public interface Host extends Container {
...
}

public StandardHost() {
        super();
        pipeline.setBasic(new StandardHostValve());
    }

public final void invoke(Request request, Response response)
        throws IOException, ServletException {

        // Select the Context to be used for this Request
        Context context = request.getContext();
        if (context == null) {
            response.sendError
                (HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
                 sm.getString("standardHost.noContext"));
            return;
        }

        // Bind the context CL to the current thread
        if( context.getLoader() != null ) {
            // Not started - it should check for availability first
            // This should eventually move to Engine, it‘s generic.
            if (Globals.IS_SECURITY_ENABLED) {
                PrivilegedAction<Void> pa = new PrivilegedSetTccl(
                        context.getLoader().getClassLoader());
                AccessController.doPrivileged(pa);                
            } else {
                Thread.currentThread().setContextClassLoader
                        (context.getLoader().getClassLoader());
            }
        }
        if (request.isAsyncSupported()) {
            request.setAsyncSupported(context.getPipeline().isAsyncSupported());
        }

        boolean asyncAtStart = request.isAsync(); 
        boolean asyncDispatching = request.isAsyncDispatching();
        if (asyncAtStart || context.fireRequestInitEvent(request)) {

            // Ask this Context to process this request. Requests that are in
            // async mode and are not being dispatched to this resource must be
            // in error and have been routed here to check for application
            // defined error pages.
            try {
                if (!asyncAtStart || asyncDispatching) {
                    context.getPipeline().getFirst().invoke(request, response);
                } else {
                    // Make sure this request/response is here because an error
                    // report is required.
                    if (!response.isErrorReportRequired()) {
                        throw new IllegalStateException(sm.getString("standardHost.asyncStateError"));
                    }
                }
            } catch (Throwable t) {
                ExceptionUtils.handleThrowable(t);
                // If a new error occurred while trying to report a previous
                // error simply log the new error and allow the original error
                // to be reported.
                if (response.isErrorReportRequired()) {
                    container.getLogger().error("Exception Processing " +
                            request.getRequestURI(), t);
                } else {
                    request.setAttribute(RequestDispatcher.ERROR_EXCEPTION, t);
                    throwable(request, response, t);
                }
            }

            // Now that the request/response pair is back under container
            // control lift the suspension so that the error handling can
            // complete and/or the container can flush any remaining data
            response.setSuspended(false);

            Throwable t = (Throwable) request.getAttribute(RequestDispatcher.ERROR_EXCEPTION);

            // Protect against NPEs if the context was destroyed during a
            // long running request.
            if (!context.getState().isAvailable()) {
                return;
            }

            // Look for (and render if found) an application level error page
            if (response.isErrorReportRequired()) {
                if (t != null) {
                    throwable(request, response, t);
                } else {
                    status(request, response);
                }
            }

            if (!request.isAsync() && (!asyncAtStart || !response.isErrorReportRequired())) {
                context.fireRequestDestroyEvent(request);
            }
        }

        // Access a session (if present) to update last accessed time, based on a
        // strict interpretation of the specification
        if (ACCESS_SESSION) {
            request.getSession(false);
        }

        // Restore the context classloader
        if (Globals.IS_SECURITY_ENABLED) {
            PrivilegedAction<Void> pa = new PrivilegedSetTccl(
                    StandardHostValve.class.getClassLoader());
            AccessController.doPrivileged(pa);                
        } else {
            Thread.currentThread().setContextClassLoader
                    (StandardHostValve.class.getClassLoader());
        }
    }

/**
     * Identify the application web.xml to be used and obtain an input source
     * for it.
     */
    protected InputSource getContextWebXmlSource() {
        InputStream stream = null;
        InputSource source = null;
        URL url = null;

        String altDDName = null;

        // Open the application web.xml file, if it exists
        ServletContext servletContext = context.getServletContext();
        if (servletContext != null) {
            altDDName = (String)servletContext.getAttribute(
                                                        Globals.ALT_DD_ATTR);
            if (altDDName != null) {
                try {
                    stream = new FileInputStream(altDDName);
                    url = new File(altDDName).toURI().toURL();
                } catch (FileNotFoundException e) {
                    log.error(sm.getString("contextConfig.altDDNotFound",
                                           altDDName));
                } catch (MalformedURLException e) {
                    log.error(sm.getString("contextConfig.applicationUrl"));
                }
            }
            else {
                stream = servletContext.getResourceAsStream
                    (Constants.ApplicationWebXml);
                try {
                    url = servletContext.getResource(
                            Constants.ApplicationWebXml);
                } catch (MalformedURLException e) {
                    log.error(sm.getString("contextConfig.applicationUrl"));
                }
            }
        }
        if (stream == null || url == null) {
            if (log.isDebugEnabled()) {
                log.debug(sm.getString("contextConfig.applicationMissing") + " " + context);
            }
        } else {
            source = new InputSource(url.toExternalForm());
            source.setByteStream(stream);
        }
        return source;
    }
   //常量类
    public final class Constants{
    ...
    public static final String ApplicationWebXml = "/WEB-INF/web.xml";
    ...
}