How Tomcat Works（十四）

我们已经知道，在tomcat中有四种类型的servlet容器，分别为Engine、Host、Context 和Wrapper，本文接下来对tomcat中Wrapper接口的标准实现进行说明。

对于每个引入的HTTP请求，连接器都会调用与其关联的servlet容器的invoke()方法；然后，servlet容器会调用所有子容器的invoke()方法

这里面的流程通常是servlet容器调用其管道对象的invoke()方法，其管道对象的invoke()方法最后调用其基础阀，管道对象的基础阀里面会调用子容器的invoke()方法；子容器的invoke()方法的调用序列与之相同。

在tomcat中，servlet类可以实现javax.servlet.SingleThreadModel接口，这样的servlet类也称为SingleThreadModel（STM）servlet类；根据Servlet规范，实现此接口的目的是保证servlet实例一次只处理一个请求。

StandardWrapper对象的主要任务是载入它所代表的servlet类，并进行实例化；不过StandardWrapper类并不调用servlet的service方法，该任务由StandardWrapperValve对象（StandardWrapper实例的管道对象中的基础阀）完成；StandardWrapperValve对象通过调用StandardWrapper实例的allocate()方法获取servlet实例，在获取实例后，StandardWrapperValve实例就会调用servlet实例的service()方法

对于STM 类型的servlet类与非STM 类型的servlet类，StandardWrapper实例的载入方式是不一样的；对于非STM 类型的servlet类，StandardWrapper实例只会载入一次，对于随后的请求都会返回servlet的同一个实例，它假设该servlet类的service()方法在多线程环境中是线程安全的。

而对于STM 类型的servlet类，StandardWrapper实例必须保证每一时刻只能有一个线程在执行STM servlet类的service()方法；StandardWrapper实例通过将STM 类型的servlet实例保存在一个java.util.Stack类型的栈中

public Servlet allocate() throws ServletException {

        if (debug >= 1)
            log("Allocating an instance");

        // If we are currently unloading this servlet, throw an exception
        if (unloading)
            throw new ServletException
              (sm.getString("standardWrapper.unloading", getName()));

        // If not SingleThreadedModel, return the same instance every time
        if (!singleThreadModel) {

            // Load and initialize our instance if necessary
            if (instance == null) {
                synchronized (this) {
                    if (instance == null) {
                        try {
                            instance = loadServlet();
                        } catch (ServletException e) {
                            throw e;
                        } catch (Throwable e) {
                            throw new ServletException
                                (sm.getString("standardWrapper.allocate"), e);
                        }
                    }
                }
            }

            if (!singleThreadModel) {
                if (debug >= 2)
                    log("  Returning non-STM instance");
                countAllocated++;
                return (instance);
            }

        }

        synchronized (instancePool) {

            while (countAllocated >= nInstances) {
                // Allocate a new instance if possible, or else wait
                if (nInstances < maxInstances) {
                    try {
                        instancePool.push(loadServlet());
                        nInstances++;
                    } catch (ServletException e) {
                        throw e;
                    } catch (Throwable e) {
                        throw new ServletException
                            (sm.getString("standardWrapper.allocate"), e);
                    }
                } else {
                    try {
                        instancePool.wait();
                    } catch (InterruptedException e) {
                        ;
                    }
                }
            }
            if (debug >= 2)
                log("  Returning allocated STM instance");
            countAllocated++;
            return (Servlet) instancePool.pop();

        }

    }

而在loadServlet()方法中，首先获取要加载的servlet的完整类名及类加载器，然后通过类加载器加载该servlet类，然后实例化该servlet类，最后调用该servlet实例的init()方法初始化（传入javax.servlet.ServletConfig对象）

public synchronized Servlet loadServlet() throws ServletException {

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

        PrintStream out = System.out;
        SystemLogHandler.startCapture();
        Servlet servlet = null;
        try {
            // If this "servlet" is really a JSP file, get the right class.
            // HOLD YOUR NOSE - this is a kludge that avoids having to do special
            // case Catalina-specific code in Jasper - it also requires that the
            // servlet path be replaced by the <jsp-file> element content in
            // order to be completely effective
            String actualClass = servletClass;
            if ((actualClass == null) && (jspFile != null)) {
                Wrapper jspWrapper = (Wrapper)
                    ((Context) getParent()).findChild(Constants.JSP_SERVLET_NAME);
                if (jspWrapper != null)
                    actualClass = jspWrapper.getServletClass();
            }

            // Complain if no servlet class has been specified
            if (actualClass == null) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.notClass", getName()));
            }

            // Acquire an instance of the class loader to be used
            Loader loader = getLoader();
            if (loader == null) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.missingLoader", getName()));
            }

