本文重点关注启动tomcat时会用到的两个类，分别为Catalina类和Bootstrap类，它们都位于org.apachae.catalina.startup包下；Catalina类用于启动或关闭Server对象，并负责解析server.xml配置文件；Bootstrap类是一个入口点，负责创建Catalina实例，并调用其process()方法。
org.apachae.catalina.startup.Catalina类是启动类，它包含一个Digester对象，用于解析位于%CATALINE_HOME%/conf目录的server.xml文件
Catalina类还封装了一个Server对象,该对象持有一个Service对象（Service对象包含一个Servlet容器和一个或多个连接器）。可以使用Catalina类来启动/关闭Server对象
可以通过实例化Catalina类，并调用其process()方法来运行Tomcat，但在调用该方法时，需要传入适当的参数（如start或stop -help -config -debug -nonaming等）。
/**
* The instance main program.
*
* @param args Command line arguments
*/
public void process(String args[]) { setCatalinaHome();
setCatalinaBase();
try {
if (arguments(args))
execute();
} catch (Exception e) {
e.printStackTrace(System.out);
}
}
上面方法会进一步调用execute()方法，execute()方法会根据传入参数调用start()方法或stop()方法
start方法会创建一个Digester对象来解析server.xml文件（Tomcat配置文件）。在解析server.xml文件之前，start方法会调用Digester对象的push方法，传入当前的Catalina对象为参数。这样，Catalina对象就成了Digester对象内部对象栈的第一个对象。解析server.xml文件后，会将变量server指向一个Server对象（默认是org.apache.catalina.core.StandardServer类型的对象）。然后，start方法会调用server的initialize和start方法。Catalina对象的start方法会调用Server对象的await方法，server对象会使用一个专用的线程来等待关闭命令。await方法会循环等待，知道接收到正确的关闭命令。当await方法返回时，Catalina对象的start方法会调用server对象的stop方法，从而关闭server对象和其他的组件。此外，start方法还会注册shutdown hook，确保服务器关闭时会执行Server对象的stop方法。
start方法的实现如下：
/**
* Start a new server instance.
*/
protected void start() { // Create and execute our Digester
Digester digester = createStartDigester();
File file = configFile();
try {
InputSource is =
new InputSource("file://" + file.getAbsolutePath());
FileInputStream fis = new FileInputStream(file);
is.setByteStream(fis);
digester.push(this);
digester.parse(is);
fis.close();
} catch (Exception e) {
System.out.println("Catalina.start: " + e);
e.printStackTrace(System.out);
System.exit(1);
} // Setting additional variables
if (!useNaming) {
System.setProperty("catalina.useNaming", "false");
} else {
System.setProperty("catalina.useNaming", "true");
String value = "org.apache.naming";
String oldValue =
System.getProperty(javax.naming.Context.URL_PKG_PREFIXES);
if (oldValue != null) {
value = value + ":" + oldValue;
}
System.setProperty(javax.naming.Context.URL_PKG_PREFIXES, value);
value = System.getProperty
(javax.naming.Context.INITIAL_CONTEXT_FACTORY);
if (value == null) {
System.setProperty
(javax.naming.Context.INITIAL_CONTEXT_FACTORY,
"org.apache.naming.java.javaURLContextFactory");
}
} // If a SecurityManager is being used, set properties for
// checkPackageAccess() and checkPackageDefinition
if( System.getSecurityManager() != null ) {
String access = Security.getProperty("package.access");
if( access != null && access.length() > 0 )
access += ",";
else
access = "sun.,";
Security.setProperty("package.access",
access + "org.apache.catalina.,org.apache.jasper.");
String definition = Security.getProperty("package.definition");
if( definition != null && definition.length() > 0 )
definition += ",";
else
definition = "sun.,";
Security.setProperty("package.definition",
// FIX ME package "javax." was removed to prevent HotSpot
// fatal internal errors
definition + "java.,org.apache.catalina.,org.apache.jasper.");
} // Replace System.out and System.err with a custom PrintStream
SystemLogHandler log = new SystemLogHandler(System.out);
System.setOut(log);
System.setErr(log); Thread shutdownHook = new CatalinaShutdownHook(); // Start the new server
if (server instanceof Lifecycle) {
try {
server.initialize();
((Lifecycle) server).start();
try {
// Register shutdown hook
Runtime.getRuntime().addShutdownHook(shutdownHook);
} catch (Throwable t) {
// This will fail on JDK 1.2. Ignoring, as Tomcat can run
// fine without the shutdown hook.
}
// Wait for the server to be told to shut down
server.await();
} catch (LifecycleException e) {
System.out.println("Catalina.start: " + e);
e.printStackTrace(System.out);
if (e.getThrowable() != null) {
System.out.println("----- Root Cause -----");
e.getThrowable().printStackTrace(System.out);
}
}
} // Shut down the server
if (server instanceof Lifecycle) {
try {
try {
// Remove the ShutdownHook first so that server.stop()
// doesn't get invoked twice
Runtime.getRuntime().removeShutdownHook(shutdownHook);
} catch (Throwable t) {
// This will fail on JDK 1.2. Ignoring, as Tomcat can run
// fine without the shutdown hook.
