Tomcat9如何加載server.xml

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1.Tomcat啟動

org.apache.catalina.startup.Bootstrap.main(String args[])

 public static void main(String args[]) {
        synchronized (daemonLock) {
            if (daemon == null) {
                Bootstrap bootstrap = new Bootstrap();
                try {
                    bootstrap.init(); //初始化類加載器
                } catch (Throwable t) {
                    handleThrowable(t);
                    t.printStackTrace();
                    return;
                }
                daemon = bootstrap;
            } else {
                Thread.currentThread().setContextClassLoader(daemon.catalinaLoader);
            }
        }
        //根據傳入的不同指令，進行相應處理
        try {
            String command = "start";
            if (args.length > 0) {
                command = args[args.length - 1];
            }
            if (command.equals("startd")) {
                args[args.length - 1] = "start";
                daemon.load(args);
                daemon.start();
            } else if (command.equals("stopd")) {
                args[args.length - 1] = "stop";
                daemon.stop();
            } else if (command.equals("start")) {
                daemon.setAwait(true);
                daemon.load(args);
                daemon.start();
                if (null == daemon.getServer()) {
                    System.exit(1);
                }
            } else if (command.equals("stop")) {
                daemon.stopServer(args);
            } else if (command.equals("configtest")) {
                daemon.load(args);
                if (null == daemon.getServer()) {
                    System.exit(1);
                }
                System.exit(0);
            } else {
                log.warn("Bootstrap: command \"" + command + "\" does not exist.");
            }
        } catch (Throwable t) {
            if (t instanceof InvocationTargetException &&
                    t.getCause() != null) {
                t = t.getCause();
            }
            handleThrowable(t);
            t.printStackTrace();
            System.exit(1);
        }
    }

在main方法中主要為兩部分邏輯：

調用bootstrap.init()進行初始化
根據傳入不同的指令進行相應的處理，本文主要分析start指定，即服務啟動。啟動服務start主要調用了org.apache.catalina.startup.Catalina.load()和start()方法

org.apache.catalina.startup.Bootstrap.init()

 public void init() throws Exception {
        initClassLoaders(); //初始化類加載
        Thread.currentThread().setContextClassLoader(catalinaLoader); //設置當前線程的類加載器為catalinaLoader
        SecurityClassLoad.securityClassLoad(catalinaLoader); //啟用java安全管理的處理
        //通過反射的方式實例化org.apache.catalina.startup.Catalina，并設置父類加載器為sharedLoader
        if (log.isDebugEnabled())
            log.debug("Loading startup class");
        Class startupClass = catalinaLoader.loadClass("org.apache.catalina.startup.Catalina");
        Object startupInstance = startupClass.getConstructor().newInstance();
        if (log.isDebugEnabled())
            log.debug("Setting startup class properties");
        String methodName = "setParentClassLoader";
        Class paramTypes[] = new Class[1];
        paramTypes[0] = Class.forName("java.lang.ClassLoader");
        Object paramValues[] = new Object[1];
        paramValues[0] = sharedLoader;
        Method method =
            startupInstance.getClass().getMethod(methodName, paramTypes);
        method.invoke(startupInstance, paramValues);
        catalinaDaemon = startupInstance;
    }

類加載器的初始化，創建commonLoader、catalinaLoader、sharedLoader，具體可參考上一篇《Tomcat9源代碼淺析-類加載體系》
啟用java安全管理的處理
通過反射的方式實例化org.apache.catalina.startup.Catalina，并設置父類加載器為sharedLoader

org.apache.catalina.security.SecurityClassLoad

public final class SecurityClassLoad {
    public static void securityClassLoad(ClassLoader loader) throws Exception {
        securityClassLoad(loader, true);
    }
    static void securityClassLoad(ClassLoader loader, boolean requireSecurityManager) throws Exception {
        if (requireSecurityManager && System.getSecurityManager() == null) {
            return;
        }
        loadCorePackage(loader);
        loadCoyotePackage(loader);
        loadLoaderPackage(loader);
        loadRealmPackage(loader);
        loadServletsPackage(loader);
        loadSessionPackage(loader);
        loadUtilPackage(loader);
        loadJavaxPackage(loader);
        loadConnectorPackage(loader);
        loadTomcatPackage(loader);
    }