            ClassLoader classLoader = loader.getClassLoader();

            // Special case class loader for a container provided servlet
            if (isContainerProvidedServlet(actualClass)) {
                classLoader = this.getClass().getClassLoader();
                log(sm.getString
                      ("standardWrapper.containerServlet", getName()));
            }

            // Load the specified servlet class from the appropriate class loader
            Class classClass = null;
            try {
                if (classLoader != null) {
                    System.out.println("Using classLoader.loadClass");
                    classClass = classLoader.loadClass(actualClass);
                } else {
                    System.out.println("Using forName");
                    classClass = Class.forName(actualClass);
                }
            } catch (ClassNotFoundException e) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.missingClass", actualClass),
                     e);
            }
            if (classClass == null) {
                unavailable(null);
                throw new ServletException
                    (sm.getString("standardWrapper.missingClass", actualClass));
            }

            // Instantiate and initialize an instance of the servlet class itself
            try {
                servlet = (Servlet) classClass.newInstance();
            } catch (ClassCastException e) {
                unavailable(null);
                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.notServlet", actualClass), e);
            } catch (Throwable e) {
                unavailable(null);
                // Restore the context ClassLoader
                throw new ServletException
                    (sm.getString("standardWrapper.instantiate", actualClass), e);
            }

            // Check if loading the servlet in this web application should be
            // allowed
            if (!isServletAllowed(servlet)) {
                throw new SecurityException
                    (sm.getString("standardWrapper.privilegedServlet",
                                  actualClass));
            }

            // Special handling for ContainerServlet instances
            if ((servlet instanceof ContainerServlet) &&
                isContainerProvidedServlet(actualClass)) {
System.out.println("calling setWrapper");
                ((ContainerServlet) servlet).setWrapper(this);
System.out.println("after calling setWrapper");
            }

            // Call the initialization method of this servlet
            try {
                instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT,
                                                  servlet);
                servlet.init(facade);
                // Invoke jspInit on JSP pages
                if ((loadOnStartup > 0) && (jspFile != null)) {
                    // Invoking jspInit
                    HttpRequestBase req = new HttpRequestBase();
                    HttpResponseBase res = new HttpResponseBase();
                    req.setServletPath(jspFile);
                    req.setQueryString("jsp_precompile=true");
                    servlet.service(req, res);
                }
                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) {
                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);
            }

            // Register our newly initialized instance
            singleThreadModel = servlet instanceof SingleThreadModel;
            if (singleThreadModel) {
                if (instancePool == null)
                    instancePool = new Stack();
            }
            fireContainerEvent("load", this);
        } finally {
            String log = SystemLogHandler.stopCapture();
            if (log != null && log.length() > 0) {
                if (getServletContext() != null) {
                    getServletContext().log(log);
                } else {
                    out.println(log);
                }
            }
        }
        return servlet;

    }

StandardWrapper类的loadServlet()方法在载入servlet类后，会调用该servlet实例的init()方法，该方法需要传入一个javax.servlet.ServletConfig实例作为参数；那么， StandardWrapper对象是如何获取ServletConfig对象的呢

答案就在StandardWrapper类本身，该类不仅实现了Wrapper接口，还实现了javax.servlet.ServletConfig接口，下面是相关方法实现

/**
     * Return the initialization parameter value for the specified name,
     * if any; otherwise return <code>null</code>.
     *
     * @param name Name of the initialization parameter to retrieve
     */
    public String getInitParameter(String name) {

        return (findInitParameter(name));

    }

    /**
     * Return the set of initialization parameter names defined for this
     * servlet.  If none are defined, an empty Enumeration is returned.
     */
    public Enumeration getInitParameterNames() {

        synchronized (parameters) {
            return (new Enumerator(parameters.keySet()));
        }

    }

    /**
     * Return the servlet context with which this servlet is associated.
     */
    public ServletContext getServletContext() {

        if (parent == null)
            return (null);
        else if (!(parent instanceof Context))
            return (null);
        else
            return (((Context) parent).getServletContext());

    }

    /**
     * Return the name of this servlet.
     */
    public String getServletName() {

        return (getName());

    }

StandardWrapper实例在它的loadServlet()方法里面调用它所载入的servlet类的实例的init()方法，该方法需要一个javax.servlet.ServletConfig类型参数，理论上StandardWrapper对象可以将自身传入init()方法，不过为了类型安全，StandardWrapper类将自身实例包装成StandardWrapperFacade类的一个实例，然后再传给init()方法

public final class StandardWrapperFacade
    implements ServletConfig {

    /**
     * Create a new facede around a StandardWrapper.
     */
    public StandardWrapperFacade(StandardWrapper config) {

        super();
        this.config = (ServletConfig) config;

