Tomcat源码解读『web.xml解析』

上篇文章我们介绍了Tomcat Context是如何构建的，了解了Context构建的两种方式：

• Host启动，触发HostConfig的Lifecycle.START_EVENT事件监听，构建Context
• 后台线程，定期去执行Host的backgroundProcess方法，触发HostConfig的Lifecycle.PERIODIC_EVENT事件监听，构建Context

这里的构建Context，指的是构建一个Context对象，并将构建好的Context对象与Host组件关联起来（调用host.addChild(context)。

但如果容器只是构建了Context容器，是无法响应一个浏览器的一次请求。就web服务器的实现来看，一次请求过来除了需要确定这次请求访问的web应用具体所对应的Context对象（根据访问的path确定），还需要包含web应用中具体的哪个Servlet来处理这次请求，中间是否还需要执行相应的过滤器(filter)、监听器(listener)等。我们知道，这些servlet、filter和listener信息，是配置在一个web应用的WEB-INF\web.xml文件中的。

所以可以推测，Tomcat启动过程伴随着Context构建之后，必然存在一个web.xml的解析过程。本篇文章我们就来介绍一下web.xml的解析过程。

1. Context启动

上篇文章在介绍Context构建的时候提到，在HostConfig的deployApps方法中，可以通过三种方式构建Context，如下：

protected void deployApps() {

    File appBase = host.getAppBaseFile();
    File configBase = host.getConfigBaseFile();
    String[] filteredAppPaths = filterAppPaths(appBase.list());
    // Deploy XML descriptors from configBase
    deployDescriptors(configBase, configBase.list());
    // Deploy WARs
    deployWARs(appBase, filteredAppPaths);
    // Deploy expanded folders
    deployDirectories(appBase, filteredAppPaths);

}

在这三种方式中，基本思想都是一样的，构建Context对象，然后调用Host的addChild方法，将构建的Context对象添加到Host。在上述三种构建方式中，在构建Context时，都会调用如下一段代码：

Class<?> clazz = Class.forName(host.getConfigClass());
LifecycleListener listener = (LifecycleListener) clazz.getConstructor().newInstance();
context.addLifecycleListener(listener);

/**
 * The Java class name of the default context configuration class
 * for deployed web applications.
 */
private String configClass =
    "org.apache.catalina.startup.ContextConfig";

同时我们也提到，Context的启动（start方法调用）就是在host的addChild方法中调用的。所以可以得出以下两点结论：

• Context构建过程中，会为Context添加一个生命周期监听器ContextConfig
• Context会在构建成功后，添加到Host组件过程中，启动（start方法调用）

而web.xml的解析就是通过ContextConfig生命周期监听器完成的。

2. ContextConfig

关于Context的启动过程，我们这里不做详细介绍，我们只需要知道最终会调用到standardContext的startInternal方法，在startInternal方法中，会触发生命周期监听器的Lifecycle.CONFIGURE_START_EVENT事件监听，如下：

// Notify our interested LifecycleListeners
fireLifecycleEvent(Lifecycle.CONFIGURE_START_EVENT, null);

关于fireLifecycleEvent的触发机制我们上篇文章介绍HostConfig时，已经介绍过，这里不重复介绍了。这里肯定会触发ContextConfig的事件监听如下：

public void lifecycleEvent(LifecycleEvent event) {

    // Identify the context we are associated with
    try {
        context = (Context) event.getLifecycle();
    } catch (ClassCastException e) {
        log.error(sm.getString("contextConfig.cce", event.getLifecycle()), e);
        return;
    }

