Servlet 的生命周期

以前只知道继承HttpServlet,然后Servlet容器就会自动调用doGet/doPost方法来提供service服务， 却不知道为什么，知道HttpServlet有生命周期init==>service==>destroy,生命周期由Servlet容器管理，却不知道怎么管理的，闲来无事就选择Tomcat作为研究对象。
其实以我做Java十几年的经验， 看源码似乎已成习惯:

1. Servlet init.

其实并不是所有的Servlet都是Servlet容器初始化就加载的，而是Servlet被第一次调用的时候加载.

web 2.4 configration

<servlet>
	<display-name>InitServlet</display-name>
	<servlet-name>InitServlet</servlet-name>
	<servlet-class>com.InitServlet</servlet-class>
	<load-on-startup>1</load-on-startup>
</servlet>

以上代码是配置Servlet load on startup, 可能有些人会对这里的数字1感兴趣，我配置成2行不行.

org.apache.catalina.core.StandardContext, 在容器启动的时候会调用startInternal方法:

// Load and initialize all "load on startup" servlets
            if (ok) {
                if (!loadOnStartup(findChildren())){
                    log.error("Error loadOnStartup");
                    ok = false;
                }
            }

/**
     * Load and initialize all servlets marked "load on startup" in the
     * web application deployment descriptor.
     *
     * @param children Array of wrappers for all currently defined
     *  servlets (including those not declared load on startup)
     */
    public boolean loadOnStartup(Container children[]) {

        // Collect "load on startup" servlets that need to be initialized
        TreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();
        for (int i = 0; i < children.length; i++) {
            Wrapper wrapper = (Wrapper) children[i];
            int loadOnStartup = wrapper.getLoadOnStartup();
            if (loadOnStartup < 0)
                continue;
            Integer key = Integer.valueOf(loadOnStartup);
            ArrayList<Wrapper> list = map.get(key);
            if (list == null) {
                list = new ArrayList<>();
                map.put(key, list);
            }
            list.add(wrapper);
        }

        // 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;

    }

/**
     * Load and initialize an instance of this servlet, if there is not already
     * at least one initialized instance.  This can be used, for example, to
     * load servlets that are marked in the deployment descriptor to be loaded
     * at server startup time.
     */
    public synchronized Servlet loadServlet() throws ServletException {

        if (unloading) {
            throw new ServletException(
                    sm.getString("standardWrapper.unloading", getName()));
        }

        // Nothing to do if we already have an instance or an instance pool
        if (!singleThreadModel && (instance != null))
            return instance;

        PrintStream out = System.out;
        if (swallowOutput) {
            SystemLogHandler.startCapture();
        }

        Servlet servlet;
        try {
            long t1=System.currentTimeMillis();
            // Complain if no servlet class has been specified
            if (servletClass == null) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.notClass", getName()));
            }

            InstanceManager instanceManager = ((StandardContext)getParent()).getInstanceManager();
            try {
                servlet = (Servlet) instanceManager.newInstance(servletClass);
            } catch (ClassCastException e) {
                unavailable(null);
                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.notServlet", servletClass), e);
            } catch (Throwable e) {
                e = ExceptionUtils.unwrapInvocationTargetException(e);
                ExceptionUtils.handleThrowable(e);
                unavailable(null);

                // Added extra log statement for Bugzilla 36630:
                // http://issues.apache.org/bugzilla/show_bug.cgi?id=36630
                if(log.isDebugEnabled()) {
                    log.debug(sm.getString("standardWrapper.instantiate", servletClass), e);
                }

                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.instantiate", servletClass), e);
            }

            if (multipartConfigElement == null) {
                MultipartConfig annotation =
                        servlet.getClass().getAnnotation(MultipartConfig.class);
                if (annotation != null) {
                    multipartConfigElement =
                            new MultipartConfigElement(annotation);
                }
            }

            processServletSecurityAnnotation(servlet.getClass());

            // Special handling for ContainerServlet instances
            if ((servlet instanceof ContainerServlet) &&
                    (isContainerProvidedServlet(servletClass) ||
                            ((Context) getParent()).getPrivileged() )) {
                ((ContainerServlet) servlet).setWrapper(this);
            }

            classLoadTime=(int) (System.currentTimeMillis() -t1);

            if (servlet instanceof SingleThreadModel) {
                if (instancePool == null) {
                    instancePool = new Stack<>();
                }
                singleThreadModel = true;
            }

            initServlet(servlet);

            fireContainerEvent("load", this);

            loadTime=System.currentTimeMillis() -t1;
        } finally {
            if (swallowOutput) {
                String log = SystemLogHandler.stopCapture();
                if (log != null && log.length() > 0) {
                    if (getServletContext() != null) {
                        getServletContext().log(log);
                    } else {
                        out.println(log);
                    }
                }
            }
        }
        return servlet;

