tomcat 爲了做到每個host中都能加載各種不同的WEB應用而不相互影響，採用的類加載機制有所特別：
加載WEB應用中我們自己寫的類的順序也是按照圖中 標示的1243順序所示。
把WebAppClassLoader.java的loadClass方法貼出來瞧瞧：
public synchronized Class loadClass(String name, boolean resolve)
throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class clazz = null;
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
// (0) Check our previously loaded local class cache.從先前已經加載的本地 cache中查找該類
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (0.1) Check our previously loaded class cache .從先前已經加載cache中查找該類
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (0.2) Try loading the class with the system class loader, to prevent
// the webapp from overriding J2SE classes
// 嘗試從system class loader中加載，保護j2se 的核心類。
try {
clazz = system.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
// (0.5) Permission to access this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
securityManager.checkPackageAccess(name.substring(0,i));
} catch (SecurityException se) {
String error = "Security Violation, attempt to use " +
"Restricted Class: " + name;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name);
// (1) Delegate to our parent if requested
// 如果delegate被設置爲true的話委託給雙親加載
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader1 " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
// (2)從WEB-INF/lib ,WEB-INF/classes加載類
if (log.isDebugEnabled())
log.debug(" Searching local repositories");
try {
clazz = findClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from local repository");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
// (3) Delegate to parent unconditionally
// 如果本地找不到需要加載的類，則還是委託雙親去加載該類
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader at end: " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = loader.loadClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
;
}
}
throw new ClassNotFoundException(name);
}那麼在哪裏用到了WebAppClassLoader呢，見下面的代碼，代碼貼的比較長。但是比較重點的：
StandardWrapper.java
/**
* Load and initialize an instance of this servlet, if there is not already at least one initialized instance. This
* can be used, for example, to load servlets that are marked in the deployment descriptor to be loaded at server
* startup time.
*/
public synchronized Servlet loadServlet() throws ServletException {
// Nothing to do if we already have an instance or an instance pool
// 初始化servlet的時候，如果發現已經初始化完畢，則不再初始化
if (!singleThreadModel && (instance != null)) return instance;
// 把標準輸出重定向
PrintStream out = System.out;
if (swallowOutput) {
SystemLogHandler.startCapture();
}
Servlet servlet;
try {
long t1 = System.currentTimeMillis();
// If this "servlet" is really a JSP file, get the right class.
// HOLD YOUR NOSE - this is a kludge that avoids having to do special
// case Catalina-specific code in Jasper - it also requires that the
// servlet path be replaced by the <jsp-file> element content in
// order to be completely effective
/**
* <pre>
* 以下代碼爲了使類似這樣的配置
* <servlet>
* <servlet-name>Test</servlet-name>
* <jsp-file>/TestPage.jsp</jsp-file>
* </servlet>
* 其中<jsp-file>是用來代替<servlet-class>的
* </pre>
*/
String actualClass = servletClass;
if ((actualClass == null) && (jspFile != null)) {
Wrapper jspWrapper = (Wrapper) ((Context) getParent()).findChild(Constants.JSP_SERVLET_NAME);
if (jspWrapper != null) {
actualClass = jspWrapper.getServletClass();
// Merge init parameters
String paramNames[] = jspWrapper.findInitParameters();
for (int i = 0; i < paramNames.length; i++) {
if (parameters.get(paramNames[i]) == null) {
parameters.put(paramNames[i], jspWrapper.findInitParameter(paramNames[i]));
}
}
}
}
// Complain if no servlet class has been specified
// 如果配置文件中<servlet-class>沒有指定的話，拋出抱怨
if (actualClass == null) {
unavailable(null);
throw new ServletException(sm.getString("standardWrapper.notClass", getName()));
}
// Acquire an instance of the class loader to be used
// 獲取一個CLASS LOADER來搞，一般獲得的是WebAppClassLoader
Loader loader = getLoader();
if (loader == null) {
unavailable(null);
throw new ServletException(sm.getString("standardWrapper.missingLoader", getName()));
}
ClassLoader classLoader = loader.getClassLoader();
// Special case class loader for a container provided servlet
if (isContainerProvidedServlet(actualClass) && !((Context) getParent()).getPrivileged()) {
// If it is a priviledged context - using its own
// class loader will work, since it's a child of the container
// loader
classLoader = this.getClass().getClassLoader();
}
// Load the specified servlet class from the appropriate class loader
Class classClass = null;
try {
if (SecurityUtil.isPackageProtectionEnabled()) {
final ClassLoader fclassLoader = classLoader;
