学步园

http://tomcat.apache.org/tomcat-4.1-doc/class-loader-howto.html

Tomcat的ClassLoader层次结构：

	Bootstrap | System | Common / / Catalina Shared / / Webapp1 Webapp2 ...

源代码如下：

org.apache.catalina.startup.Bootstrap类（tomcat主类）

public final class Bootstrap {<br /> // ------------------------------------------------------- Static Variables<br /> /**<br /> * Debugging detail level for processing the startup.<br /> */<br /> private static int debug = 0;<br /> // ----------------------------------------------------------- Main Program<br /> /**<br /> * The main program for the bootstrap.<br /> *<br /> * @param args Command line arguments to be processed<br /> */<br /> public static void main(String args[]) {<br /> // Set the debug flag appropriately<br /> for (int i = 0; i < args.length; i++) {<br /> if ("-debug".equals(args[i]))<br /> debug = 1;<br /> }<br /> // Configure catalina.base from catalina.home if not yet set<br /> if (System.getProperty("catalina.base") == null)<br /> System.setProperty("catalina.base", getCatalinaHome());<br /> // Construct the class loaders we will need<br /> ClassLoader commonLoader = null;<br /> ClassLoader catalinaLoader = null;<br /> ClassLoader sharedLoader = null;<br /> try {<br /> File unpacked[] = new File[1];<br /> File packed[] = new File[1];<br /> File packed2[] = new File[2];<br /> ClassLoaderFactory.setDebug(debug);<br /> unpacked[0] = new File(getCatalinaHome(),<br /> "common" + File.separator + "classes");<br /> packed2[0] = new File(getCatalinaHome(),<br /> "common" + File.separator + "endorsed");<br /> packed2[1] = new File(getCatalinaHome(),<br /> "common" + File.separator + "lib");<br /> commonLoader =<br /> ClassLoaderFactory.createClassLoader(unpacked, packed2, null);<br /> unpacked[0] = new File(getCatalinaHome(),<br /> "server" + File.separator + "classes");<br /> packed[0] = new File(getCatalinaHome(),<br /> "server" + File.separator + "lib");<br /> catalinaLoader =<br /> ClassLoaderFactory.createClassLoader(unpacked, packed,<br /> commonLoader);<br /> unpacked[0] = new File(getCatalinaBase(),<br /> "shared" + File.separator + "classes");<br /> packed[0] = new File(getCatalinaBase(),<br /> "shared" + File.separator + "lib");<br /> sharedLoader =<br /> ClassLoaderFactory.createClassLoader(unpacked, packed,<br /> commonLoader);<br /> } catch (Throwable t) {<br /> log("Class loader creation threw exception", t);<br /> System.exit(1);<br /> }<br /> Thread.currentThread().setContextClassLoader(catalinaLoader);<br /> // Load our startup class and call its process() method<br /> try {<br /> SecurityClassLoad.securityClassLoad(catalinaLoader);<br /> // Instantiate a startup class instance<br /> if (debug >= 1)<br /> log("Loading startup class");<br /> Class startupClass =<br /> catalinaLoader.loadClass<br /> ("org.apache.catalina.startup.Catalina");<br /> Object startupInstance = startupClass.newInstance();<br /> // Set the shared extensions class loader<br /> if (debug >= 1)<br /> log("Setting startup class properties");<br /> String methodName = "setParentClassLoader";<br /> Class paramTypes[] = new Class[1];<br /> paramTypes[0] = Class.forName("java.lang.ClassLoader");<br /> Object paramValues[] = new Object[1];<br /> paramValues[0] = sharedLoader;<br /> Method method =<br /> startupInstance.getClass().getMethod(methodName, paramTypes);<br /> method.invoke(startupInstance, paramValues);<br /> // Call the process() method<br /> if (debug >= 1)<br /> log("Calling startup class process() method");<br /> methodName = "process";<br /> paramTypes = new Class[1];<br /> paramTypes[0] = args.getClass();<br /> paramValues = new Object[1];<br /> paramValues[0] = args;<br /> method =<br /> startupInstance.getClass().getMethod(methodName, paramTypes);<br /> method.invoke(startupInstance, paramValues);<br /> } catch (Exception e) {<br /> System.out.println("Exception during startup processing");<br /> e.printStackTrace(System.out);<br /> System.exit(2);<br /> }<br /> }

其中：

commonLoader =
                ClassLoaderFactory.createClassLoader(unpacked, packed2, null);

创建common classloader，以AppClassLoader为父ClassLoader

catalinaLoader =
                ClassLoaderFactory.createClassLoader(unpacked, packed,
                                                     commonLoader);