    // Process the event that has occurred
    if (event.getType().equals(Lifecycle.CONFIGURE_START_EVENT)) {
        configureStart();
    } else if (event.getType().equals(Lifecycle.BEFORE_START_EVENT)) {
        beforeStart();
    } else if (event.getType().equals(Lifecycle.AFTER_START_EVENT)) {
        // Restore docBase for management tools
        if (originalDocBase != null) {
            context.setDocBase(originalDocBase);
        }
    } else if (event.getType().equals(Lifecycle.CONFIGURE_STOP_EVENT)) {
        configureStop();
    } else if (event.getType().equals(Lifecycle.AFTER_INIT_EVENT)) {
        init();
    } else if (event.getType().equals(Lifecycle.AFTER_DESTROY_EVENT)) {
        destroy();
    }

}

所以会调用ContextConfig的configureStart()方法。

2.1 configureStart()

/**
 * Process a "contextConfig" event for this Context.
 */
protected synchronized void configureStart() {
    // Called from StandardContext.start()

    if (log.isDebugEnabled()) {
        log.debug(sm.getString("contextConfig.start"));
    }

    if (log.isDebugEnabled()) {
        log.debug(sm.getString("contextConfig.xmlSettings",
                context.getName(),
                Boolean.valueOf(context.getXmlValidation()),
                Boolean.valueOf(context.getXmlNamespaceAware())));
    }

    webConfig();

    if (!context.getIgnoreAnnotations()) {
        applicationAnnotationsConfig();
    }
    if (ok) {
        validateSecurityRoles();
    }

    // Configure an authenticator if we need one
    if (ok) {
        authenticatorConfig();
    }

    // Dump the contents of this pipeline if requested
    if (log.isDebugEnabled()) {
        log.debug("Pipeline Configuration:");
        Pipeline pipeline = context.getPipeline();
        Valve valves[] = null;
        if (pipeline != null) {
            valves = pipeline.getValves();
        }
        if (valves != null) {
            for (Valve valve : valves) {
                log.debug("  " + valve.getClass().getName());
            }
        }
        log.debug("======================");
    }

    // Make our application available if no problems were encountered
    if (ok) {
        context.setConfigured(true);
    } else {
        log.error(sm.getString("contextConfig.unavailable"));
        context.setConfigured(false);
    }

}

不难发现，核心逻辑肯定在webConfig中。

2.2 webConfig()

/**
 * Scan the web.xml files that apply to the web application and merge them
 * using the rules defined in the spec. For the global web.xml files,
 * where there is duplicate configuration, the most specific level wins. ie
 * an application's web.xml takes precedence over the host level or global
 * web.xml file.
 */
protected void webConfig() {
    /*
     * Anything and everything can override the global and host defaults.
     * This is implemented in two parts
     * - Handle as a web fragment that gets added after everything else so
     *   everything else takes priority
     * - Mark Servlets as overridable so SCI configuration can replace
     *   configuration from the defaults
     */

    /*
     * The rules for annotation scanning are not as clear-cut as one might
     * think. Tomcat implements the following process:
     * - As per SRV.1.6.2, Tomcat will scan for annotations regardless of
     *   which Servlet spec version is declared in web.xml. The EG has
     *   confirmed this is the expected behaviour.
     * - As per http://java.net/jira/browse/SERVLET_SPEC-36, if the main
     *   web.xml is marked as metadata-complete, JARs are still processed
     *   for SCIs.
     * - If metadata-complete=true and an absolute ordering is specified,
     *   JARs excluded from the ordering are also excluded from the SCI
     *   processing.
     * - If an SCI has a @HandlesType annotation then all classes (except
     *   those in JARs excluded from an absolute ordering) need to be
     *   scanned to check if they match.
     */
    WebXmlParser webXmlParser = new WebXmlParser(context.getXmlNamespaceAware(),
            context.getXmlValidation(), context.getXmlBlockExternal());

    Set<WebXml> defaults = new HashSet<>();
    defaults.add(getDefaultWebXmlFragment(webXmlParser));

    Set<WebXml> tomcatWebXml = new HashSet<>();
    tomcatWebXml.add(getTomcatWebXmlFragment(webXmlParser));

    WebXml webXml = createWebXml();