當時使用Java SecurityManager時，會提前加載一些必要的java類，以避免觸發權限異常AccessControlException

2.server.xml解析框架

2.1 SAX

Tomcat中使用SAX解析server.xml文件。SAX解析方式會逐行的解析XML文檔，當遇到標簽時會觸發解析處理器，采用事件處理的方式解析XML，它的優點是不需要將完整的XML文檔加載進內存，可以在讀取文檔的同時就進行解析，節省內存，適合解析超大XML，主要方法有：

startDocument()：文檔解析開始時調用，該方法只會調用一次
startElement(String uri, String localName, String qName, Attributes attributes):標簽解析開始時調用
endElement(String uri, String localName, String qName)：標簽（節點）解析結束后調用
endDocument()：文檔解析結束后調用，該方法只會調用一次

2.2 規則Rules

Tomcat9如何加載server.xml
Tomcat將server.xml的解析抽象為規則，利用Java的引用傳遞，通過有副作用的void方法，對xml進行解析，規則調用的順序與xml解析的順序是一致的，即start方法是正序，end方法是逆序。
規則中包含以下方法：

begin：Degister.startElement 方法調用
body、end：Degister.endElement方法中調用，先調用body，再調用end
finish：Degister.endDocument方法中調用

Tomcat中常見的規則類型：

ObjectCreateRule 創建對應class的對象實例，并放到Designer的堆棧成員屬性中
SetPropertiesRule 獲取堆棧中棧頂的元素，并將xml元素的屬性賦值給對象實例
SetNextRule 調用父節點的實例對象，將當前對象作為參數，反射調用某個方法
ListenerCreateRule 當Listener標簽有optional屬性為true時，創建實例異常時，強制添加OptionalListener實例
ConnectorCreateRule 創建Connector實例
SetAllPropertiesRule 主體功能與SetPropertiesRule 一致，這個Rule可以排除一些屬性的設置
AddPortOffsetRule Set portOffset on all the connectors based on portOffset in the Server
CertificateCreateRule 實例化SSLHostConfigCertificate

3.server.xml解析源代碼解析

org.apache.catalina.startup.Catalina.load()

public void load() {
        if (loaded) {
            return;
        }
        loaded = true;
        long t1 = System.nanoTime();
        initDirs();
        // Before digester - it may be needed
        initNaming();
        // 讀取conf/server.xml
        ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(Bootstrap.getCatalinaBaseFile(), getConfigFile()));
        File file = configFile();
        // 創建xml解析Digester 
        Digester digester = createStartDigester();
        try (ConfigurationSource.Resource resource = ConfigFileLoader.getSource().getServerXml()) {
            InputStream inputStream = resource.getInputStream();
            InputSource inputSource = new InputSource(resource.getURI().toURL().toString());
            inputSource.setByteStream(inputStream);
            digester.push(this);
            digester.parse(inputSource); //解析xml
        } catch (Exception e) {
            log.warn(sm.getString("catalina.configFail", file.getAbsolutePath()), e);
            if (file.exists() && !file.canRead()) {
                log.warn(sm.getString("catalina.incorrectPermissions"));
            }
            return;
        }
        //設置server的屬性
        getServer().setCatalina(this);
        getServer().setCatalinaHome(Bootstrap.getCatalinaHomeFile());
        getServer().setCatalinaBase(Bootstrap.getCatalinaBaseFile());
        // Stream redirection
        initStreams();
        // 初始化server
        try {
            getServer().init();
        } catch (LifecycleException e) {
            if (Boolean.getBoolean("org.apache.catalina.startup.EXIT_ON_INIT_FAILURE")) {
                throw new java.lang.Error(e);
            } else {
                log.error(sm.getString("catalina.initError"), e);
            }
        }
        long t2 = System.nanoTime();
        if(log.isInfoEnabled()) {
            log.info(sm.getString("catalina.init", Long.valueOf((t2 - t1) / 1000000)));
        }
    }