创建catalina classloader，以common classloader为父classloader

sharedLoader =
                ClassLoaderFactory.createClassLoader(unpacked, packed,
                                                     commonLoader);

创建share classloader，以common classloader为父classloader

Thread.currentThread().setContextClassLoader(catalinaLoader);

设置ContextClassLoader为catalina classloader

org.apache.catalina.startup.ClassLoaderFactory类

public static ClassLoader createClassLoader(File unpacked[],<br /> File packed[],<br /> ClassLoader parent)<br /> throws Exception {<br /> if (debug >= 1)<br /> log("Creating new class loader");<br /> // Construct the "class path" for this class loader<br /> ArrayList list = new ArrayList();<br /> // Add unpacked directories<br /> if (unpacked != null) {<br /> for (int i = 0; i < unpacked.length; i++) {<br /> File file = unpacked[i];<br /> if (!file.isDirectory() || !file.exists() || !file.canRead())<br /> continue;<br /> if (debug >= 1)<br /> log(" Including directory " + file.getAbsolutePath());<br /> URL url = new URL("file", null,<br /> file.getCanonicalPath() + File.separator);<br /> list.add(url.toString());<br /> }<br /> }<br /> // Add packed directory JAR files<br /> if (packed != null) {<br /> for (int i = 0; i < packed.length; i++) {<br /> File directory = packed[i];<br /> if (!directory.isDirectory() || !directory.exists() ||<br /> !directory.canRead())<br /> continue;<br /> String filenames[] = directory.list();<br /> for (int j = 0; j < filenames.length; j++) {<br /> String filename = filenames[j].toLowerCase();<br /> if (!filename.endsWith(".jar"))<br /> continue;<br /> File file = new File(directory, filenames[j]);<br /> if (debug >= 1)<br /> log(" Including jar file " + file.getAbsolutePath());<br /> URL url = new URL("file", null,<br /> file.getCanonicalPath());<br /> list.add(url.toString());<br /> }<br /> }<br /> }<br /> // Construct the class loader itself<br /> String array[] = (String[]) list.toArray(new String[list.size()]);<br /> StandardClassLoader classLoader = null;<br /> if (parent == null)<br /> classLoader = new StandardClassLoader(array);<br /> else<br /> classLoader = new StandardClassLoader(array, parent);<br /> classLoader.setDelegate(true);<br /> return (classLoader);<br /> }

ClassLoaderFactory创建的是StandardClassLoader(org.apache.catalina.loader包中)

由Bootstrap类调用Catalina类的process()方法，再调用execute()方法，再调用start()来启动Server实例。

当Tomcat开启每个Context时，是调用的StandardContext的start()方法，其中：

设置Loader为WebappLoader

if (getLoader() == null) { // (2) Required by Manager<br /> if (getPrivileged()) {<br /> if (debug >= 1)<br /> log("Configuring privileged default Loader");<br /> setLoader(new WebappLoader(this.getClass().getClassLoader()));<br /> } else {<br /> if (debug >= 1)<br /> log("Configuring non-privileged default Loader");<br /> setLoader(new WebappLoader(getParentClassLoader()));<br /> }<br /> }

然后调用：bindThread()，设置当前线程的ClassLoader为WebappLoader的ClassLoader，即为WebappClassLoader

private ClassLoader bindThread() {<br /> ClassLoader oldContextClassLoader =<br /> Thread.currentThread().getContextClassLoader();<br /> if (getResources() == null)<br /> return oldContextClassLoader;<br /> Thread.currentThread().setContextClassLoader<br /> (getLoader().getClassLoader());<br /> DirContextURLStreamHandler.bind(getResources());<br /> if (isUseNaming()) {<br /> try {<br /> ContextBindings.bindThread(this, this);<br /> } catch (NamingException e) {<br /> // Silent catch, as this is a normal case during the early<br /> // startup stages<br /> }<br /> }<br /> return oldContextClassLoader;<br /> }