    // Parse context level web.xml
    InputSource contextWebXml = getContextWebXmlSource();
    if (!webXmlParser.parseWebXml(contextWebXml, webXml, false)) {
        ok = false;
    }

    ServletContext sContext = context.getServletContext();

    // Ordering is important here

    // Step 1. Identify all the JARs packaged with the application and those
    // provided by the container. If any of the application JARs have a
    // web-fragment.xml it will be parsed at this point. web-fragment.xml
    // files are ignored for container provided JARs.
    Map<String,WebXml> fragments = processJarsForWebFragments(webXml, webXmlParser);

    // Step 2. Order the fragments.
    Set<WebXml> orderedFragments = null;
    orderedFragments =
            WebXml.orderWebFragments(webXml, fragments, sContext);

    // Step 3. Look for ServletContainerInitializer implementations
    if (ok) {
        processServletContainerInitializers();
    }

    if  (!webXml.isMetadataComplete() || typeInitializerMap.size() > 0) {
        // Steps 4 & 5.
        processClasses(webXml, orderedFragments);
    }

    if (!webXml.isMetadataComplete()) {
        // Step 6. Merge web-fragment.xml files into the main web.xml
        // file.
        if (ok) {
            ok = webXml.merge(orderedFragments);
        }

        // Step 7a
        // merge tomcat-web.xml
        webXml.merge(tomcatWebXml);

        // Step 7b. Apply global defaults
        // Have to merge defaults before JSP conversion since defaults
        // provide JSP servlet definition.
        webXml.merge(defaults);

        // Step 8. Convert explicitly mentioned jsps to servlets
        if (ok) {
            convertJsps(webXml);
        }
    
        // Step 9. Apply merged web.xml to Context
        if (ok) {
            configureContext(webXml);
        }
    } else {
        webXml.merge(tomcatWebXml);
        webXml.merge(defaults);
        convertJsps(webXml);
        configureContext(webXml);
    }

    if (context.getLogEffectiveWebXml()) {
        log.info(sm.getString("contextConfig.effectiveWebXml", webXml.toXml()));
    }

    // Always need to look for static resources
    // Step 10. Look for static resources packaged in JARs
    if (ok) {
        // Spec does not define an order.
        // Use ordered JARs followed by remaining JARs
        Set<WebXml> resourceJars = new LinkedHashSet<>(orderedFragments);
        for (WebXml fragment : fragments.values()) {
            if (!resourceJars.contains(fragment)) {
                resourceJars.add(fragment);
            }
        }
        processResourceJARs(resourceJars);
        // See also StandardContext.resourcesStart() for
        // WEB-INF/classes/META-INF/resources configuration
    }

    // Step 11. Apply the ServletContainerInitializer config to the
    // context
    if (ok) {
        for (Map.Entry<ServletContainerInitializer,
                Set<Class<?>>> entry :
                    initializerClassMap.entrySet()) {
            if (entry.getValue().isEmpty()) {
                context.addServletContainerInitializer(
                        entry.getKey(), null);
            } else {
                context.addServletContainerInitializer(
                        entry.getKey(), entry.getValue());
            }
        }
    }
}

这个方法的具体逻辑，在英文注释中说的很清楚了，概括起来包括以下两点：

• 合并Tomcat全局web.xml 、当前应用中的web.xml 、web-fragment.xml和web应用的注解中的配置信息
• 将解析出的各种配置信息（如Servlet配置、Filter配置、Listener配置等）关联到Context对象中

而上面提到的第二点，是通过configureContext方法完成的。

2.3 configureContext

private void configureContext(WebXml webxml) {
    // As far as possible, process in alphabetical order so it is easy to
    // check everything is present
    // Some validation depends on correct public ID
    context.setPublicId(webxml.getPublicId());