    }

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);

            if( Globals.IS_SECURITY_ENABLED) {
                boolean success = false;
                try {
                    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 {
                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);
        }
    }

我想不需要我解释，大家也知道这个数字(1)的意义了吧，服务器内部也只是用treeMap来排序罢了，但是不管你配置了多少，只要是数字并且大于0，那么都回在服务器启动的时候初始化.
其实这种例子可是很多的哦，servlet继承struts1.x的时候就是这么配置的:

servlet>
    <servlet-name>action</servlet-name>
    <servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
    <init-param>
      <param-name>config</param-name>
      <param-value>/WEB-INF/struts-config.xml</param-value>
    </init-param>
    <init-param>
      <param-name>debug</param-name>
      <param-value>3</param-value>
    </init-param>
    <init-param>
      <param-name>detail</param-name>
      <param-value>3</param-value>
    </init-param>
    <load-on-startup>0</load-on-startup>
  </servlet>

struts中的ActionServlet必须在容器初始化的时候实例化，这样就能在Servlet提供service服务前执行.

2. Servlet service.

经过以上实例化的Servlet就能提供service服务了.
org.apache.catalina.core.StandardWrapperValve

servlet = null;
        Context context = (Context) wrapper.getParent();

        // Check for the application being marked unavailable
        if (!context.getState().isAvailable()) {
            response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardContext.isUnavailable"));
            unavailable = true;
        }

        // Check for the servlet being marked unavailable
        if (!unavailable && wrapper.isUnavailable()) {
            container.getLogger().info(sm.getString("standardWrapper.isUnavailable",
                    wrapper.getName()));
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                        sm.getString("standardWrapper.isUnavailable",
                                wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                        sm.getString("standardWrapper.notFound",
                                wrapper.getName()));
            }
            unavailable = true;
        }

        // Allocate a servlet instance to process this request
        try {
            if (!unavailable) {
                servlet = wrapper.allocate();
            }
        } catch (UnavailableException e) {
            container.getLogger().error(
                    sm.getString("standardWrapper.allocateException",
                            wrapper.getName()), e);
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardWrapper.isUnavailable",
                                        wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                           sm.getString("standardWrapper.notFound",
                                        wrapper.getName()));
            }
        } catch (ServletException e) {
            container.getLogger().error(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), StandardWrapper.getRootCause(e));
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
            servlet = null;
        }

        // Identify if the request is Comet related now that the servlet has been allocated
        boolean comet = false;
        if (servlet instanceof CometProcessor && request.getAttribute(
                Globals.COMET_SUPPORTED_ATTR) == Boolean.TRUE) {
            comet = true;
            request.setComet(true);
        }

        MessageBytes requestPathMB = request.getRequestPathMB();
        DispatcherType dispatcherType = DispatcherType.REQUEST;
        if (request.getDispatcherType()==DispatcherType.ASYNC) dispatcherType = DispatcherType.ASYNC;
        request.setAttribute(Globals.DISPATCHER_TYPE_ATTR,dispatcherType);
        request.setAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR,
                requestPathMB);
        // Create the filter chain for this request
        ApplicationFilterFactory factory =
            ApplicationFilterFactory.getInstance();
        ApplicationFilterChain filterChain =
            factory.createFilterChain(request, wrapper, servlet);

        // Reset comet flag value after creating the filter chain
        request.setComet(false);

        // Call the filter chain for this request
        // NOTE: This also calls the servlet's service() method
        try {
            if ((servlet != null) && (filterChain != null)) {
                // Swallow output if needed
                if (context.getSwallowOutput()) {
                    try {
                        SystemLogHandler.startCapture();
                        if (request.isAsyncDispatching()) {
                            ((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
                        } else if (comet) {
                            filterChain.doFilterEvent(request.getEvent());
                            request.setComet(true);
                        } else {
                            filterChain.doFilter(request.getRequest(),
                                    response.getResponse());
                        }
                    } finally {
                        String log = SystemLogHandler.stopCapture();
                        if (log != null && log.length() > 0) {
                            context.getLogger().info(log);
                        }
                    }
                } else {
                    if (request.isAsyncDispatching()) {
                        ((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
                    } else if (comet) {
                        request.setComet(true);
                        filterChain.doFilterEvent(request.getEvent());
                    } else {
                        filterChain.doFilter
                            (request.getRequest(), response.getResponse());
                    }
                }