WebappLoader的ClassLoader的赋值如下：

private String loaderClass =<br /> "org.apache.catalina.loader.WebappClassLoader";<br /> ......<br /> public void start() throws LifecycleException {<br /> // Validate and update our current component state<br /> if (started)<br /> throw new LifecycleException<br /> (sm.getString("webappLoader.alreadyStarted"));<br /> if (debug >= 1)<br /> log(sm.getString("webappLoader.starting"));<br /> lifecycle.fireLifecycleEvent(START_EVENT, null);<br /> started = true;<br /> if (container.getResources() == null)<br /> return;<br /> // Register a stream handler factory for the JNDI protocol<br /> URLStreamHandlerFactory streamHandlerFactory =<br /> new DirContextURLStreamHandlerFactory();<br /> try {<br /> URL.setURLStreamHandlerFactory(streamHandlerFactory);<br /> } catch (Throwable t) {<br /> // Ignore the error here.<br /> }<br /> // Construct a class loader based on our current repositories list<br /> try {<br /> classLoader = createClassLoader();<br /> classLoader.setResources(container.getResources());<br /> classLoader.setDebug(this.debug);<br /> classLoader.setDelegate(this.delegate);<br /> if (container instanceof StandardContext)<br /> classLoader.setAntiJARLocking(((StandardContext) container).getAntiJARLocking());<br /> for (int i = 0; i < repositories.length; i++) {<br /> classLoader.addRepository(repositories[i]);<br /> }<br /> // Configure our repositories<br /> setRepositories();<br /> setClassPath();<br /> setPermissions();<br /> if (classLoader instanceof Lifecycle)<br /> ((Lifecycle) classLoader).start();<br /> // Binding the Webapp class loader to the directory context<br /> DirContextURLStreamHandler.bind<br /> ((ClassLoader) classLoader, this.container.getResources());<br /> } catch (Throwable t) {<br /> throw new LifecycleException("start: ", t);<br /> }<br /> // Validate that all required packages are actually available<br /> validatePackages();<br /> // Start our background thread if we are reloadable<br /> if (reloadable) {<br /> log(sm.getString("webappLoader.reloading"));<br /> try {<br /> threadStart();<br /> } catch (IllegalStateException e) {<br /> throw new LifecycleException(e);<br /> }<br /> }<br /> }<br /> ......<br /> private WebappClassLoader createClassLoader()<br /> throws Exception {<br /> Class clazz = Class.forName(loaderClass);<br /> WebappClassLoader classLoader = null;<br /> if (parentClassLoader == null) {<br /> // Will cause a ClassCast is the class does not extend WCL, but<br /> // this is on purpose (the exception will be caught and rethrown)<br /> classLoader = (WebappClassLoader) clazz.newInstance();<br /> } else {<br /> Class[] argTypes = { ClassLoader.class };<br /> Object[] args = { parentClassLoader };<br /> Constructor constr = clazz.getConstructor(argTypes);<br /> classLoader = (WebappClassLoader) constr.newInstance(args);<br /> }<br /> return classLoader;<br /> }

在WebappClassLoader中，其findClass的搜索顺序与一般的ClassLoader的搜索顺序不同。

一般的ClassLoader的搜索顺序为：

将其委托给父ClassLoader，如果父ClassLoader不能载入相应类，则才交给自己处理

但是WebappClassLoader中，其是先由自己来处理，如果不行再委托给父ClassLoader

相关源代码如下：

Class clazz = null;<br /> try {<br /> if (debug >= 4)<br /> log(" findClassInternal(" + name + ")");<br /> try {<br /> clazz = findClassInternal(name);<br /> } catch(ClassNotFoundException cnfe) {<br /> if (!hasExternalRepositories) {<br /> throw cnfe;<br /> }<br /> } catch(AccessControlException ace) {<br /> ace.printStackTrace();<br /> throw new ClassNotFoundException(name);<br /> } catch (RuntimeException e) {<br /> if (debug >= 4)<br /> log(" -->RuntimeException Rethrown", e);<br /> throw e;<br /> }<br /> if ((clazz == null) && hasExternalRepositories) {<br /> try {<br /> clazz = super.findClass(name);<br /> } catch(AccessControlException ace) {<br /> throw new ClassNotFoundException(name);<br /> } catch (RuntimeException e) {<br /> if (debug >= 4)<br /> log(" -->RuntimeException Rethrown", e);<br /> throw e;<br /> }<br /> }<br /> if (clazz == null) {<br /> if (debug >= 3)<br /> log(" --> Returning ClassNotFoundException");<br /> throw new ClassNotFoundException(name);<br /> }<br /> } catch (ClassNotFoundException e) {<br /> if (debug >= 3)<br /> log(" --> Passing on ClassNotFoundException", e);<br /> throw e;<br /> }</p> <p>

以下引自tomcat的说明文档，说明了加载类的顺序

Therefore, from the perspective of a web application, class or resource
loading looks in the following repositories, in this order:

• /WEB-INF/classes
  of your web application
• /WEB-INF/lib/*.jar
  of your web application
• Bootstrap classes of your JVM
• System class loader classses (described above)
• $CATALINA_HOME/common/classes
• $CATALINA_HOME/common/endorsed/*.jar
• $CATALINA_HOME/common/lib/*.jar
• $CATALINA_BASE/shared/classes
• $CATALINA_BASE/shared/lib/*.jar