final String factualClass = actualClass;
try {
classClass = (Class) AccessController.doPrivileged(new PrivilegedExceptionAction() {
public Object run() throws Exception {
if (fclassLoader != null) {
return fclassLoader.loadClass(factualClass);
} else {
return Class.forName(factualClass);
}
}
});
} catch (PrivilegedActionException pax) {
Exception ex = pax.getException();
if (ex instanceof ClassNotFoundException) {
throw (ClassNotFoundException) ex;
} else {
getServletContext().log("Error loading " + fclassLoader + " " + factualClass, ex);
}
}
} else {
if (classLoader != null) {
// 使用WebAppClassLoader加載實際的類
classClass = classLoader.loadClass(actualClass);
} else {
classClass = Class.forName(actualClass);
}
}
} catch (ClassNotFoundException e) {
unavailable(null);
getServletContext().log("Error loading " + classLoader + " " + actualClass, e);
throw new ServletException(sm.getString("standardWrapper.missingClass", actualClass), e);
}
if (classClass == null) {
unavailable(null);
throw new ServletException(sm.getString("standardWrapper.missingClass", actualClass));
}
// Instantiate and initialize an instance of the servlet class itself
try {
// 創建一個Servlet實例
servlet = (Servlet) classClass.newInstance();
// Annotation processing
if (!((Context) getParent()).getIgnoreAnnotations()) {
if (getParent() instanceof StandardContext) {
((StandardContext) getParent()).getAnnotationProcessor().processAnnotations(servlet);
((StandardContext) getParent()).getAnnotationProcessor().postConstruct(servlet);
}
}
} catch (ClassCastException e) {
unavailable(null);
// Restore the context ClassLoader
throw new ServletException(sm.getString("standardWrapper.notServlet", actualClass), e);
} catch (Throwable e) {
unavailable(null);
// Added extra log statement for Bugzilla 36630:
// http://issues.apache.org/bugzilla/show_bug.cgi?id=36630
if (log.isDebugEnabled()) {
log.debug(sm.getString("standardWrapper.instantiate", actualClass), e);
}
// Restore the context ClassLoader
throw new ServletException(sm.getString("standardWrapper.instantiate", actualClass), e);
}
// Check if loading the servlet in this web application should be
// allowed
if (!isServletAllowed(servlet)) {
throw new SecurityException(sm.getString("standardWrapper.privilegedServlet", actualClass));
}
// Special handling for ContainerServlet instances
if ((servlet instanceof ContainerServlet)
&& (isContainerProvidedServlet(actualClass) || ((Context) getParent()).getPrivileged())) {
((ContainerServlet) servlet).setWrapper(this);
}
classLoadTime = (int) (System.currentTimeMillis() - t1);
// Call the initialization method of this servlet
try {
// 觸發servlet的beforeInit事件
instanceSupport.fireInstanceEvent(InstanceEvent.BEFORE_INIT_EVENT, servlet);
if (Globals.IS_SECURITY_ENABLED) {
Object[] args = new Object[] { ((ServletConfig) facade) };
SecurityUtil.doAsPrivilege("init", servlet, classType, args);
args = null;
} else {
// 觸發servlet初始化操作
servlet.init(facade);
}
// Invoke jspInit on JSP pages
if ((loadOnStartup >= 0) && (jspFile != null)) {
// Invoking jspInit
DummyRequest req = new DummyRequest();
req.setServletPath(jspFile);
req.setQueryString(Constants.PRECOMPILE + "=true");
DummyResponse res = new DummyResponse();
if (Globals.IS_SECURITY_ENABLED) {
Object[] args = new Object[] { req, res };
SecurityUtil.doAsPrivilege("service", servlet, classTypeUsedInService, args);
args = null;
} else {
servlet.service(req, res);
}
}
// 觸發afterInit初始化操作
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT, servlet);
} catch (UnavailableException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT, servlet, f);
unavailable(f);
throw f;
} catch (ServletException f) {
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT, servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw f;
} catch (Throwable f) {
getServletContext().log("StandardWrapper.Throwable", f);
instanceSupport.fireInstanceEvent(InstanceEvent.AFTER_INIT_EVENT, servlet, f);
// If the servlet wanted to be unavailable it would have
// said so, so do not call unavailable(null).
throw new ServletException(sm.getString("standardWrapper.initException", getName()), f);
}
// Register our newly initialized instance
// singleTheadModel純粹沒用的東西，不用考慮了
singleThreadModel = servlet instanceof SingleThreadModel;
if (singleThreadModel) {
if (instancePool == null) instancePool = new Stack();
}
fireContainerEvent("load", this);
loadTime = System.currentTimeMillis() - t1;
} finally {
if (swallowOutput) {
String log = SystemLogHandler.stopCapture();
if (log != null && log.length() > 0) {
if (getServletContext() != null) {
getServletContext().log(log);
} else {
out.println(log);
}
}
}
}
return servlet;
}