tomcat源码研究之 web.xml中load-on-startup标签的含义

(1)

在servlet的配置当中，<load-on-startup>5</load-on-startup>的含义是：

标记容器是否在启动的时候就加载这个servlet。

当值为0或者大于0时，表示容器在应用启动时就加载这个servlet；

当是一个负数时或者没有指定时，则指示容器在该servlet被选择时才加载。

正数的值越小，启动该servlet的优先级越高。

web.xml的配置如下:

<servlet>  
 <servlet-name>initservlet</servlet-name>  
 <servlet-class>com.ccc.lxy.util.initServlet</servlet-class>  
 <load-on-startup>1</load-on-startup>  
</servlet>  

（2） 在我的tomcat源码研究中我已经介绍到，tomcat本身就是组件构成的容器，我们的应用程序程序将对应到Context组件

而Context组件实现是StandardContext这个类，而Servlet对应的组件是Wrapper，它的实现是StandardWrapper，

既然当load-on-startup值大于等于0时候 在启动时候 那么我看StandardContext 这个类启动时的情况 

  protected synchronized void startInternal() throws LifecycleException {

        if(log.isDebugEnabled())
            log.debug("Starting " + getBaseName());

        // Send j2ee.state.starting notification 
        if (this.getObjectName() != null) {
            Notification notification = new Notification("j2ee.state.starting",
                    this.getObjectName(), sequenceNumber.getAndIncrement());
            broadcaster.sendNotification(notification);
        }

        setConfigured(false);
        boolean ok = true;

        // Currently this is effectively a NO-OP but needs to be called to
        // ensure the NamingResources follows the correct lifecycle
        if (namingResources != null) {
            namingResources.start();
        }
        
        // Add missing components as necessary
        if (webappResources == null) {   // (1) Required by Loader
            if (log.isDebugEnabled())
                log.debug("Configuring default Resources");
            try {
                if ((getDocBase() != null) && (getDocBase().endsWith(".war")) &&
                        (!(new File(getBasePath())).isDirectory()))
                    setResources(new WARDirContext());
                else
                    setResources(new FileDirContext());
            } catch (IllegalArgumentException e) {
                log.error("Error initializing resources: " + e.getMessage());
                ok = false;
            }
        }
// 代码省略...

    if (ok) {
                if (!listenerStart()) {
                    log.error( "Error listenerStart");
                    ok = false;
                }
            }
            
            try {
                // Start manager
                if ((manager != null) && (manager instanceof Lifecycle)) {
                    ((Lifecycle) getManager()).start();
                }
            } catch(Exception e) {
                log.error("Error manager.start()", e);
                ok = false;
            }

            // Configure and call application filters
            if (ok) {
                if (!filterStart()) {
                    log.error("Error filterStart");
                    ok = false;
                }
            }
            
            // Load and initialize all "load on startup" servlets
            
            //加载servlet的loadOnStardup属性
            if (ok) {
                if (!loadOnStartup(findChildren())){
                    log.error("Error loadOnStartup");
                    ok = false;
                }
            }
            
            // Start ContainerBackgroundProcessor thread
            super.threadStart();
//代码省略

 从上述代码中，我可知Context组件在启动时调用方法 loadOnStartup这个方法

public boolean loadOnStartup(Container children[]) {

        // Collect "load on startup" servlets that need to be initialized
        TreeMap<Integer, ArrayList<Wrapper>> map =
            new TreeMap<Integer, ArrayList<Wrapper>>();
        for (int i = 0; i < children.length; i++) {
            Wrapper wrapper = (Wrapper) children[i];
            int loadOnStartup = wrapper.getLoadOnStartup();
            //从这儿可知loadonStartup 小于0时 跳过，
            //也就是只有大于等于0时才进行的 初始化化加载了
            if (loadOnStartup < 0)
                continue;
            Integer key = Integer.valueOf(loadOnStartup);
            ArrayList<Wrapper> list = map.get(key);
            if (list == null) {
                list = new ArrayList<Wrapper>();
                map.put(key, list);
            }
            list.add(wrapper);
        }

        //下面是具体初始化加载的loadonStartup>=0的Servlet
        
        // Load the collected "load on startup" servlets
        for (ArrayList<Wrapper> list : map.values()) {
            for (Wrapper wrapper : list) {
                try {
                    wrapper.load();
                } catch (ServletException e) {
                    getLogger().error(sm.getString("standardWrapper.loadException",
                                      getName()), StandardWrapper.getRootCause(e));
                    // NOTE: load errors (including a servlet that throws
                    // UnavailableException from tht init() method) are NOT
                    // fatal to application startup, excepted if failDeploymentIfServletLoadedOnStartupFails is specified
                    if(getComputedFailCtxIfServletStartFails()) {
                        return false;
                    }
                }
            }
        }
        return true;

    }

  从上面可知只有loadonStartup>=0时才调用wrapper的load

 而且从上面可知将这些大于等于0的Servlet都放入一个treemap， treemap恰恰是一个排序map且升序map 所以loadonstartup 越小越先遍历到 越先执行load

 

而Wrapper的load就是StandardWrapper的load方法具体如下

  public synchronized void load() throws ServletException {
    	//实例化一个Servlet
        instance = loadServlet();
        //实例化完成以后开始调用InitServlet方法
        if (!instanceInitialized) {
            initServlet(instance);
        }

        if (isJspServlet) {
            StringBuilder oname =
                new StringBuilder(MBeanUtils.getDomain(getParent()));
            
            oname.append(":type=JspMonitor,name=");
            oname.append(getName());
            
            oname.append(getWebModuleKeyProperties());
            
            try {
                jspMonitorON = new ObjectName(oname.toString());
                Registry.getRegistry(null, null)
                    .registerComponent(instance, jspMonitorON, null);
            } catch( Exception ex ) {
                log.info("Error registering JSP monitoring with jmx " +
                         instance);
            }
        }
    }

 具体调用InitServlet方法如下：

   

 private synchronized void initServlet(Servlet servlet)
            throws ServletException {
        
        if (instanceInitialized && !singleThreadModel) return;

        // Call the initialization method of this servlet
        try {
            instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT,
                                              servlet);

            //调用Servlet的init方法
            if( Globals.IS_SECURITY_ENABLED) {
                boolean success = false;
                try {
                	//反射调用init方法
                    Object[] args = new Object[] { facade };
                    SecurityUtil.doAsPrivilege("init",
                                               servlet,
                                               classType,
                                               args);
                    success = true;
                } finally {
                    if (!success) {
                        // destroy() will not be called, thus clear the reference now
                        SecurityUtil.remove(servlet);
                    }
                }
            } else {
            	//{初始化servlet的init（ServletConfig config）方法然后 在这方法中init()方法了了}
                servlet.init(facade);
            }

            instanceInitialized = true;

            instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
                                              servlet);
        } catch (UnavailableException f) {
            instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
                                              servlet, f);
            unavailable(f);
            throw f;
        } catch (ServletException f) {
            instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
                                              servlet, f);
            // If the servlet wanted to be unavailable it would have
            // said so, so do not call unavailable(null).
            throw f;
        } catch (Throwable f) {
            ExceptionUtils.handleThrowable(f);
            getServletContext().log("StandardWrapper.Throwable", f );
            instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT,
                                              servlet, f);
            // If the servlet wanted to be unavailable it would have
            // said so, so do not call unavailable(null).
            throw new ServletException
                (sm.getString("standardWrapper.initException", getName()), f);
        }
    }

 从这里可知我们Servlet调用情况