    // Everything else in order
    context.setEffectiveMajorVersion(webxml.getMajorVersion());
    context.setEffectiveMinorVersion(webxml.getMinorVersion());

    for (Entry<String, String> entry : webxml.getContextParams().entrySet()) {
        context.addParameter(entry.getKey(), entry.getValue());
    }
    context.setDenyUncoveredHttpMethods(
            webxml.getDenyUncoveredHttpMethods());
    context.setDisplayName(webxml.getDisplayName());
    context.setDistributable(webxml.isDistributable());
    for (ContextLocalEjb ejbLocalRef : webxml.getEjbLocalRefs().values()) {
        context.getNamingResources().addLocalEjb(ejbLocalRef);
    }
    for (ContextEjb ejbRef : webxml.getEjbRefs().values()) {
        context.getNamingResources().addEjb(ejbRef);
    }
    for (ContextEnvironment environment : webxml.getEnvEntries().values()) {
        context.getNamingResources().addEnvironment(environment);
    }
    for (ErrorPage errorPage : webxml.getErrorPages().values()) {
        context.addErrorPage(errorPage);
    }
    for (FilterDef filter : webxml.getFilters().values()) {
        if (filter.getAsyncSupported() == null) {
            filter.setAsyncSupported("false");
        }
        context.addFilterDef(filter);
    }
    for (FilterMap filterMap : webxml.getFilterMappings()) {
        context.addFilterMap(filterMap);
    }
    context.setJspConfigDescriptor(webxml.getJspConfigDescriptor());
    for (String listener : webxml.getListeners()) {
        context.addApplicationListener(listener);
    }
    for (Entry<String, String> entry :
            webxml.getLocaleEncodingMappings().entrySet()) {
        context.addLocaleEncodingMappingParameter(entry.getKey(),
                entry.getValue());
    }
    // Prevents IAE
    if (webxml.getLoginConfig() != null) {
        context.setLoginConfig(webxml.getLoginConfig());
    }
    for (MessageDestinationRef mdr :
            webxml.getMessageDestinationRefs().values()) {
        context.getNamingResources().addMessageDestinationRef(mdr);
    }

    // messageDestinations were ignored in Tomcat 6, so ignore here

    context.setIgnoreAnnotations(webxml.isMetadataComplete());
    for (Entry<String, String> entry :
            webxml.getMimeMappings().entrySet()) {
        context.addMimeMapping(entry.getKey(), entry.getValue());
    }
    context.setRequestCharacterEncoding(webxml.getRequestCharacterEncoding());
    // Name is just used for ordering
    for (ContextResourceEnvRef resource :
            webxml.getResourceEnvRefs().values()) {
        context.getNamingResources().addResourceEnvRef(resource);
    }
    for (ContextResource resource : webxml.getResourceRefs().values()) {
        context.getNamingResources().addResource(resource);
    }
    context.setResponseCharacterEncoding(webxml.getResponseCharacterEncoding());
    boolean allAuthenticatedUsersIsAppRole =
            webxml.getSecurityRoles().contains(
                    SecurityConstraint.ROLE_ALL_AUTHENTICATED_USERS);
    for (SecurityConstraint constraint : webxml.getSecurityConstraints()) {
        if (allAuthenticatedUsersIsAppRole) {
            constraint.treatAllAuthenticatedUsersAsApplicationRole();
        }
        context.addConstraint(constraint);
    }
    for (String role : webxml.getSecurityRoles()) {
        context.addSecurityRole(role);
    }
    for (ContextService service : webxml.getServiceRefs().values()) {
        context.getNamingResources().addService(service);
    }
    for (ServletDef servlet : webxml.getServlets().values()) {
        Wrapper wrapper = context.createWrapper();
        // Description is ignored
        // Display name is ignored
        // Icons are ignored