    }

    /**
     * Wrapped config.
     */
    private ServletConfig config = null;

    public String getServletName() {
        return config.getServletName();
    }

    public ServletContext getServletContext() {
        ServletContext theContext = config.getServletContext();
        if ((theContext != null) &&
            (theContext instanceof ApplicationContext))
            theContext = ((ApplicationContext) theContext).getFacade();
        return (theContext);
    }

    public String getInitParameter(String name) {
        return config.getInitParameter(name);
    }

    public Enumeration getInitParameterNames() {
        return config.getInitParameterNames();
    }

}

StandardWrapperValve类是StandardWrapper实例中的基础阀，要完成两个操作

（1）执行与该servlet相关联的全部过滤器

（2）调用servlet实例的service()方法

public void invoke(Request request, Response response,
                       ValveContext valveContext)
        throws IOException, ServletException {
        // Initialize local variables we may need
        boolean unavailable = false;
        Throwable throwable = null;
        StandardWrapper wrapper = (StandardWrapper) getContainer();
        ServletRequest sreq = request.getRequest();
        ServletResponse sres = response.getResponse();
        Servlet servlet = null;
        HttpServletRequest hreq = null;
        if (sreq instanceof HttpServletRequest)
            hreq = (HttpServletRequest) sreq;
        HttpServletResponse hres = null;
        if (sres instanceof HttpServletResponse)
            hres = (HttpServletResponse) sres;

        // Check for the application being marked unavailable
        if (!((Context) wrapper.getParent()).getAvailable()) {
            hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardContext.isUnavailable"));
            unavailable = true;
        }

        // Check for the servlet being marked unavailable
        if (!unavailable && wrapper.isUnavailable()) {
            log(sm.getString("standardWrapper.isUnavailable",
                             wrapper.getName()));
            if (hres == null) {
                ;       // NOTE - Not much we can do generically
            } else {
                long available = wrapper.getAvailable();
                if ((available > 0L) && (available < Long.MAX_VALUE))
                    hres.setDateHeader("Retry-After", available);
                hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                               sm.getString("standardWrapper.isUnavailable",
                                            wrapper.getName()));
            }
            unavailable = true;
        }

        // Allocate a servlet instance to process this request
        try {
            if (!unavailable) {
                servlet = wrapper.allocate();
            }
        } catch (ServletException e) {
            log(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
            servlet = null;
        } catch (Throwable e) {
            log(sm.getString("standardWrapper.allocateException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
            servlet = null;
        }

        // Acknowlege the request
        try {
            response.sendAcknowledgement();
        } catch (IOException e) {
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            log(sm.getString("standardWrapper.acknowledgeException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            log(sm.getString("standardWrapper.acknowledgeException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
            servlet = null;
        }

        // Create the filter chain for this request
        ApplicationFilterChain filterChain =
            createFilterChain(request, servlet);

        // Call the filter chain for this request
        // NOTE: This also calls the servlet's service() method
        try {
            String jspFile = wrapper.getJspFile();
            if (jspFile != null)
                sreq.setAttribute(Globals.JSP_FILE_ATTR, jspFile);
            else
                sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            if ((servlet != null) && (filterChain != null)) {
                filterChain.doFilter(sreq, sres);
            }
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
        } catch (IOException e) {
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            log(sm.getString("standardWrapper.serviceException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
        } catch (UnavailableException e) {
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            log(sm.getString("standardWrapper.serviceException",
                             wrapper.getName()), e);
            //            throwable = e;
            //            exception(request, response, e);
            wrapper.unavailable(e);
            long available = wrapper.getAvailable();
            if ((available > 0L) && (available < Long.MAX_VALUE))
                hres.setDateHeader("Retry-After", available);
            hres.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE,
                           sm.getString("standardWrapper.isUnavailable",
                                        wrapper.getName()));
            // Do not save exception in 'throwable', because we
            // do not want to do exception(request, response, e) processing
        } catch (ServletException e) {
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            log(sm.getString("standardWrapper.serviceException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
        } catch (Throwable e) {
            sreq.removeAttribute(Globals.JSP_FILE_ATTR);
            log(sm.getString("standardWrapper.serviceException",
                             wrapper.getName()), e);
            throwable = e;
            exception(request, response, e);
        }

        // Release the filter chain (if any) for this request
        try {
            if (filterChain != null)
                filterChain.release();
        } catch (Throwable e) {
            log(sm.getString("standardWrapper.releaseFilters",
                             wrapper.getName()), e);
            if (throwable == null) {
                throwable = e;
                exception(request, response, e);
            }
        }

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

    }

上述创建过滤器链的过程本人后来作了补充：How Tomcat Works（十四）补充

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