}
((Lifecycle) server).stop();
} catch (LifecycleException e) {
System.out.println("Catalina.stop: " + e);
e.printStackTrace(System.out);
if (e.getThrowable() != null) {
System.out.println("----- Root Cause -----");
e.getThrowable().printStackTrace(System.out);
}
}
} }
其中的CatalinaShutdownHook类为关闭钩子
/**
* Shutdown hook which will perform a clean shutdown of Catalina if needed.
*/
protected class CatalinaShutdownHook extends Thread { public void run() { if (server != null) {
try {
((Lifecycle) server).stop();
} catch (LifecycleException e) {
System.out.println("Catalina.stop: " + e);
e.printStackTrace(System.out);
if (e.getThrowable() != null) {
System.out.println("----- Root Cause -----");
e.getThrowable().printStackTrace(System.out);
}
}
} }
}
Catalina对象的stop方法会关闭Server对象，其实现如下：
/**
* Stop an existing server instance.
*/
protected void stop() { // Create and execute our Digester
Digester digester = createStopDigester();
File file = configFile();
try {
InputSource is =
new InputSource("file://" + file.getAbsolutePath());
FileInputStream fis = new FileInputStream(file);
is.setByteStream(fis);
digester.push(this);
digester.parse(is);
fis.close();
} catch (Exception e) {
System.out.println("Catalina.stop: " + e);
e.printStackTrace(System.out);
System.exit(1);
} // Stop the existing server
try {
Socket socket = new Socket("127.0.0.1", server.getPort());
OutputStream stream = socket.getOutputStream();
String shutdown = server.getShutdown();
for (int i = 0; i < shutdown.length(); i++)
stream.write(shutdown.charAt(i));
stream.flush();
stream.close();
socket.close();
} catch (IOException e) {
System.out.println("Catalina.stop: " + e);
e.printStackTrace(System.out);
System.exit(1);
} }
注意，stop方法通过调用createStopDigester方法创建一个Digester对象，然后将Catalina对象push到Digester对象的内部对象栈中。
createStartDigester方法创建了一个Digester对象，然后将规则添加到其中，解析server.xml文件。添加到Digester对象中的规则是理解tomcat配置的关键。
createStartDigester方法的实现如下：
/**
* Create and configure the Digester we will be using for startup.
*/
protected Digester createStartDigester() { // Initialize the digester
Digester digester = new Digester();
if (debug)
digester.setDebug(999);
digester.setValidating(false); // 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.NamingResources");
digester.addSetProperties("Server/GlobalNamingResources");
digester.addSetNext("Server/GlobalNamingResources",
"setGlobalNamingResources",
"org.apache.catalina.deploy.NamingResources"); 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"); digester.addObjectCreate("Server/Service/Connector",
"org.apache.catalina.connector.http.HttpConnector",
"className");
digester.addSetProperties("Server/Service/Connector");
digester.addSetNext("Server/Service/Connector",
"addConnector",
"org.apache.catalina.Connector"); digester.addObjectCreate("Server/Service/Connector/Factory",
"org.apache.catalina.net.DefaultServerSocketFactory",
"className");
digester.addSetProperties("Server/Service/Connector/Factory");
digester.addSetNext("Server/Service/Connector/Factory",
"setFactory",
"org.apache.catalina.net.ServerSocketFactory"); 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"); // 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/Default"));
digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/DefaultContext/"));
digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/Default"));
digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/DefaultContext/"));
digester.addRuleSet(new ContextRuleSet("Server/Service/Engine/Host/"));
digester.addRuleSet(new NamingRuleSet("Server/Service/Engine/Host/Context/")); digester.addRule("Server/Service/Engine",
new SetParentClassLoaderRule(digester,
parentClassLoader)); return (digester); }
这里需要注意的是 digester.addSetNext("Server", "setServer","org.apache.catalina.Server")方法，该方法将Server对象压入到Digester对象的内部栈中，并与栈中的下一个对象相关联；在这里，下一个对象是Catalina实例，调用其setServer()方法与Server对象相关联。
createStopDigester方法返回一个Digester对象来关闭Server对象。createStopDigester方法实现如下：
/**
* Create and configure the Digester we will be using for shutdown.
*/
protected Digester createStopDigester() { // Initialize the digester
Digester digester = new Digester();
if (debug)
digester.setDebug(999); // Configure the rules we need for shutting down
digester.addObjectCreate("Server",
"org.apache.catalina.core.StandardServer",
"className");
digester.addSetProperties("Server");
digester.addSetNext("Server",
"setServer",
"org.apache.catalina.Server"); return (digester); }
与启动Digester对象不同，关闭Digester对象只对XML文件的根元素感兴趣
org.apache.catalina.startup.Bootstrap类提供了启动tomcat的切入点（还有一些其他的类也有此功能）。当使用bat或sh启动tomcat时，实际上会调用该类的main方法。在main方法中会创建三个loader，并实例化Catalina对象，然后调用Catalina对象的process方法。
Bootstrap类的定义如下所示：
public final class Bootstrap {
// ------------------------------------------------------- Static Variables
/**
* Debugging detail level for processing the startup.