            }
        } catch (ClientAbortException e) {
            throwable = e;
            exception(request, response, e);
        } catch (IOException e) {
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            throwable = e;
            exception(request, response, e);
        } catch (UnavailableException e) {
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            //            throwable = e;
            //            exception(request, response, e);
            wrapper.unavailable(e);
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE)) {
                response.setDateHeader("Retry-After", available);
                response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardWrapper.isUnavailable",
                                        wrapper.getName()));
            } else if (available == Long.MAX_VALUE) {
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                            sm.getString("standardWrapper.notFound",
                                        wrapper.getName()));
            }
            // Do not save exception in 'throwable', because we
            // do not want to do exception(request, response, e) processing
        } catch (ServletException e) {
            Throwable rootCause = StandardWrapper.getRootCause(e);
            if (!(rootCause instanceof ClientAbortException)) {
                container.getLogger().error(sm.getString(
                        "standardWrapper.serviceExceptionRoot",
                        wrapper.getName(), context.getName(), e.getMessage()),
                        rootCause);
            }
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString(
                    "standardWrapper.serviceException", wrapper.getName(),
                    context.getName()), e);
            throwable = e;
            exception(request, response, e);
        }

        // Release the filter chain (if any) for this request
        if (filterChain != null) {
            if (request.isComet()) {
                // If this is a Comet request, then the same chain will be used for the
                // processing of all subsequent events.
                filterChain.reuse();
            } else {
                filterChain.release();
            }
        }

        // Deallocate the allocated servlet instance
        try {
            if (servlet != null) {
                wrapper.deallocate(servlet);
            }
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.deallocateException",
                             wrapper.getName()), e);
            if (throwable == null) {
                throwable = e;
                exception(request, response, e);
            }
        }

        // If this servlet has been marked permanently unavailable,
        // unload it and release this instance
        try {
            if ((servlet != null) &&
                (wrapper.getAvailable() == Long.MAX_VALUE)) {
                wrapper.unload();
            }
        } catch (Throwable e) {
            ExceptionUtils.handleThrowable(e);
            container.getLogger().error(sm.getString("standardWrapper.unloadException",
                             wrapper.getName()), e);
            if (throwable == null) {
                throwable = e;
                exception(request, response, e);
            }
        }
        long t2=System.currentTimeMillis();

        long time=t2-t1;
        processingTime += time;
        if( time > maxTime) maxTime=time;
        if( time < minTime) minTime=time;

    }

如果Servlet没有在容器初始化的时候实例化，那么会执行初始化。使用完servlet之后把当前Servlet实例放回到对象池中.

    /**
     * Return this previously allocated servlet to the pool of available
     * instances.  If this servlet class does not implement SingleThreadModel,
     * no action is actually required.
     *
     * @param servlet The servlet to be returned
     *
     * @exception ServletException if a deallocation error occurs
     */
    @Override
    public void deallocate(Servlet servlet) throws ServletException {

        // If not SingleThreadModel, no action is required
        if (!singleThreadModel) {
            countAllocated.decrementAndGet();
            return;
        }

        // Unlock and free this instance
        synchronized (instancePool) {
            countAllocated.decrementAndGet();
            instancePool.push(servlet);
            instancePool.notify();
        }

    }

3. Servlet destroy.

当容器停止，或Servlet到期时，容器会调用servlet.destroy().

/**
     * Stop this component and implement the requirements
     * of {@link org.apache.catalina.util.LifecycleBase#stopInternal()}.
     *
     * @exception LifecycleException if this component detects a fatal error
     *  that prevents this component from being used
     */
    @Override
    protected synchronized void stopInternal() throws LifecycleException {

        setAvailable(Long.MAX_VALUE);

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

        // Shut down our servlet instance (if it has been initialized)
        try {
            unload();
        } catch (ServletException e) {
            getServletContext().log(sm.getString
                      ("standardWrapper.unloadException", getName()), e);
        }

        // Shut down this component
        super.stopInternal();

