagerServlet.serverOSArch");
writer.print(MessageFormat
.format(Constants.SERVER_HEADER_SECTION, args));

// Server Row Section
args = new Object[6];
args[0] = ServerInfo.getServerInfo();
args[1] = System.getProperty("java.runtime.version");
args[2] = System.getProperty("java.vm.vendor");
args[3] = System.getProperty("os.name");
args[4] = System.getProperty("os.version");
args[5] = System.getProperty("os.arch");
writer.print(MessageFormat.format(Constants.SERVER_ROW_SECTION, args));

// HTML Tail Section
writer.print(Constants.HTML_TAIL_SECTION);

// Finish up the response
writer.flush();
writer.close();
}

　　注意：context.getManager().findSessions()可以取得所有session，但这是个org.apache.catalina.Session[]数组，不是HttpSession[]数组，但这个Session接口里面有个getSession方法，返回结果正是HttpSession类型，没错，就是循环这个数组并调用其getSession方法就可以取得所有在线用户了

上面的Session[]数组是从context对象里面来的，而context是从host对象来的，host是个初始值为NULL的成员变量，是什么时候赋上值的？是在init方法执行前，setWrapper方法执行时赋的值，请看setWrapper方法代码

public class HostManagerServlet
extends HttpServlet implements ContainerServlet

/**
* Set the Wrapper with which we are associated.
*
* @param wrapper The new wrapper
*/
public void setWrapper(Wrapper wrapper) {

//这里所有需要的对象都有了，其实下面我们需要拿到wrapper就够了
this.wrapper = wrapper;
if (wrapper == null) {
context = null;
host = null;
engine = null;
} else {
context = (Context) wrapper.getParent();
host = (Host) context.getParent();
engine = (Engine) host.getParent();
}

// Retrieve the MBean server
mBeanServer = Registry.getRegistry(null, null).getMBeanServer();

}

　　setWrapper会在初始化时被调用，怎么实现的，首先看web.xml中对此servlet的配置，没什么特别，我们可以发散一下思维，struts2里面action如何能自动注入request对象？Spring如何让service监听事件？答案是一样的，那就是让你的类实现某个接口，你要的东西就给你了，对的，这里也一样，此servlet实现了ContainerServlet接口，初始的时候setWrapper方法才会被调用。

　　是JAVA新手的看这里，我提出上面这些问题，不是想卖什么关子，只是想启发JAVA初学者们，当某天你们做设计时，可以参考这种方法，一句概括就是：只要你实现我的接口，我就可以让你做某事，而不需要任何额外的配置。当然这种设计的缺点就是入侵、偶合。举个简单的应用场景：每天晚上，我用一个定时器通过Spring搜索所有实现了“GarbageCleaner”接口的service bean，并调用其clean方法清理对应模块的垃圾数据，那么任何模块的service只要实现了此接口，就会被调用。

回到正题，我也自已写个servlet并且实ContainerServlet接口吧，使用静态方法取得所有的session，具体代码如下：

package manager.session.http.servlet;

import java.io.IOException;
import java.util.LinkedHashMap;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.apache.catalina.ContainerServlet;
import org.apache.catalina.Context;
import org.apache.catalina.Session;
import org.apache.catalina.Wrapper;

public class TomcatWrapperServlet extends HttpServlet implements
ContainerServlet {
private static final long serialVersionUID = 1L;

// 弄个静态变量，初始化后就记下来，以备随时使用
private static Wrapper wrapper = null;

public Wrapper getWrapper() {
return wrapper;
}

public void setWrapper(Wrapper wrapper) {
TomcatWrapperServlet.wrapper = wrapper;
}

// doGet不做任何事情，只需要接收第一次请求，触发初始动作就完成它的使命了
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
resp.getWriter().println("Hello world!");
resp.getWriter().flush();
resp.getWriter().close();
}

// 初始化后可通过此静态方法取得所有session
public static Map<String, HttpSession> fillSessions() {
if (wrapper == null) {// 没有初始化
throw new RuntimeException(
"本servlet未被初始化，您必须先通过URL访问本servlet后，才可以调用这个方法");
}
Map<String, HttpSession> sessions = new LinkedHashMap<String, HttpSession>();

// 取得本应用
Context context = (Context) wrapper.getParent();
// 取得Session[]数组
Session[] temps = context.getManager().findSessions();
if (temps != null && temps.length > 0) {
for (int j = 0; j < temps.length; j++) {
// Map<sessionId,session>
sessions.put(temps[j].getSession().getId(), temps[j]
.getSession());
}
}
return sessions;
}

}

　

　在web.xml配置一下，然后启动应用，访问之，结果出现异常，是一个安全异常：TomcatWrapperServlet is privileged and cannot be loaded by this web application(想想如下)，说我的类是个特权类，不能被普通的web应用加载，为何manager这个应用又可以呢？把manager/META-INF/context.xml复制到我的应用，再加载，再访问，一切搞定，此文件内容只有一句

　　Xml代码

<Context antiResourceLocking="false" privileged="true" />

HTTP Status 500 - Error allocating a servlet instance

type Exception report

message Error allocating a servlet instance

description The server encountered an internal error that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Error allocating a servlet instance
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)
org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1760)
java.lang.Thread.run(Thread.java:722)

root cause

java.lang.SecurityException: Servlet of class manager.session.http.servlet.TomcatWrapperServlet is privileged and cannot be loaded by this web application
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)
org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1760)
java.lang.Thread.run(Thread.java:722)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.37 logs.

Apache Tomcat/6.0.37

来自:http://www.jspspace.com/ResearchTopics/Art-1757-17.html