*/
private static int debug = 0;
// ----------------------------------------------------------- Main Program
/**
* The main program for the bootstrap.
*
* @param args Command line arguments to be processed
*/
public static void main(String args[]) {
// Set the debug flag appropriately
for (int i = 0; i < args.length; i++) {
if ("-debug".equals(args[i]))
debug = 1;
}
// Configure catalina.base from catalina.home if not yet set
if (System.getProperty("catalina.base") == null)
System.setProperty("catalina.base", getCatalinaHome());
// Construct the class loaders we will need
ClassLoader commonLoader = null;
ClassLoader catalinaLoader = null;
ClassLoader sharedLoader = null;
try {
File unpacked[] = new File[1];
File packed[] = new File[1];
File packed2[] = new File[2];
ClassLoaderFactory.setDebug(debug);
unpacked[0] = new File(getCatalinaHome(),
"common" + File.separator + "classes");
packed2[0] = new File(getCatalinaHome(),
"common" + File.separator + "endorsed");
packed2[1] = new File(getCatalinaHome(),
"common" + File.separator + "lib");
commonLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed2, null);
unpacked[0] = new File(getCatalinaHome(),
"server" + File.separator + "classes");
packed[0] = new File(getCatalinaHome(),
"server" + File.separator + "lib");
catalinaLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed,
commonLoader);
unpacked[0] = new File(getCatalinaBase(),
"shared" + File.separator + "classes");
packed[0] = new File(getCatalinaBase(),
"shared" + File.separator + "lib");
sharedLoader =
ClassLoaderFactory.createClassLoader(unpacked, packed,
commonLoader);
} catch (Throwable t) {
log("Class loader creation threw exception", t);
System.exit(1);
}
Thread.currentThread().setContextClassLoader(catalinaLoader);
// Load our startup class and call its process() method
try {
SecurityClassLoad.securityClassLoad(catalinaLoader);
// Instantiate a startup class instance
if (debug >= 1)
log("Loading startup class");
Class startupClass =
catalinaLoader.loadClass
("org.apache.catalina.startup.Catalina");
Object startupInstance = startupClass.newInstance();
// Set the shared extensions class loader
if (debug >= 1)
log("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);
// Call the process() method
if (debug >= 1)
log("Calling startup class process() method");
methodName = "process";
paramTypes = new Class[1];
paramTypes[0] = args.getClass();
paramValues = new Object[1];
paramValues[0] = args;
method =
startupInstance.getClass().getMethod(methodName, paramTypes);
method.invoke(startupInstance, paramValues);
} catch (Exception e) {
System.out.println("Exception during startup processing");
e.printStackTrace(System.out);
System.exit(2);
}
}
/**
* Get the value of the catalina.home environment variable.
*/
private static String getCatalinaHome() {
return System.getProperty("catalina.home",
System.getProperty("user.dir"));
}
/**
* Get the value of the catalina.base environment variable.
*/
private static String getCatalinaBase() {
return System.getProperty("catalina.base", getCatalinaHome());
}
/**
* Log a debugging detail message.
*
* @param message The message to be logged
*/
private static void log(String message) {
System.out.print("Bootstrap: ");
System.out.println(message);
}
/**
* Log a debugging detail message with an exception.
*
* @param message The message to be logged
* @param exception The exception to be logged
*/
private static void log(String message, Throwable exception) {
log(message);
exception.printStackTrace(System.out);
}
}
Bootstrap类定义了四个静态方法，其中getCatalinaHome返回catalina.home属性的值，若没有，则返回user.dir属性的值。getCatalinaBase方法与getCatalinaHome方法类似。
Bootstrap类的main方法构造了三个loader，之所以如此做是为了防止运行WEB-INF/classes和WEB-INF/lib目录外的类。
其中commonLoader允许从%CATALINA_HOME%/common/classes，%CATALINA_HOME%/common/endorsed和%CATALINA_HOME%/common/lib目录下载入类。
catalinaLoader负责载入servlet容器需要使用的类，它只会从%CATALINA_HOME%/server/classes和%CATALINA_HOME%/server/lib目录下查找。
sharedLoader会从%CATALINA_HOME%/shared/classes和%CATALJNA_HOME%/shared/lib目录，以及对commonLoader可用的目录下查找需要的类。
然后，将sharedLoader设置为每个web应用的类载入器的父类载入器。（注意，sharedLoader并不访问catalina的内部类，或CLASSPATH中的类）
在创建了三个loader之后，main方法会载入Catalina类，实例化，并将之赋值给startupInstance变量。然后调用setParentClassLoader方法。
最后，main方法调用Catalina对象的process对象。
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