        // jsp-file gets passed to the JSP Servlet as an init-param

        if (servlet.getLoadOnStartup() != null) {
            wrapper.setLoadOnStartup(servlet.getLoadOnStartup().intValue());
        }
        if (servlet.getEnabled() != null) {
            wrapper.setEnabled(servlet.getEnabled().booleanValue());
        }
        wrapper.setName(servlet.getServletName());
        Map<String,String> params = servlet.getParameterMap();
        for (Entry<String, String> entry : params.entrySet()) {
            wrapper.addInitParameter(entry.getKey(), entry.getValue());
        }
        wrapper.setRunAs(servlet.getRunAs());
        Set<SecurityRoleRef> roleRefs = servlet.getSecurityRoleRefs();
        for (SecurityRoleRef roleRef : roleRefs) {
            wrapper.addSecurityReference(
                    roleRef.getName(), roleRef.getLink());
        }
        wrapper.setServletClass(servlet.getServletClass());
        MultipartDef multipartdef = servlet.getMultipartDef();
        if (multipartdef != null) {
            long maxFileSize = -1;
            long maxRequestSize = -1;
            int fileSizeThreshold = 0;

            if(null != multipartdef.getMaxFileSize()) {
                maxFileSize = Long.parseLong(multipartdef.getMaxFileSize());
            }
            if(null != multipartdef.getMaxRequestSize()) {
                maxRequestSize = Long.parseLong(multipartdef.getMaxRequestSize());
            }
            if(null != multipartdef.getFileSizeThreshold()) {
                fileSizeThreshold = Integer.parseInt(multipartdef.getFileSizeThreshold());
            }

            wrapper.setMultipartConfigElement(new MultipartConfigElement(
                    multipartdef.getLocation(),
                    maxFileSize,
                    maxRequestSize,
                    fileSizeThreshold));
        }
        if (servlet.getAsyncSupported() != null) {
            wrapper.setAsyncSupported(
                    servlet.getAsyncSupported().booleanValue());
        }
        wrapper.setOverridable(servlet.isOverridable());
        context.addChild(wrapper);
    }
    for (Entry<String, String> entry :
            webxml.getServletMappings().entrySet()) {
        context.addServletMappingDecoded(entry.getKey(), entry.getValue());
    }
    SessionConfig sessionConfig = webxml.getSessionConfig();
    if (sessionConfig != null) {
        if (sessionConfig.getSessionTimeout() != null) {
            context.setSessionTimeout(
                    sessionConfig.getSessionTimeout().intValue());
        }
        SessionCookieConfig scc =
            context.getServletContext().getSessionCookieConfig();
        scc.setName(sessionConfig.getCookieName());
        scc.setDomain(sessionConfig.getCookieDomain());
        scc.setPath(sessionConfig.getCookiePath());
        scc.setComment(sessionConfig.getCookieComment());
        if (sessionConfig.getCookieHttpOnly() != null) {
            scc.setHttpOnly(sessionConfig.getCookieHttpOnly().booleanValue());
        }
        if (sessionConfig.getCookieSecure() != null) {
            scc.setSecure(sessionConfig.getCookieSecure().booleanValue());
        }
        if (sessionConfig.getCookieMaxAge() != null) {
            scc.setMaxAge(sessionConfig.getCookieMaxAge().intValue());
        }
        if (sessionConfig.getSessionTrackingModes().size() > 0) {
            context.getServletContext().setSessionTrackingModes(
                    sessionConfig.getSessionTrackingModes());
        }
    }

    // Context doesn't use version directly

    for (String welcomeFile : webxml.getWelcomeFiles()) {
        /*
         * The following will result in a welcome file of "" so don't add
         * that to the context
         * <welcome-file-list>
         *   <welcome-file/>
         * </welcome-file-list>
         */
        if (welcomeFile != null && welcomeFile.length() > 0) {
            context.addWelcomeFile(welcomeFile);
        }
    }