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

        // Send j2ee.object.deleted notification
        Notification notification =
            new Notification("j2ee.object.deleted", this.getObjectName(),
                            sequenceNumber++);
        broadcaster.sendNotification(notification);

    }

/**
     * Unload all initialized instances of this servlet, after calling the
     * <code>destroy()</code> method for each instance.  This can be used,
     * for example, prior to shutting down the entire servlet engine, or
     * prior to reloading all of the classes from the Loader associated with
     * our Loader's repository.
     *
     * @exception ServletException if an exception is thrown by the
     *  destroy() method
     */
    @Override
    public synchronized void unload() throws ServletException {

        // Nothing to do if we have never loaded the instance
        if (!singleThreadModel && (instance == null))
            return;
        unloading = true;

        // Loaf a while if the current instance is allocated
        // (possibly more than once if non-STM)
        if (countAllocated.get() > 0) {
            int nRetries = 0;
            long delay = unloadDelay / 20;
            while ((nRetries < 21) && (countAllocated.get() > 0)) {
                if ((nRetries % 10) == 0) {
                    log.info(sm.getString("standardWrapper.waiting",
                                          countAllocated.toString(),
                                          getName()));
                }
                try {
                    Thread.sleep(delay);
                } catch (InterruptedException e) {
                    // Ignore
                }
                nRetries++;
            }
        }

        if (instanceInitialized) {
            PrintStream out = System.out;
            if (swallowOutput) {
                SystemLogHandler.startCapture();
            }

            // Call the servlet destroy() method
            try {
                instanceSupport.fireInstanceEvent
                  (InstanceEvent.BEFORE_DESTROY_EVENT, instance);

                if( Globals.IS_SECURITY_ENABLED) {
                    try {
                        SecurityUtil.doAsPrivilege("destroy",
                                                   instance);
                    } finally {
                        SecurityUtil.remove(instance);
                    }
                } else {
                    instance.destroy();
                }

                instanceSupport.fireInstanceEvent
                  (InstanceEvent.AFTER_DESTROY_EVENT, instance);

                // Annotation processing
                if (!((Context) getParent()).getIgnoreAnnotations()) {
                   ((StandardContext)getParent()).getInstanceManager().destroyInstance(instance);
                }

            } catch (Throwable t) {
                t = ExceptionUtils.unwrapInvocationTargetException(t);
                ExceptionUtils.handleThrowable(t);
                instanceSupport.fireInstanceEvent
                  (InstanceEvent.AFTER_DESTROY_EVENT, instance, t);
                instance = null;
                instancePool = null;
                nInstances = 0;
                fireContainerEvent("unload", this);
                unloading = false;
                throw new ServletException
                    (sm.getString("standardWrapper.destroyException", getName()),
                     t);
            } finally {
                // Write captured output
                if (swallowOutput) {
                    String log = SystemLogHandler.stopCapture();
                    if (log != null && log.length() > 0) {
                        if (getServletContext() != null) {
                            getServletContext().log(log);
                        } else {
                            out.println(log);
                        }
                    }
                }
            }
        }

        // Deregister the destroyed instance
        instance = null;

        if (isJspServlet && jspMonitorON != null ) {
            Registry.getRegistry(null, null).unregisterComponent(jspMonitorON);
        }

        if (singleThreadModel && (instancePool != null)) {
            try {
                while (!instancePool.isEmpty()) {
                    Servlet s = instancePool.pop();
                    if (Globals.IS_SECURITY_ENABLED) {
                        try {
                            SecurityUtil.doAsPrivilege("destroy", s);
                        } finally {
                            SecurityUtil.remove(s);
                        }
                    } else {
                        s.destroy();
                    }
                    // Annotation processing
                    if (!((Context) getParent()).getIgnoreAnnotations()) {
                       ((StandardContext)getParent()).getInstanceManager().destroyInstance(s);
                    }
                }
            } catch (Throwable t) {
                t = ExceptionUtils.unwrapInvocationTargetException(t);
                ExceptionUtils.handleThrowable(t);
                instancePool = null;
                nInstances = 0;
                unloading = false;
                fireContainerEvent("unload", this);
                throw new ServletException
                    (sm.getString("standardWrapper.destroyException",
                                  getName()), t);
            }
            instancePool = null;
            nInstances = 0;
        }

        singleThreadModel = false;

        unloading = false;
        fireContainerEvent("unload", this);

    }

这就是HttpServlet的生命周期，init方法可以放一些资源的初始化，destroy方法可以放一些资源的释放.