Bootstrap中start指令邏輯，通過反射調用Catalina.load()
Catalina.load() 讀取conf/server.xml，創建解析xml的Digester
開始初始化server

org.apache.catalina.startup.Catalina.createStartDigester()

 protected Digester createStartDigester() {
        long t1=System.currentTimeMillis();
        // Initialize the digester
        Digester digester = new Digester();
        digester.setValidating(false);
        digester.setRulesValidation(true);
        Map, List> fakeAttributes = new HashMap<>();
        // Ignore className on all elements
        List objectAttrs = new ArrayList<>();
        objectAttrs.add("className");
        fakeAttributes.put(Object.class, objectAttrs);
        // Ignore attribute added by Eclipse for its internal tracking
        List contextAttrs = new ArrayList<>();
        contextAttrs.add("source");
        fakeAttributes.put(StandardContext.class, contextAttrs);
        // Ignore Connector attribute used internally but set on Server
        List connectorAttrs = new ArrayList<>();
        connectorAttrs.add("portOffset");
        fakeAttributes.put(Connector.class, connectorAttrs);
        digester.setFakeAttributes(fakeAttributes);
        digester.setUseContextClassLoader(true);
        // Configure the actions we will be using
        digester.addObjectCreate("Server",
                                 "org.apache.catalina.core.StandardServer",
                                 "className");
        digester.addSetProperties("Server");
        digester.addSetNext("Server",
                            "setServer",
                            "org.apache.catalina.Server");
        digester.addObjectCreate("Server/GlobalNamingResources",
                                 "org.apache.catalina.deploy.NamingResourcesImpl");
        digester.addSetProperties("Server/GlobalNamingResources");
        digester.addSetNext("Server/GlobalNamingResources",
                            "setGlobalNamingResources",
                            "org.apache.catalina.deploy.NamingResourcesImpl");
        digester.addRule("Server/Listener",
                new ListenerCreateRule(null, "className"));
        digester.addSetProperties("Server/Listener");
        digester.addSetNext("Server/Listener",
                            "addLifecycleListener",
                            "org.apache.catalina.LifecycleListener");
        digester.addObjectCreate("Server/Service",
                                 "org.apache.catalina.core.StandardService",
                                 "className");
        digester.addSetProperties("Server/Service");
        digester.addSetNext("Server/Service",
                            "addService",
                            "org.apache.catalina.Service");
        digester.addObjectCreate("Server/Service/Listener",
                                 null, // MUST be specified in the element
                                 "className");
        digester.addSetProperties("Server/Service/Listener");
        digester.addSetNext("Server/Service/Listener",
                            "addLifecycleListener",
                            "org.apache.catalina.LifecycleListener");
        //Executor
        digester.addObjectCreate("Server/Service/Executor",
                         "org.apache.catalina.core.StandardThreadExecutor",
                         "className");
        digester.addSetProperties("Server/Service/Executor");
        digester.addSetNext("Server/Service/Executor",
                            "addExecutor",
                            "org.apache.catalina.Executor");
        digester.addRule("Server/Service/Connector",
                         new ConnectorCreateRule());
        digester.addRule("Server/Service/Connector", new SetAllPropertiesRule(
                new String[]{"executor", "sslImplementationName", "protocol"}));
        digester.addSetNext("Server/Service/Connector",
                            "addConnector",
                            "org.apache.catalina.connector.Connector");
        digester.addRule("Server/Service/Connector", new AddPortOffsetRule());
        digester.addObjectCreate("Server/Service/Connector/SSLHostConfig",