    // Do this last as it depends on servlets
    for (JspPropertyGroup jspPropertyGroup :
            webxml.getJspPropertyGroups()) {
        String jspServletName = context.findServletMapping("*.jsp");
        if (jspServletName == null) {
            jspServletName = "jsp";
        }
        if (context.findChild(jspServletName) != null) {
            for (String urlPattern : jspPropertyGroup.getUrlPatterns()) {
                context.addServletMappingDecoded(urlPattern, jspServletName, true);
            }
        } else {
            if(log.isDebugEnabled()) {
                for (String urlPattern : jspPropertyGroup.getUrlPatterns()) {
                    log.debug("Skipping " + urlPattern + " , no servlet " +
                            jspServletName);
                }
            }
        }
    }

    for (Entry<String, String> entry :
            webxml.getPostConstructMethods().entrySet()) {
        context.addPostConstructMethod(entry.getKey(), entry.getValue());
    }

    for (Entry<String, String> entry :
        webxml.getPreDestroyMethods().entrySet()) {
        context.addPreDestroyMethod(entry.getKey(), entry.getValue());
    }
}

这里逻辑就比较清晰了，就是调用Context的add方法、set方法配置Context，其中我们比较熟悉的，比如添加Filter：

for (FilterDef filter : webxml.getFilters().values()) {
    if (filter.getAsyncSupported() == null) {
        filter.setAsyncSupported("false");
    }
    context.addFilterDef(filter);
}

for (FilterMap filterMap : webxml.getFilterMappings()) {
    context.addFilterMap(filterMap);
}

添加Listener：

for (String listener : webxml.getListeners()) {
    context.addApplicationListener(listener);
}

添加servlet wrapper：

for (ServletDef servlet : webxml.getServlets().values()) {
    Wrapper wrapper = context.createWrapper();
    // Description is ignored
    // Display name is ignored
    // Icons are ignored

    // jsp-file gets passed to the JSP Servlet as an init-param

    if (servlet.getLoadOnStartup() != null) {
        wrapper.setLoadOnStartup(servlet.getLoadOnStartup().intValue());
    }
    if (servlet.getEnabled() != null) {
        wrapper.setEnabled(servlet.getEnabled().booleanValue());
    }
    wrapper.setName(servlet.getServletName());
    Map<String,String> params = servlet.getParameterMap();
    for (Entry<String, String> entry : params.entrySet()) {
        wrapper.addInitParameter(entry.getKey(), entry.getValue());
    }
    wrapper.setRunAs(servlet.getRunAs());
    Set<SecurityRoleRef> roleRefs = servlet.getSecurityRoleRefs();
    for (SecurityRoleRef roleRef : roleRefs) {
        wrapper.addSecurityReference(
                roleRef.getName(), roleRef.getLink());
    }
    wrapper.setServletClass(servlet.getServletClass());
    MultipartDef multipartdef = servlet.getMultipartDef();
    if (multipartdef != null) {
        long maxFileSize = -1;
        long maxRequestSize = -1;
        int fileSizeThreshold = 0;

        if(null != multipartdef.getMaxFileSize()) {
            maxFileSize = Long.parseLong(multipartdef.getMaxFileSize());
        }
        if(null != multipartdef.getMaxRequestSize()) {
            maxRequestSize = Long.parseLong(multipartdef.getMaxRequestSize());
        }
        if(null != multipartdef.getFileSizeThreshold()) {
            fileSizeThreshold = Integer.parseInt(multipartdef.getFileSizeThreshold());
        }

        wrapper.setMultipartConfigElement(new MultipartConfigElement(
                multipartdef.getLocation(),
                maxFileSize,
                maxRequestSize,
                fileSizeThreshold));
    }
    if (servlet.getAsyncSupported() != null) {
        wrapper.setAsyncSupported(
                servlet.getAsyncSupported().booleanValue());
    }
    wrapper.setOverridable(servlet.isOverridable());
    context.addChild(wrapper);
}

通过ContextConfig，实现了web.xml的解析，及与Context的关联。

参考链接：

1. Tomcat源码