Tomcat之XML解析器Digester

痛定思痛。 2022-05-25 03:15 201阅读 0赞

Catalina中使用Digester解析XML(server.xml)配置文件并创建应用服务器。

Digester是一款用于将XML转换为Java对象的事件驱动型工具，是对SAX的高层次封装，它提供了更加友好的接口，隐藏了XML节点具体的层次细节，使开发者可以更加专注于处理过程。

Digester最早作为Appache Struts的一部分，后来考虑其通用性，将其移动到了Appache Commons项目。

Tomcat中使用Digester API，但并不依赖Appache Commons包，而是将其源代码直接移植到Tomcat项目中。

Digester匹配模式(server.xml)

Digester定义操作规则（Rule）

/** * Create and configure the Digester we will be using for startup. * @return the main digester to parse server.xml */
        protected Digester createStartDigester() {
            long t1=System.currentTimeMillis();
            // Initialize the digester
            Digester digester = new Digester();
            digester.setValidating(false);
            digester.setRulesValidation(true);
            HashMap<Class<?>, List<String>> fakeAttributes = new HashMap<>();
            ArrayList<String> attrs = new ArrayList<>();
            attrs.add("className");
            fakeAttributes.put(Object.class, attrs);
            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.addObjectCreate("Server/Listener",
                                     null, // MUST be specified in the element
                                     "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"}));
            digester.addSetNext("Server/Service/Connector",
                                "addConnector",
                                "org.apache.catalina.connector.Connector");
    
            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);
    
        }

**1.addObjectCreate(String rule,Class class)**  
设置节点与Java对象的映射规则，rule指定节点的筛选规则，class设置映射对象。SAX解析时，遇到rule指定的节节点，会创建一个class实例放入堆栈中。

比如：digester.addObjectCreate(“Server”,  
“org.apache.catalina.core.StandardServer”,  
“className”);.解析遇到Server节点时，会创建一个org.apache.catalina.core.StandardServer实例并放入堆栈中。

**2.addSetProperties(String rule)**  
设置节点的属性设置规则。当解析遇到符合rule的节点时，根据属性列表中的属性值对，使用Java反射机制使用标准的JavaBean方法设置栈顶对象实例；

比如：digester.addSetProperties(“Server”),解析遇到Server节点时，会获取键值对，获得栈顶的StandardServer对象，设置实例的globalNamingResources、port等属性；解析元素上的属性

**3.addBeanPropertySetter(String rule)**  
该方法的作用及使用方法类似于addSetProperties，只不过它是用rule所指定的标签来调用对象的setter。

**4.addSetNext(String rule,String methodName)**  
设置当前rule节点与父节点的调用规则，当遇到rule节点时，调用堆栈中的次栈顶元素调用methodName方法。将栈顶元素作为次顶元素指定方法的输入参数。

比如: digester.addSetNext(“Server”,  
“setServer”,  
“org.apache.catalina.Server”),调用Server实例的setServer，org.apache.catalina.Server为参数类型。

**5.addCallMethod(String rule,String methodName,int paraNumber)**  
该方法同样设置对象的属性，但更加灵活，不需要对象具有setter

根据rule规则指定的属性，调用对象的methodName方法，paraNumber参数是表示方法需要的参数个数，当paraNumber=0时，可以单独使用，不然需要配合addCallParam方法

**6.addCallParam(String rule,int paraIndex,String attributeName)**  
该方法与addCallMethod配合使用，根据rule指定的标签属性来调用方法

paraIndex表明需要填充的方法形参序号，从0开始，方法由addCallMethdo指定，attributeName指定标签属性名；