                                 "org.apache.tomcat.util.net.SSLHostConfig");
        digester.addSetProperties("Server/Service/Connector/SSLHostConfig");
        digester.addSetNext("Server/Service/Connector/SSLHostConfig",
                "addSslHostConfig",
                "org.apache.tomcat.util.net.SSLHostConfig");
        digester.addRule("Server/Service/Connector/SSLHostConfig/Certificate",
                         new CertificateCreateRule());
        digester.addRule("Server/Service/Connector/SSLHostConfig/Certificate",
                         new SetAllPropertiesRule(new String[]{"type"}));
        digester.addSetNext("Server/Service/Connector/SSLHostConfig/Certificate",
                            "addCertificate",
                            "org.apache.tomcat.util.net.SSLHostConfigCertificate");
        digester.addObjectCreate("Server/Service/Connector/SSLHostConfig/OpenSSLConf",
                                 "org.apache.tomcat.util.net.openssl.OpenSSLConf");
        digester.addSetProperties("Server/Service/Connector/SSLHostConfig/OpenSSLConf");
        digester.addSetNext("Server/Service/Connector/SSLHostConfig/OpenSSLConf",
                            "setOpenSslConf",
                            "org.apache.tomcat.util.net.openssl.OpenSSLConf");
        digester.addObjectCreate("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd",
                                 "org.apache.tomcat.util.net.openssl.OpenSSLConfCmd");
        digester.addSetProperties("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd");
        digester.addSetNext("Server/Service/Connector/SSLHostConfig/OpenSSLConf/OpenSSLConfCmd",
                            "addCmd",
                            "org.apache.tomcat.util.net.openssl.OpenSSLConfCmd");
        digester.addObjectCreate("Server/Service/Connector/Listener",
                                 null, // MUST be specified in the element
                                 "className");
        digester.addSetProperties("Server/Service/Connector/Listener");
        digester.addSetNext("Server/Service/Connector/Listener",
                            "addLifecycleListener",
                            "org.apache.catalina.LifecycleListener");
        digester.addObjectCreate("Server/Service/Connector/UpgradeProtocol",
                                  null, // MUST be specified in the element
                                  "className");
        digester.addSetProperties("Server/Service/Connector/UpgradeProtocol");
        digester.addSetNext("Server/Service/Connector/UpgradeProtocol",
                            "addUpgradeProtocol",
                            "org.apache.coyote.UpgradeProtocol");
        // Add RuleSets for nested elements
        digester.addRuleSet(new NamingRuleSet("Server/GlobalNamingResources/"));
        digester.addRuleSet(new EngineRuleSet("Server/Service/"));
        digester.addRuleSet(new HostRuleSet("Server/Service/Engine/"));
        digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/"));
        addClusterRuleSet(digester, "Server/Service/Engine/Host/Cluster/");
        digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/Context/"));
        // When the 'engine' is found, set the parentClassLoader.
        digester.addRule("Server/Service/Engine",
                         new SetParentClassLoaderRule(parentClassLoader));
        addClusterRuleSet(digester, "Server/Service/Engine/Cluster/");
        long t2=System.currentTimeMillis();
        if (log.isDebugEnabled()) {
            log.debug("Digester for server.xml created " + ( t2-t1 ));
        }
        return digester;
    }

此方法創建解析server.xml的Digester，根據server.xml的元素標簽，為每個標簽設置相應的規則組，在解析標簽時進行調用。
由此也可以得到結論，server.xml的結構就是Tomcat容器內部的結構，通過對server.xml的解析規則的執行，實例化出Tomcat容器結構。

以下為Tomcat9默認的server.xml

其結構見下圖：
Tomcat9如何加載server.xml

Server代表服務器,一個Tomcat只有一個Server
Service 代表服務: 一個Server可以對外提供多個服務
Connector連接器: service服務的核心組成之一,主要是鏈接客戶端請求
Container容器：service服務的核心組成之一，主要是執行業務邏輯，這里按層級為Engine、Host、Context
Wrapper：對應Servlet的定義

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